refactor(common,core,proprietary): standardize Locale.ROOT usage for case/format & safer string handling (#4628)

# Description of Changes

- Standardized all locale-sensitive operations to use `Locale.ROOT`:
- Replaced `toLowerCase()/toUpperCase()` and `String.format(...)` with
`Locale.ROOT` variants across services, controllers, utils, and tests
(e.g., `InstallationPathConfig`, `ApplicationProperties`,
`ResourceMonitor`, `ChecksumUtils`, `PdfUtils`, `UploadLimitService`).
- Hardened comparisons and parsing:
- Normalized host/domain and file-extension checks with
`toLowerCase(Locale.ROOT)`; switched several `equals` calls to
constant-first style (e.g., content types, security domain checks).
- Logging & formatting improvements:
- Ensured percent/size values and hex formatting use root-locale
formatting to avoid locale-dependent output.
- Code quality & readability:
  - Converted multiple if/else ladders to modern `switch` expressions.
- Minor refactors (method references, early returns), removed redundant
returns, and clarified log messages.
- Minor fixes/behavioral nits:
- Normalized printer selection by lowercasing the searched name once;
made some equality checks null-safe/constant-first; added missing
`@Override` annotations where appropriate.

## Why

- Consistent use of `Locale.ROOT` avoids surprises in different
user/system locales (e.g., Turkish-I issues), makes string comparisons
deterministic, and keeps numeric/hex formatting stable across
environments.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>
Co-authored-by: James Brunton <jbrunton96@gmail.com>

app/common/src/main/java/stirling/software/common/configuration/InstallationPathConfig.java

@@ -2,6 +2,7 @@ package stirling.software.common.configuration;
 
 import java.io.File;
 import java.nio.file.Paths;
+import java.util.Locale;
 
 import lombok.extern.slf4j.Slf4j;
 
@@ -59,7 +60,7 @@ public class InstallationPathConfig {
 
     private static String initializeBasePath() {
         if (Boolean.parseBoolean(System.getProperty("STIRLING_PDF_DESKTOP_UI", "false"))) {
-            String os = System.getProperty("os.name").toLowerCase();
+            String os = System.getProperty("os.name").toLowerCase(Locale.ROOT);
             if (os.contains("win")) {
                 return Paths.get(
                                 System.getenv("APPDATA"), // parent path

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -12,6 +12,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
+import java.util.Locale;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.context.annotation.Bean;
@@ -286,7 +287,7 @@ public class ApplicationProperties {
                 private KeycloakProvider keycloak = new KeycloakProvider();
 
                 public Provider get(String registrationId) throws UnsupportedProviderException {
-                    return switch (registrationId.toLowerCase()) {
+                    return switch (registrationId.toLowerCase(Locale.ROOT)) {
                         case "google" -> getGoogle();
                         case "github" -> getGithub();
                         case "keycloak" -> getKeycloak();

app/common/src/main/java/stirling/software/common/model/FileInfo.java

@@ -30,11 +30,11 @@ public class FileInfo {
     // Formats the file size into a human-readable string.
     public String getFormattedFileSize() {
         if (fileSize >= 1024 * 1024 * 1024) {
-            return String.format(Locale.US, "%.2f GB", fileSize / (1024.0 * 1024 * 1024));
+            return String.format(Locale.ROOT, "%.2f GB", fileSize / (1024.0 * 1024 * 1024));
         } else if (fileSize >= 1024 * 1024) {
-            return String.format(Locale.US, "%.2f MB", fileSize / (1024.0 * 1024));
+            return String.format(Locale.ROOT, "%.2f MB", fileSize / (1024.0 * 1024));
         } else if (fileSize >= 1024) {
-            return String.format(Locale.US, "%.2f KB", fileSize / 1024.0);
+            return String.format(Locale.ROOT, "%.2f KB", fileSize / 1024.0);
         } else {
             return String.format("%d Bytes", fileSize);
         }

app/common/src/main/java/stirling/software/common/service/CustomPDFDocumentFactory.java

@@ -7,6 +7,7 @@ import java.io.InputStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
+import java.util.Locale;
 import java.util.concurrent.atomic.AtomicLong;
 
 import org.apache.pdfbox.Loader;
@@ -249,7 +250,7 @@ public class CustomPDFDocumentFactory {
         log.debug(
                 "Memory status - Free: {}MB ({}%), Used: {}MB, Max: {}MB",
                 actualFreeMemory / (1024 * 1024),
-                String.format("%.2f", freeMemoryPercent),
+                String.format(Locale.ROOT, "%.2f", freeMemoryPercent),
                 usedMemory / (1024 * 1024),
                 maxMemory / (1024 * 1024));
 
@@ -258,7 +259,7 @@ public class CustomPDFDocumentFactory {
                 || actualFreeMemory < MIN_FREE_MEMORY_BYTES) {
             log.debug(
                     "Low memory detected ({}%), forcing file-based cache",
-                    String.format("%.2f", freeMemoryPercent));
+                    String.format(Locale.ROOT, "%.2f", freeMemoryPercent));
             return createScratchFileCacheFunction(MemoryUsageSetting.setupTempFileOnly());
         } else if (contentSize < SMALL_FILE_THRESHOLD) {
             log.debug("Using memory-only cache for small document ({}KB)", contentSize / 1024);

app/common/src/main/java/stirling/software/common/service/JobExecutorService.java

@@ -1,6 +1,7 @@
 package stirling.software.common.service;
 
 import java.io.IOException;
+import java.util.Locale;
 import java.util.Map;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
@@ -440,7 +441,7 @@ public class JobExecutorService {
 
             double numericValue = Double.parseDouble(value);
 
-            return switch (unit.toLowerCase()) {
+            return switch (unit.toLowerCase(Locale.ROOT)) {
                 case "s" -> (long) (numericValue * 1000);
                 case "m" -> (long) (numericValue * 60 * 1000);
                 case "h" -> (long) (numericValue * 60 * 60 * 1000);

app/common/src/main/java/stirling/software/common/service/PostHogService.java

@@ -397,7 +397,7 @@ public class PostHogService {
                 if (hardwareAddress != null) {
                     String[] hexadecimal = new String[hardwareAddress.length];
                     for (int i = 0; i < hardwareAddress.length; i++) {
-                        hexadecimal[i] = String.format("%02X", hardwareAddress[i]);
+                        hexadecimal[i] = String.format(Locale.ROOT, "%02X", hardwareAddress[i]);
                     }
                     return String.join("-", hexadecimal);
                 }

app/common/src/main/java/stirling/software/common/service/ResourceMonitor.java

@@ -5,6 +5,7 @@ import java.lang.management.MemoryMXBean;
 import java.lang.management.OperatingSystemMXBean;
 import java.time.Duration;
 import java.time.Instant;
+import java.util.Locale;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
@@ -173,8 +174,8 @@ public class ResourceMonitor {
                 log.info("System resource status changed from {} to {}", oldStatus, newStatus);
                 log.info(
                         "Current metrics - CPU: {}%, Memory: {}%, Free Memory: {} MB",
-                        String.format("%.1f", cpuUsage * 100),
-                        String.format("%.1f", memoryUsage * 100),
+                        String.format(Locale.ROOT, "%.1f", cpuUsage * 100),
+                        String.format(Locale.ROOT, "%.1f", memoryUsage * 100),
                         freeMemory / (1024 * 1024));
             }
         } catch (Exception e) {

app/common/src/main/java/stirling/software/common/service/SsrfProtectionService.java

@@ -5,6 +5,7 @@ import java.net.Inet6Address;
 import java.net.InetAddress;
 import java.net.URI;
 import java.net.UnknownHostException;
+import java.util.Locale;
 import java.util.regex.Pattern;
 
 import org.springframework.stereotype.Service;
@@ -83,7 +84,7 @@ public class SsrfProtectionService {
                 return false;
             }
 
-            return config.getAllowedDomains().contains(host.toLowerCase());
+            return config.getAllowedDomains().contains(host.toLowerCase(Locale.ROOT));
 
         } catch (Exception e) {
             log.debug("Failed to parse URL for MAX security check: {}", url, e);
@@ -101,7 +102,7 @@ public class SsrfProtectionService {
                 return false;
             }
 
-            String hostLower = host.toLowerCase();
+            String hostLower = host.toLowerCase(Locale.ROOT);
 
             // Check explicit blocked domains
             if (config.getBlockedDomains().contains(hostLower)) {
@@ -111,7 +112,7 @@ public class SsrfProtectionService {
 
             // Check internal TLD patterns
             for (String tld : config.getInternalTlds()) {
-                if (hostLower.endsWith(tld.toLowerCase())) {
+                if (hostLower.endsWith(tld.toLowerCase(Locale.ROOT))) {
                     log.debug("URL blocked by internal TLD pattern '{}': {}", tld, url);
                     return false;
                 }
@@ -123,9 +124,11 @@ public class SsrfProtectionService {
                         config.getAllowedDomains().stream()
                                 .anyMatch(
                                         domain ->
-                                                hostLower.equals(domain.toLowerCase())
+                                                hostLower.equals(domain.toLowerCase(Locale.ROOT))
                                                         || hostLower.endsWith(
-                                                                "." + domain.toLowerCase()));
+                                                                "."
+                                                                        + domain.toLowerCase(
+                                                                                Locale.ROOT)));
 
                 if (!isAllowed) {
                     log.debug("URL not in allowed domains list: {}", url);

app/common/src/main/java/stirling/software/common/service/TaskManager.java

@@ -7,6 +7,7 @@ import java.time.LocalDateTime;
 import java.time.temporal.ChronoUnit;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executors;
@@ -101,14 +102,16 @@ public class TaskManager {
                 if (!extractedFiles.isEmpty()) {
                     jobResult.completeWithFiles(extractedFiles);
                     log.debug(
-                            "Set multiple file results for job ID: {} with {} files extracted from ZIP",
+                            "Set multiple file results for job ID: {} with {} files extracted from"
+                                    + " ZIP",
                             jobId,
                             extractedFiles.size());
                     return;
                 }
             } catch (Exception e) {
                 log.warn(
-                        "Failed to extract ZIP file for job {}: {}. Falling back to single file result.",
+                        "Failed to extract ZIP file for job {}: {}. Falling back to single file"
+                                + " result.",
                         jobId,
                         e.getMessage());
             }
@@ -342,12 +345,12 @@ public class TaskManager {
     /** Check if a file is a ZIP file based on content type and filename */
     private boolean isZipFile(String contentType, String fileName) {
         if (contentType != null
-                && (contentType.equals("application/zip")
-                        || contentType.equals("application/x-zip-compressed"))) {
+                && ("application/zip".equals(contentType)
+                        || "application/x-zip-compressed".equals(contentType))) {
             return true;
         }
 
-        if (fileName != null && fileName.toLowerCase().endsWith(".zip")) {
+        if (fileName != null && fileName.toLowerCase(Locale.ROOT).endsWith(".zip")) {
             return true;
         }
 
@@ -414,7 +417,7 @@ public class TaskManager {
             return MediaType.APPLICATION_OCTET_STREAM_VALUE;
         }
 
-        String lowerName = fileName.toLowerCase();
+        String lowerName = fileName.toLowerCase(Locale.ROOT);
         if (lowerName.endsWith(".pdf")) {
             return MediaType.APPLICATION_PDF_VALUE;
         } else if (lowerName.endsWith(".txt")) {

app/common/src/main/java/stirling/software/common/util/CbrUtils.java

@@ -6,6 +6,7 @@ import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
+import java.util.Locale;
 
 import org.apache.commons.io.FilenameUtils;
 import org.apache.pdfbox.pdmodel.PDDocument;
@@ -60,10 +61,9 @@ public class CbrUtils {
                             e.getMessage());
                     throw ExceptionUtils.createIllegalArgumentException(
                             "error.invalidFormat",
-                            "Invalid or corrupted CBR/RAR archive. "
-                                    + "The file may be corrupted, use an unsupported RAR format (RAR5+), "
-                                    + "or may not be a valid RAR archive. "
-                                    + "Please ensure the file is a valid RAR archive.");
+                            "Invalid or corrupted CBR/RAR archive. The file may be corrupted, use"
+                                    + " an unsupported RAR format (RAR5+), or may not be a valid RAR"
+                                    + " archive. Please ensure the file is a valid RAR archive.");
                 } catch (RarException e) {
                     log.warn("Failed to open CBR/RAR archive: {}", e.getMessage());
                     String errorMessage;
@@ -73,13 +73,14 @@ public class CbrUtils {
                         errorMessage = "Encrypted CBR/RAR archives are not supported.";
                     } else if (exMessage.isEmpty()) {
                         errorMessage =
-                                "Invalid CBR/RAR archive. "
-                                        + "The file may be encrypted, corrupted, or use an unsupported format.";
+                                "Invalid CBR/RAR archive. The file may be encrypted, corrupted, or"
+                                        + " use an unsupported format.";
                     } else {
                         errorMessage =
                                 "Invalid CBR/RAR archive: "
                                         + exMessage
-                                        + ". The file may be encrypted, corrupted, or use an unsupported format.";
+                                        + ". The file may be encrypted, corrupted, or use an"
+                                        + " unsupported format.";
                     }
                     throw ExceptionUtils.createIllegalArgumentException(
                             "error.invalidFormat", errorMessage);
@@ -121,7 +122,8 @@ public class CbrUtils {
                 if (imageEntries.isEmpty()) {
                     throw ExceptionUtils.createIllegalArgumentException(
                             "error.fileProcessing",
-                            "No valid images found in the CBR file. The archive may be empty or contain no supported image formats.");
+                            "No valid images found in the CBR file. The archive may be empty or"
+                                    + " contain no supported image formats.");
                 }
 
                 for (ImageEntryData imageEntry : imageEntries) {
@@ -146,7 +148,8 @@ public class CbrUtils {
                 if (document.getNumberOfPages() == 0) {
                     throw ExceptionUtils.createIllegalArgumentException(
                             "error.fileProcessing",
-                            "No images could be processed from the CBR file. All images may be corrupted or in unsupported formats.");
+                            "No images could be processed from the CBR file. All images may be"
+                                    + " corrupted or in unsupported formats.");
                 }
 
                 ByteArrayOutputStream baos = new ByteArrayOutputStream();
@@ -159,7 +162,6 @@ public class CbrUtils {
                         return GeneralUtils.optimizePdfWithGhostscript(pdfBytes);
                     } catch (IOException e) {
                         log.warn("Ghostscript optimization failed, returning unoptimized PDF", e);
-                        return pdfBytes;
                     }
                 }
 
@@ -178,7 +180,7 @@ public class CbrUtils {
             throw new IllegalArgumentException("File must have a name");
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         if (!"cbr".equals(extension) && !"rar".equals(extension)) {
             throw new IllegalArgumentException("File must be a CBR or RAR archive");
         }
@@ -190,7 +192,7 @@ public class CbrUtils {
             return false;
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         return "cbr".equals(extension) || "rar".equals(extension);
     }
 

app/common/src/main/java/stirling/software/common/util/CbzUtils.java

@@ -8,6 +8,7 @@ import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.Enumeration;
 import java.util.List;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipFile;
 import java.util.zip.ZipInputStream;
@@ -119,7 +120,6 @@ public class CbzUtils {
                         return GeneralUtils.optimizePdfWithGhostscript(pdfBytes);
                     } catch (IOException e) {
                         log.warn("Ghostscript optimization failed, returning unoptimized PDF", e);
-                        return pdfBytes;
                     }
                 }
 
@@ -138,7 +138,7 @@ public class CbzUtils {
             throw new IllegalArgumentException("File must have a name");
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         if (!"cbz".equals(extension) && !"zip".equals(extension)) {
             throw new IllegalArgumentException("File must be a CBZ or ZIP archive");
         }
@@ -150,7 +150,7 @@ public class CbzUtils {
             return false;
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         return "cbz".equals(extension) || "zip".equals(extension);
     }
 
@@ -160,7 +160,7 @@ public class CbzUtils {
             return false;
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         return "cbz".equals(extension)
                 || "zip".equals(extension)
                 || "cbr".equals(extension)

app/common/src/main/java/stirling/software/common/util/ChecksumUtils.java

@@ -58,14 +58,11 @@ public class ChecksumUtils {
      * @throws IOException if reading from the stream fails
      */
     public static String checksum(InputStream is, String algorithm) throws IOException {
-        switch (algorithm.toUpperCase(Locale.ROOT)) {
-            case "CRC32":
-                return checksumChecksum(is, new CRC32());
-            case "ADLER32":
-                return checksumChecksum(is, new Adler32());
-            default:
-                return toHex(checksumBytes(is, algorithm));
-        }
+        return switch (algorithm.toUpperCase(Locale.ROOT)) {
+            case "CRC32" -> checksumChecksum(is, new CRC32());
+            case "ADLER32" -> checksumChecksum(is, new Adler32());
+            default -> toHex(checksumBytes(is, algorithm));
+        };
     }
 
     /**
@@ -98,14 +95,13 @@ public class ChecksumUtils {
      * @throws IOException if reading from the stream fails
      */
     public static String checksumBase64(InputStream is, String algorithm) throws IOException {
-        switch (algorithm.toUpperCase(Locale.ROOT)) {
-            case "CRC32":
-                return Base64.getEncoder().encodeToString(checksumChecksumBytes(is, new CRC32()));
-            case "ADLER32":
-                return Base64.getEncoder().encodeToString(checksumChecksumBytes(is, new Adler32()));
-            default:
-                return Base64.getEncoder().encodeToString(checksumBytes(is, algorithm));
-        }
+        return switch (algorithm.toUpperCase(Locale.ROOT)) {
+            case "CRC32" ->
+                    Base64.getEncoder().encodeToString(checksumChecksumBytes(is, new CRC32()));
+            case "ADLER32" ->
+                    Base64.getEncoder().encodeToString(checksumChecksumBytes(is, new Adler32()));
+            default -> Base64.getEncoder().encodeToString(checksumBytes(is, algorithm));
+        };
     }
 
     /**
@@ -179,7 +175,7 @@ public class ChecksumUtils {
         for (Map.Entry<String, Checksum> entry : checksums.entrySet()) {
             // Keep value as long and mask to ensure unsigned hex formatting.
             long unsigned32 = entry.getValue().getValue() & UNSIGNED_32_BIT_MASK;
-            results.put(entry.getKey(), String.format("%08x", unsigned32));
+            results.put(entry.getKey(), String.format(Locale.ROOT, "%08x", unsigned32));
         }
         return results;
     }
@@ -258,7 +254,7 @@ public class ChecksumUtils {
         }
         // Keep as long and mask to ensure correct unsigned representation.
         long unsigned32 = checksum.getValue() & UNSIGNED_32_BIT_MASK;
-        return String.format("%08x", unsigned32);
+        return String.format(Locale.ROOT, "%08x", unsigned32);
     }
 
     /**
@@ -294,7 +290,7 @@ public class ChecksumUtils {
     private static String toHex(byte[] hash) {
         StringBuilder sb = new StringBuilder(hash.length * 2);
         for (byte b : hash) {
-            sb.append(String.format("%02x", b));
+            sb.append(String.format(Locale.ROOT, "%02x", b));
         }
         return sb.toString();
     }

app/common/src/main/java/stirling/software/common/util/EmlParser.java

@@ -11,6 +11,7 @@ import java.time.ZonedDateTime;
 import java.util.ArrayList;
 import java.util.Date;
 import java.util.List;
+import java.util.Locale;
 import java.util.Properties;
 import java.util.regex.Pattern;
 
@@ -229,7 +230,8 @@ public class EmlParser {
                 Method getContentType = message.getClass().getMethod("getContentType");
                 String contentType = (String) getContentType.invoke(message);
 
-                if (contentType != null && contentType.toLowerCase().contains(TEXT_HTML)) {
+                if (contentType != null
+                        && contentType.toLowerCase(Locale.ROOT).contains(TEXT_HTML)) {
                     content.setHtmlBody(stringContent);
                 } else {
                     content.setTextBody(stringContent);
@@ -296,7 +298,7 @@ public class EmlParser {
             String contentType = (String) getContentType.invoke(part);
 
             String normalizedDisposition =
-                    disposition != null ? ((String) disposition).toLowerCase() : null;
+                    disposition != null ? ((String) disposition).toLowerCase(Locale.ROOT) : null;
 
             if ((Boolean) isMimeType.invoke(part, TEXT_PLAIN) && normalizedDisposition == null) {
                 Object partContent = getContent.invoke(part);
@@ -422,7 +424,7 @@ public class EmlParser {
                     RegexPatternUtils.getInstance().getNewlineSplitPattern().split(emlContent);
             for (int i = 0; i < lines.length; i++) {
                 String line = lines[i];
-                if (line.toLowerCase().startsWith(headerName.toLowerCase())) {
+                if (line.toLowerCase(Locale.ROOT).startsWith(headerName.toLowerCase(Locale.ROOT))) {
                     StringBuilder value =
                             new StringBuilder(line.substring(headerName.length()).trim());
                     for (int j = i + 1; j < lines.length; j++) {
@@ -444,7 +446,7 @@ public class EmlParser {
 
     private static String extractHtmlBody(String emlContent) {
         try {
-            String lowerContent = emlContent.toLowerCase();
+            String lowerContent = emlContent.toLowerCase(Locale.ROOT);
             int htmlStart = lowerContent.indexOf(HEADER_CONTENT_TYPE + " " + TEXT_HTML);
             if (htmlStart == -1) return null;
 
@@ -463,7 +465,7 @@ public class EmlParser {
 
     private static String extractTextBody(String emlContent) {
         try {
-            String lowerContent = emlContent.toLowerCase();
+            String lowerContent = emlContent.toLowerCase(Locale.ROOT);
             int textStart = lowerContent.indexOf(HEADER_CONTENT_TYPE + " " + TEXT_PLAIN);
             if (textStart == -1) {
                 int bodyStart = emlContent.indexOf("\r\n\r\n");
@@ -516,7 +518,7 @@ public class EmlParser {
             String currentEncoding = "";
 
             for (String line : lines) {
-                String lowerLine = line.toLowerCase().trim();
+                String lowerLine = line.toLowerCase(Locale.ROOT).trim();
 
                 if (line.trim().isEmpty()) {
                     inHeaders = false;
@@ -554,9 +556,12 @@ public class EmlParser {
     }
 
     private static boolean isAttachment(String disposition, String filename, String contentType) {
-        return (disposition.toLowerCase().contains(DISPOSITION_ATTACHMENT) && !filename.isEmpty())
-                || (!filename.isEmpty() && !contentType.toLowerCase().startsWith("text/"))
-                || (contentType.toLowerCase().contains("application/") && !filename.isEmpty());
+        return (disposition.toLowerCase(Locale.ROOT).contains(DISPOSITION_ATTACHMENT)
+                        && !filename.isEmpty())
+                || (!filename.isEmpty()
+                        && !contentType.toLowerCase(Locale.ROOT).startsWith("text/"))
+                || (contentType.toLowerCase(Locale.ROOT).contains("application/")
+                        && !filename.isEmpty());
     }
 
     private static String extractFilenameFromDisposition(String disposition) {
@@ -565,8 +570,8 @@ public class EmlParser {
         }
 
         // Handle filename*= (RFC 2231 encoded filename)
-        if (disposition.toLowerCase().contains("filename*=")) {
-            int filenameStarStart = disposition.toLowerCase().indexOf("filename*=") + 10;
+        if (disposition.toLowerCase(Locale.ROOT).contains("filename*=")) {
+            int filenameStarStart = disposition.toLowerCase(Locale.ROOT).indexOf("filename*=") + 10;
             int filenameStarEnd = disposition.indexOf(";", filenameStarStart);
             if (filenameStarEnd == -1) filenameStarEnd = disposition.length();
             String extendedFilename =
@@ -586,7 +591,7 @@ public class EmlParser {
         }
 
         // Handle regular filename=
-        int filenameStart = disposition.toLowerCase().indexOf("filename=") + 9;
+        int filenameStart = disposition.toLowerCase(Locale.ROOT).indexOf("filename=") + 9;
         int filenameEnd = disposition.indexOf(";", filenameStart);
         if (filenameEnd == -1) filenameEnd = disposition.length();
         String filename = disposition.substring(filenameStart, filenameEnd).trim();

app/common/src/main/java/stirling/software/common/util/EmlProcessingUtils.java

@@ -109,12 +109,13 @@ public class EmlProcessingUtils {
 
         html.append(
                 String.format(
+                        Locale.ROOT,
                         """
-                <!DOCTYPE html>
-                <html lang="en"><head><meta charset="UTF-8">
-                <title>%s</title>
-                <style>
-                """,
+                        <!DOCTYPE html>
+                        <html lang="en"><head><meta charset="UTF-8">
+                        <title>%s</title>
+                        <style>
+                        """,
                         sanitizeText(content.getSubject(), customHtmlSanitizer)));
 
         appendEnhancedStyles(html);
@@ -127,14 +128,15 @@ public class EmlProcessingUtils {
 
         html.append(
                 String.format(
+                        Locale.ROOT,
                         """
-                <div class="email-container">
-                <div class="email-header">
-                <h1>%s</h1>
-                <div class="email-meta">
-                <div><strong>From:</strong> %s</div>
-                <div><strong>To:</strong> %s</div>
-                """,
+                        <div class="email-container">
+                        <div class="email-header">
+                        <h1>%s</h1>
+                        <div class="email-meta">
+                        <div><strong>From:</strong> %s</div>
+                        <div><strong>To:</strong> %s</div>
+                        """,
                         sanitizeText(content.getSubject(), customHtmlSanitizer),
                         sanitizeText(content.getFrom(), customHtmlSanitizer),
                         sanitizeText(content.getTo(), customHtmlSanitizer)));
@@ -142,6 +144,7 @@ public class EmlProcessingUtils {
         if (content.getCc() != null && !content.getCc().trim().isEmpty()) {
             html.append(
                     String.format(
+                            Locale.ROOT,
                             "<div><strong>CC:</strong> %s</div>\n",
                             sanitizeText(content.getCc(), customHtmlSanitizer)));
         }
@@ -149,6 +152,7 @@ public class EmlProcessingUtils {
         if (content.getBcc() != null && !content.getBcc().trim().isEmpty()) {
             html.append(
                     String.format(
+                            Locale.ROOT,
                             "<div><strong>BCC:</strong> %s</div>\n",
                             sanitizeText(content.getBcc(), customHtmlSanitizer)));
         }
@@ -156,11 +160,13 @@ public class EmlProcessingUtils {
         if (content.getDate() != null) {
             html.append(
                     String.format(
+                            Locale.ROOT,
                             "<div><strong>Date:</strong> %s</div>\n",
                             PdfAttachmentHandler.formatEmailDate(content.getDate())));
         } else if (content.getDateString() != null && !content.getDateString().trim().isEmpty()) {
             html.append(
                     String.format(
+                            Locale.ROOT,
                             "<div><strong>Date:</strong> %s</div>\n",
                             sanitizeText(content.getDateString(), customHtmlSanitizer)));
         }
@@ -175,6 +181,7 @@ public class EmlProcessingUtils {
         } else if (content.getTextBody() != null && !content.getTextBody().trim().isEmpty()) {
             html.append(
                     String.format(
+                            Locale.ROOT,
                             "<div class=\"text-body\">%s</div>",
                             convertTextToHtml(content.getTextBody(), customHtmlSanitizer)));
         } else {
@@ -234,14 +241,16 @@ public class EmlProcessingUtils {
                         .getUrlLinkPattern()
                         .matcher(html)
                         .replaceAll(
-                                "<a href=\"$1\" style=\"color: #1a73e8; text-decoration: underline;\">$1</a>");
+                                "<a href=\"$1\" style=\"color: #1a73e8; text-decoration:"
+                                        + " underline;\">$1</a>");
 
         html =
                 RegexPatternUtils.getInstance()
                         .getEmailLinkPattern()
                         .matcher(html)
                         .replaceAll(
-                                "<a href=\"mailto:$1\" style=\"color: #1a73e8; text-decoration: underline;\">$1</a>");
+                                "<a href=\"mailto:$1\" style=\"color: #1a73e8; text-decoration:"
+                                        + " underline;\">$1</a>");
 
         return html;
     }
@@ -249,127 +258,128 @@ public class EmlProcessingUtils {
     private static void appendEnhancedStyles(StringBuilder html) {
         String css =
                 String.format(
+                        Locale.ROOT,
                         """
-                body {
-                  font-family: %s;
-                  font-size: %dpx;
-                  line-height: %s;
-                  color: %s;
-                  margin: 0;
-                  padding: 16px;
-                  background-color: %s;
-                }
+                        body {
+                          font-family: %s;
+                          font-size: %dpx;
+                          line-height: %s;
+                          color: %s;
+                          margin: 0;
+                          padding: 16px;
+                          background-color: %s;
+                        }
 
-                .email-container {
-                  width: 100%%;
-                  max-width: 100%%;
-                  margin: 0 auto;
-                }
+                        .email-container {
+                          width: 100%%;
+                          max-width: 100%%;
+                          margin: 0 auto;
+                        }
 
-                .email-header {
-                  padding-bottom: 10px;
-                  border-bottom: 1px solid %s;
-                  margin-bottom: 10px;
-                }
+                        .email-header {
+                          padding-bottom: 10px;
+                          border-bottom: 1px solid %s;
+                          margin-bottom: 10px;
+                        }
 
-                .email-header h1 {
-                  margin: 0 0 10px 0;
-                  font-size: %dpx;
-                  font-weight: bold;
-                }
+                        .email-header h1 {
+                          margin: 0 0 10px 0;
+                          font-size: %dpx;
+                          font-weight: bold;
+                        }
 
-                .email-meta div {
-                  margin-bottom: 2px;
-                  font-size: %dpx;
-                }
+                        .email-meta div {
+                          margin-bottom: 2px;
+                          font-size: %dpx;
+                        }
 
-                .email-body {
-                  word-wrap: break-word;
-                }
+                        .email-body {
+                          word-wrap: break-word;
+                        }
 
-                .attachment-section {
-                  margin-top: 15px;
-                  padding: 10px;
-                  background-color: %s;
-                  border: 1px solid %s;
-                  border-radius: 3px;
-                }
+                        .attachment-section {
+                          margin-top: 15px;
+                          padding: 10px;
+                          background-color: %s;
+                          border: 1px solid %s;
+                          border-radius: 3px;
+                        }
 
-                .attachment-section h3 {
-                  margin: 0 0 8px 0;
-                  font-size: %dpx;
-                }
+                        .attachment-section h3 {
+                          margin: 0 0 8px 0;
+                          font-size: %dpx;
+                        }
 
-                .attachment-item {
-                  padding: 5px 0;
-                }
+                        .attachment-item {
+                          padding: 5px 0;
+                        }
 
-                .attachment-icon {
-                  margin-right: 5px;
-                }
+                        .attachment-icon {
+                          margin-right: 5px;
+                        }
 
-                .attachment-details, .attachment-type {
-                  font-size: %dpx;
-                  color: #555555;
-                }
+                        .attachment-details, .attachment-type {
+                          font-size: %dpx;
+                          color: #555555;
+                        }
 
-                .attachment-inclusion-note, .attachment-info-note {
-                  margin-top: 8px;
-                  padding: 6px;
-                  font-size: %dpx;
-                  border-radius: 3px;
-                }
+                        .attachment-inclusion-note, .attachment-info-note {
+                          margin-top: 8px;
+                          padding: 6px;
+                          font-size: %dpx;
+                          border-radius: 3px;
+                        }
 
-                .attachment-inclusion-note {
-                  background-color: #e6ffed;
-                  border: 1px solid #d4f7dc;
-                  color: #006420;
-                }
+                        .attachment-inclusion-note {
+                          background-color: #e6ffed;
+                          border: 1px solid #d4f7dc;
+                          color: #006420;
+                        }
 
-                .attachment-info-note {
-                  background-color: #fff9e6;
-                  border: 1px solid #fff0c2;
-                  color: #664d00;
-                }
+                        .attachment-info-note {
+                          background-color: #fff9e6;
+                          border: 1px solid #fff0c2;
+                          color: #664d00;
+                        }
 
-                .attachment-link-container {
-                  display: flex;
-                  align-items: center;
-                  padding: 8px;
-                  background-color: #f8f9fa;
-                  border: 1px solid #dee2e6;
-                  border-radius: 4px;
-                  margin: 4px 0;
-                }
+                        .attachment-link-container {
+                          display: flex;
+                          align-items: center;
+                          padding: 8px;
+                          background-color: #f8f9fa;
+                          border: 1px solid #dee2e6;
+                          border-radius: 4px;
+                          margin: 4px 0;
+                        }
 
-                .attachment-link-container:hover {
-                  background-color: #e9ecef;
-                }
+                        .attachment-link-container:hover {
+                          background-color: #e9ecef;
+                        }
 
-                .attachment-note {
-                  font-size: %dpx;
-                  color: #6c757d;
-                  font-style: italic;
-                  margin-left: 8px;
-                }
+                        .attachment-note {
+                          font-size: %dpx;
+                          color: #6c757d;
+                          font-style: italic;
+                          margin-left: 8px;
+                        }
 
-                .no-content {
-                  padding: 20px;
-                  text-align: center;
-                  color: #666;
-                  font-style: italic;
-                }
+                        .no-content {
+                          padding: 20px;
+                          text-align: center;
+                          color: #666;
+                          font-style: italic;
+                        }
 
-                .text-body {
-                  white-space: pre-wrap;
-                }
+                        .text-body {
+                          white-space: pre-wrap;
+                        }
 
-                img {
-                  max-width: 100%%;
-                  height: auto;
-                  display: block;
-                }
-                """,
+                        img {
+                          max-width: 100%%;
+                          height: auto;
+                          display: block;
+                        }
+                        """,
                         DEFAULT_FONT_FAMILY,
                         DEFAULT_FONT_SIZE,
                         DEFAULT_LINE_HEIGHT,
@@ -420,13 +430,14 @@ public class EmlProcessingUtils {
                 String attachmentId = "attachment_" + i;
                 html.append(
                         String.format(
+                                Locale.ROOT,
                                 """
-                        <div class="attachment-item" id="%s">
-                        <span class="attachment-icon" data-filename="%s">@</span>
-                        <span class="attachment-name">%s</span>
-                        <span class="attachment-details">(%s%s)</span>
-                        </div>
-                        """,
+                                <div class="attachment-item" id="%s">
+                                <span class="attachment-icon" data-filename="%s">@</span>
+                                <span class="attachment-name">%s</span>
+                                <span class="attachment-details">(%s%s)</span>
+                                </div>
+                                """,
                                 attachmentId,
                                 escapeHtml(embeddedFilename),
                                 escapeHtml(EmlParser.safeMimeDecode(attachment.getFilename())),
@@ -470,7 +481,7 @@ public class EmlProcessingUtils {
         }
 
         if (filename != null) {
-            String lowerFilename = filename.toLowerCase();
+            String lowerFilename = filename.toLowerCase(Locale.ROOT);
             for (Map.Entry<String, String> entry : EXTENSION_TO_MIME_TYPE.entrySet()) {
                 if (lowerFilename.endsWith(entry.getKey())) {
                     return entry.getValue();
@@ -516,7 +527,7 @@ public class EmlProcessingUtils {
                 result.append(processedText, lastEnd, matcher.start());
 
                 String charset = matcher.group(1);
-                String encoding = matcher.group(2).toUpperCase();
+                String encoding = matcher.group(2).toUpperCase(Locale.ROOT);
                 String encodedValue = matcher.group(3);
 
                 try {

app/common/src/main/java/stirling/software/common/util/ExceptionUtils.java

@@ -2,6 +2,7 @@ package stirling.software.common.util;
 
 import java.io.IOException;
 import java.text.MessageFormat;
+import java.util.Locale;
 
 import lombok.extern.slf4j.Slf4j;
 
@@ -34,9 +35,8 @@ public class ExceptionUtils {
         if (context != null && !context.isEmpty()) {
             message =
                     String.format(
-                            "Error %s: PDF file appears to be corrupted or damaged. Please try"
-                                    + " using the 'Repair PDF' feature first to fix the file before"
-                                    + " proceeding with this operation.",
+                            Locale.ROOT,
+                            "Error %s: PDF file appears to be corrupted or damaged. Please try using the 'Repair PDF' feature first to fix the file before proceeding with this operation.",
                             context);
         } else {
             message =
@@ -97,8 +97,10 @@ public class ExceptionUtils {
     public static IOException createFileProcessingException(String operation, Exception cause) {
         String message =
                 String.format(
+                        Locale.ROOT,
                         "An error occurred while processing the file during %s operation: %s",
-                        operation, cause.getMessage());
+                        operation,
+                        cause.getMessage());
         return new IOException(message, cause);
     }
 

app/common/src/main/java/stirling/software/common/util/FileToPdf.java

@@ -12,6 +12,7 @@ import java.nio.file.SimpleFileVisitor;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.stream.Stream;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
@@ -37,9 +38,11 @@ public class FileToPdf {
             try (TempFile tempInputFile =
                     new TempFile(
                             tempFileManager,
-                            fileName.toLowerCase().endsWith(".html") ? ".html" : ".zip")) {
+                            fileName.toLowerCase(Locale.ROOT).endsWith(".html")
+                                    ? ".html"
+                                    : ".zip")) {
 
-                if (fileName.toLowerCase().endsWith(".html")) {
+                if (fileName.toLowerCase(Locale.ROOT).endsWith(".html")) {
                     String sanitizedHtml =
                             sanitizeHtmlContent(
                                     new String(fileBytes, StandardCharsets.UTF_8),
@@ -47,7 +50,7 @@ public class FileToPdf {
                     Files.write(
                             tempInputFile.getPath(),
                             sanitizedHtml.getBytes(StandardCharsets.UTF_8));
-                } else if (fileName.toLowerCase().endsWith(".zip")) {
+                } else if (fileName.toLowerCase(Locale.ROOT).endsWith(".zip")) {
                     Files.write(tempInputFile.getPath(), fileBytes);
                     sanitizeHtmlFilesInZip(
                             tempInputFile.getPath(), tempFileManager, customHtmlSanitizer);
@@ -102,8 +105,8 @@ public class FileToPdf {
                             tempUnzippedDir.getPath().resolve(sanitizeZipFilename(entry.getName()));
                     if (!entry.isDirectory()) {
                         Files.createDirectories(filePath.getParent());
-                        if (entry.getName().toLowerCase().endsWith(".html")
-                                || entry.getName().toLowerCase().endsWith(".htm")) {
+                        if (entry.getName().toLowerCase(Locale.ROOT).endsWith(".html")
+                                || entry.getName().toLowerCase(Locale.ROOT).endsWith(".htm")) {
                             String content =
                                     new String(zipIn.readAllBytes(), StandardCharsets.UTF_8);
                             String sanitizedContent =

app/common/src/main/java/stirling/software/common/util/GeneralUtils.java

@@ -545,7 +545,7 @@ public class GeneralUtils {
             throw new IllegalArgumentException("Invalid default unit: " + defaultUnit);
         }
 
-        sizeStr = sizeStr.trim().toUpperCase();
+        sizeStr = sizeStr.trim().toUpperCase(Locale.ROOT);
         sizeStr = sizeStr.replace(",", ".").replace(" ", "");
 
         try {
@@ -574,7 +574,7 @@ public class GeneralUtils {
                 return Long.parseLong(sizeStr.substring(0, sizeStr.length() - 1));
             } else {
                 // Use provided default unit or fall back to MB
-                String unit = defaultUnit != null ? defaultUnit.toUpperCase() : "MB";
+                String unit = defaultUnit != null ? defaultUnit.toUpperCase(Locale.ROOT) : "MB";
                 double value = Double.parseDouble(sizeStr);
                 return switch (unit) {
                     case "TB" -> (long) (value * 1024L * 1024L * 1024L * 1024L);
@@ -616,13 +616,14 @@ public class GeneralUtils {
         if (bytes < 1024) {
             return bytes + " B";
         } else if (bytes < 1024L * 1024L) {
-            return String.format(Locale.US, "%.2f KB", bytes / 1024.0);
+            return String.format(Locale.ROOT, "%.2f KB", bytes / 1024.0);
         } else if (bytes < 1024L * 1024L * 1024L) {
-            return String.format(Locale.US, "%.2f MB", bytes / (1024.0 * 1024.0));
+            return String.format(Locale.ROOT, "%.2f MB", bytes / (1024.0 * 1024.0));
         } else if (bytes < 1024L * 1024L * 1024L * 1024L) {
-            return String.format(Locale.US, "%.2f GB", bytes / (1024.0 * 1024.0 * 1024.0));
+            return String.format(Locale.ROOT, "%.2f GB", bytes / (1024.0 * 1024.0 * 1024.0));
         } else {
-            return String.format(Locale.US, "%.2f TB", bytes / (1024.0 * 1024.0 * 1024.0 * 1024.0));
+            return String.format(
+                    Locale.ROOT, "%.2f TB", bytes / (1024.0 * 1024.0 * 1024.0 * 1024.0));
         }
     }
 
@@ -882,7 +883,7 @@ public class GeneralUtils {
                         byte[] mac = net.getHardwareAddress();
                         if (mac != null && mac.length > 0) {
                             for (byte b : mac) {
-                                sb.append(String.format("%02X", b));
+                                sb.append(String.format(Locale.ROOT, "%02X", b));
                             }
                             break; // Use the first valid network interface
                         }
@@ -892,7 +893,7 @@ public class GeneralUtils {
                 byte[] mac = network.getHardwareAddress();
                 if (mac != null) {
                     for (byte b : mac) {
-                        sb.append(String.format("%02X", b));
+                        sb.append(String.format(Locale.ROOT, "%02X", b));
                     }
                 }
             }

app/common/src/main/java/stirling/software/common/util/ImageProcessingUtils.java

@@ -6,6 +6,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.nio.ByteBuffer;
 import java.util.Iterator;
+import java.util.Locale;
 
 import javax.imageio.ImageIO;
 import javax.imageio.ImageReader;
@@ -118,7 +119,7 @@ public class ImageProcessingUtils {
         BufferedImage image = null;
         String filename = file.getOriginalFilename();
 
-        if (filename != null && filename.toLowerCase().endsWith(".psd")) {
+        if (filename != null && filename.toLowerCase(Locale.ROOT).endsWith(".psd")) {
             // For PSD files, try explicit ImageReader
             Iterator<ImageReader> readers = ImageIO.getImageReadersByFormatName("PSD");
             if (readers.hasNext()) {
@@ -134,7 +135,8 @@ public class ImageProcessingUtils {
                 throw new IOException(
                         "Unable to read image from file: "
                                 + filename
-                                + ". Supported PSD formats: RGB/CMYK/Gray 8-32 bit, RLE/ZIP compression");
+                                + ". Supported PSD formats: RGB/CMYK/Gray 8-32 bit, RLE/ZIP"
+                                + " compression");
             }
         } else {
             // For non-PSD files, use standard ImageIO

app/common/src/main/java/stirling/software/common/util/PdfAttachmentHandler.java

@@ -318,7 +318,7 @@ public class PdfAttachmentHandler {
 
     private static String normalizeFilename(String filename) {
         if (filename == null) return "";
-        String normalized = filename.toLowerCase().trim();
+        String normalized = filename.toLowerCase(Locale.ROOT).trim();
         normalized =
                 RegexPatternUtils.getInstance()
                         .getWhitespacePattern()
@@ -560,7 +560,7 @@ public class PdfAttachmentHandler {
         @Override
         protected void writeString(String string, List<TextPosition> textPositions)
                 throws IOException {
-            String lowerString = string.toLowerCase();
+            String lowerString = string.toLowerCase(Locale.ROOT);
 
             if (ATTACHMENT_SECTION_PATTERN.matcher(lowerString).find()) {
                 isInAttachmentSection = true;

app/common/src/main/java/stirling/software/common/util/PdfToCbrUtils.java

@@ -9,6 +9,7 @@ import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
+import java.util.Locale;
 
 import javax.imageio.ImageIO;
 
@@ -51,7 +52,7 @@ public class PdfToCbrUtils {
             throw new IllegalArgumentException("File must have a name");
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         if (!"pdf".equals(extension)) {
             throw new IllegalArgumentException("File must be a PDF");
         }
@@ -70,7 +71,8 @@ public class PdfToCbrUtils {
                     BufferedImage image =
                             pdfRenderer.renderImageWithDPI(pageIndex, dpi, ImageType.RGB);
 
-                    String imageFilename = String.format("page_%03d.png", pageIndex + 1);
+                    String imageFilename =
+                            String.format(Locale.ROOT, "page_%03d.png", pageIndex + 1);
                     Path imagePath = tempDir.resolve(imageFilename);
 
                     ImageIO.write(image, "PNG", imagePath.toFile());
@@ -167,7 +169,7 @@ public class PdfToCbrUtils {
             return false;
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         return "pdf".equals(extension);
     }
 }

app/common/src/main/java/stirling/software/common/util/PdfToCbzUtils.java

@@ -3,6 +3,7 @@ package stirling.software.common.util;
 import java.awt.image.BufferedImage;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -46,7 +47,7 @@ public class PdfToCbzUtils {
             throw new IllegalArgumentException("File must have a name");
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         if (!"pdf".equals(extension)) {
             throw new IllegalArgumentException("File must be a PDF");
         }
@@ -65,7 +66,8 @@ public class PdfToCbzUtils {
                     BufferedImage image =
                             pdfRenderer.renderImageWithDPI(pageIndex, dpi, ImageType.RGB);
 
-                    String imageFilename = String.format("page_%03d.png", pageIndex + 1);
+                    String imageFilename =
+                            String.format(Locale.ROOT, "page_%03d.png", pageIndex + 1);
 
                     ZipEntry zipEntry = new ZipEntry(imageFilename);
                     zipOut.putNextEntry(zipEntry);
@@ -93,7 +95,7 @@ public class PdfToCbzUtils {
             return false;
         }
 
-        String extension = FilenameUtils.getExtension(filename).toLowerCase();
+        String extension = FilenameUtils.getExtension(filename).toLowerCase(Locale.ROOT);
         return "pdf".equals(extension);
     }
 }

app/common/src/main/java/stirling/software/common/util/PdfUtils.java

@@ -8,6 +8,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.zip.ZipEntry;
@@ -50,36 +51,18 @@ public class PdfUtils {
 
     public PDRectangle textToPageSize(String size) {
 
-        switch (size.toUpperCase()) {
-            case "A0" -> {
-                return PDRectangle.A0;
-            }
-            case "A1" -> {
-                return PDRectangle.A1;
-            }
-            case "A2" -> {
-                return PDRectangle.A2;
-            }
-            case "A3" -> {
-                return PDRectangle.A3;
-            }
-            case "A4" -> {
-                return PDRectangle.A4;
-            }
-            case "A5" -> {
-                return PDRectangle.A5;
-            }
-            case "A6" -> {
-                return PDRectangle.A6;
-            }
-            case "LETTER" -> {
-                return PDRectangle.LETTER;
-            }
-            case "LEGAL" -> {
-                return PDRectangle.LEGAL;
-            }
+        return switch (size.toUpperCase(Locale.ROOT)) {
+            case "A0" -> PDRectangle.A0;
+            case "A1" -> PDRectangle.A1;
+            case "A2" -> PDRectangle.A2;
+            case "A3" -> PDRectangle.A3;
+            case "A4" -> PDRectangle.A4;
+            case "A5" -> PDRectangle.A5;
+            case "A6" -> PDRectangle.A6;
+            case "LETTER" -> PDRectangle.LETTER;
+            case "LEGAL" -> PDRectangle.LEGAL;
             default -> throw ExceptionUtils.createInvalidPageSizeException(size);
-        }
+        };
     }
 
     public List<RenderedImage> getAllImages(PDResources resources) throws IOException {
@@ -182,8 +165,8 @@ public class PdfUtils {
             ByteArrayOutputStream baos = new ByteArrayOutputStream();
 
             if (singleImage) {
-                if ("tiff".equals(imageType.toLowerCase())
-                        || "tif".equals(imageType.toLowerCase())) {
+                if ("tiff".equals(imageType.toLowerCase(Locale.ROOT))
+                        || "tif".equals(imageType.toLowerCase(Locale.ROOT))) {
                     // Write the images to the output stream as a TIFF with multiple frames
                     ImageWriter writer = ImageIO.getImageWritersByFormatName("tiff").next();
                     ImageWriteParam param = writer.getDefaultWriteParam();
@@ -361,9 +344,10 @@ public class PdfUtils {
                             zos.putNextEntry(
                                     new ZipEntry(
                                             String.format(
+                                                    Locale.ROOT,
                                                     filename + "_%d.%s",
                                                     i + 1,
-                                                    imageType.toLowerCase())));
+                                                    imageType.toLowerCase(Locale.ROOT))));
                             zos.write(baosImage.toByteArray());
                         }
                     }
@@ -463,8 +447,8 @@ public class PdfUtils {
                 String contentType = file.getContentType();
                 String originalFilename = Filenames.toSimpleFileName(file.getOriginalFilename());
                 if (originalFilename != null
-                        && (originalFilename.toLowerCase().endsWith(".tiff")
-                                || originalFilename.toLowerCase().endsWith(".tif"))) {
+                        && (originalFilename.toLowerCase(Locale.ROOT).endsWith(".tiff")
+                                || originalFilename.toLowerCase(Locale.ROOT).endsWith(".tif"))) {
                     ImageReader reader = ImageIO.getImageReadersByFormatName("tiff").next();
                     reader.setInput(ImageIO.createImageInputStream(file.getInputStream()));
                     int numPages = reader.getNumImages(true);
@@ -631,7 +615,7 @@ public class PdfUtils {
         int actualPageCount = pdfDocument.getNumberOfPages();
         pdfDocument.close();
 
-        return switch (comparator.toLowerCase()) {
+        return switch (comparator.toLowerCase(Locale.ROOT)) {
             case "greater" -> actualPageCount > pageCount;
             case "equal" -> actualPageCount == pageCount;
             case "less" -> actualPageCount < pageCount;

app/common/src/test/java/stirling/software/common/model/ApplicationPropertiesLogicTest.java

@@ -7,6 +7,7 @@ import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.Locale;
 import java.util.function.Function;
 
 import org.junit.jupiter.api.Test;
@@ -114,7 +115,7 @@ class ApplicationPropertiesLogicTest {
 
         UnsupportedProviderException ex =
                 assertThrows(UnsupportedProviderException.class, () -> client.get("unknown"));
-        assertTrue(ex.getMessage().toLowerCase().contains("not supported"));
+        assertTrue(ex.getMessage().toLowerCase(Locale.ROOT).contains("not supported"));
     }
 
     @Test

app/common/src/test/java/stirling/software/common/service/ResourceMonitorTest.java

@@ -5,6 +5,7 @@ import static org.junit.jupiter.api.Assertions.*;
 import java.lang.management.MemoryMXBean;
 import java.lang.management.OperatingSystemMXBean;
 import java.time.Instant;
+import java.util.Locale;
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -116,8 +117,11 @@ class ResourceMonitorTest {
                 shouldQueue,
                 result,
                 String.format(
+                        Locale.ROOT,
                         "For weight %d and status %s, shouldQueue should be %s",
-                        weight, status, shouldQueue));
+                        weight,
+                        status,
+                        shouldQueue));
     }
 
     @Test

app/common/src/test/java/stirling/software/common/util/EmlToPdfTest.java

@@ -12,6 +12,7 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.util.Base64;
+import java.util.Locale;
 
 import org.apache.pdfbox.pdmodel.PDDocument;
 import org.apache.pdfbox.pdmodel.PDPage;
@@ -773,20 +774,33 @@ class EmlToPdfTest {
     private String createSimpleTextEmailWithCharset(
             String from, String to, String subject, String body, String charset) {
         return String.format(
+                Locale.ROOT,
                 "From: %s\nTo: %s\nSubject: %s\nDate: %s\nContent-Type: text/plain; charset=%s\nContent-Transfer-Encoding: 8bit\n\n%s",
-                from, to, subject, getTimestamp(), charset, body);
+                from,
+                to,
+                subject,
+                getTimestamp(),
+                charset,
+                body);
     }
 
     private String createEmailWithCustomHeaders() {
         return String.format(
+                Locale.ROOT,
                 "From: sender@example.com\nDate: %s\nContent-Type: text/plain; charset=UTF-8\nContent-Transfer-Encoding: 8bit\n\n%s",
-                getTimestamp(), "This is an email body with some headers missing.");
+                getTimestamp(),
+                "This is an email body with some headers missing.");
     }
 
     private String createHtmlEmail(String from, String to, String subject, String htmlBody) {
         return String.format(
+                Locale.ROOT,
                 "From: %s\nTo: %s\nSubject: %s\nDate: %s\nContent-Type: text/html; charset=UTF-8\nContent-Transfer-Encoding: 8bit\n\n%s",
-                from, to, subject, getTimestamp(), htmlBody);
+                from,
+                to,
+                subject,
+                getTimestamp(),
+                htmlBody);
     }
 
     private String createMultipartEmailWithAttachment(
@@ -801,6 +815,7 @@ class EmlToPdfTest {
                 Base64.getEncoder()
                         .encodeToString(attachmentContent.getBytes(StandardCharsets.UTF_8));
         return String.format(
+                Locale.ROOT,
                 """
                     From: %s
                     To: %s
@@ -840,6 +855,7 @@ class EmlToPdfTest {
                 Base64.getEncoder()
                         .encodeToString(attachmentEmlContent.getBytes(StandardCharsets.UTF_8));
         return String.format(
+                Locale.ROOT,
                 """
                     From: %s
                     To: %s
@@ -878,6 +894,7 @@ class EmlToPdfTest {
     private String createMultipartAlternativeEmail(
             String textBody, String htmlBody, String boundary) {
         return String.format(
+                Locale.ROOT,
                 """
                     From: %s
                     To: %s
@@ -913,6 +930,7 @@ class EmlToPdfTest {
 
     private String createQuotedPrintableEmail() {
         return String.format(
+                Locale.ROOT,
                 "From: %s\nTo: %s\nSubject: %s\nDate: %s\nMIME-Version: 1.0\nContent-Type: text/plain; charset=UTF-8\nContent-Transfer-Encoding: quoted-printable\n\n%s",
                 "sender@example.com",
                 "recipient@example.com",
@@ -925,6 +943,7 @@ class EmlToPdfTest {
         String encodedBody =
                 Base64.getEncoder().encodeToString(body.getBytes(StandardCharsets.UTF_8));
         return String.format(
+                Locale.ROOT,
                 "From: %s\nTo: %s\nSubject: %s\nDate: %s\nMIME-Version: 1.0\nContent-Type: text/plain; charset=UTF-8\nContent-Transfer-Encoding: base64\n\n%s",
                 "sender@example.com",
                 "recipient@example.com",
@@ -936,6 +955,7 @@ class EmlToPdfTest {
     private String createEmailWithInlineImage(
             String htmlBody, String boundary, String contentId, String base64Image) {
         return String.format(
+                Locale.ROOT,
                 """
                     From: %s
                     To: %s
@@ -980,6 +1000,7 @@ class EmlToPdfTest {
         String encodedAttachment =
                 Base64.getEncoder().encodeToString(attachmentBody.getBytes(StandardCharsets.UTF_8));
         return String.format(
+                Locale.ROOT,
                 """
                     From: %s
                     To: %s

app/core/src/main/java/stirling/software/SPDF/SPDFApplication.java

@@ -7,6 +7,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Properties;
 
@@ -155,7 +156,7 @@ public class SPDFApplication {
             boolean browserOpen = browserOpenEnv != null && "true".equalsIgnoreCase(browserOpenEnv);
             if (browserOpen) {
                 try {
-                    String os = System.getProperty("os.name").toLowerCase();
+                    String os = System.getProperty("os.name").toLowerCase(Locale.ROOT);
                     Runtime rt = Runtime.getRuntime();
 
                     if (os.contains("win")) {

app/core/src/main/java/stirling/software/SPDF/controller/api/RearrangePagesPDFController.java

@@ -4,6 +4,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 
 import org.apache.pdfbox.pdmodel.PDDocument;
 import org.apache.pdfbox.pdmodel.PDPage;
@@ -204,7 +205,7 @@ public class RearrangePagesPDFController {
 
     private List<Integer> processSortTypes(String sortTypes, int totalPages, String pageOrder) {
         try {
-            SortTypes mode = SortTypes.valueOf(sortTypes.toUpperCase());
+            SortTypes mode = SortTypes.valueOf(sortTypes.toUpperCase(Locale.ROOT));
             return switch (mode) {
                 case REVERSE_ORDER -> reverseOrder(totalPages);
                 case DUPLEX_SORT -> duplexSort(totalPages);
@@ -247,7 +248,7 @@ public class RearrangePagesPDFController {
             List<Integer> newPageOrder;
             if (sortType != null
                     && !sortType.isEmpty()
-                    && !"custom".equals(sortType.toLowerCase())) {
+                    && !"custom".equals(sortType.toLowerCase(Locale.ROOT))) {
                 newPageOrder = processSortTypes(sortType, totalPages, pageOrder);
             } else {
                 newPageOrder = GeneralUtils.parsePageList(pageOrderArr, totalPages, false);

app/core/src/main/java/stirling/software/SPDF/controller/api/SplitPdfByChaptersController.java

@@ -5,6 +5,7 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -242,7 +243,7 @@ public class SplitPdfByChaptersController {
                 // split files will be named as "[FILE_NUMBER] [BOOKMARK_TITLE].pdf"
 
                 String fileName =
-                        String.format(fileNumberFormatter, i)
+                        String.format(Locale.ROOT, fileNumberFormatter, i)
                                 + bookmarks.get(i).getTitle()
                                 + ".pdf";
                 ByteArrayOutputStream baos = splitDocumentsBoas.get(i);

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ConvertEmlToPDF.java

@@ -2,6 +2,7 @@ package stirling.software.SPDF.controller.api.converters;
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.util.Locale;
 
 import org.jetbrains.annotations.NotNull;
 import org.springframework.http.HttpStatus;
@@ -45,9 +46,9 @@ public class ConvertEmlToPDF {
             summary = "Convert EML to PDF",
             description =
                     "This endpoint converts EML (email) files to PDF format with extensive"
-                            + " customization options. Features include font settings, image constraints, display modes, attachment handling,"
-                            + " and HTML debug output. Input: EML file, Output: PDF"
-                            + " or HTML file. Type: SISO")
+                            + " customization options. Features include font settings, image"
+                            + " constraints, display modes, attachment handling, and HTML debug output."
+                            + " Input: EML file, Output: PDF or HTML file. Type: SISO")
     public ResponseEntity<byte[]> convertEmlToPdf(@ModelAttribute EmlToPdfRequest request) {
 
         MultipartFile inputFile = request.getFileInput();
@@ -67,7 +68,7 @@ public class ConvertEmlToPDF {
         }
 
         // Validate file type - support EML
-        String lowerFilename = originalFilename.toLowerCase();
+        String lowerFilename = originalFilename.toLowerCase(Locale.ROOT);
         if (!lowerFilename.endsWith(".eml")) {
             log.error("Invalid file type for EML to PDF: {}", originalFilename);
             return ResponseEntity.badRequest()

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ConvertImgPDFController.java

@@ -8,6 +8,7 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.regex.Pattern;
 import java.util.stream.Stream;
@@ -118,7 +119,7 @@ public class ConvertImgPDFController {
                             newPdfBytes,
                             "webp".equalsIgnoreCase(imageFormat)
                                     ? "png"
-                                    : imageFormat.toUpperCase(),
+                                    : imageFormat.toUpperCase(Locale.ROOT),
                             colorTypeResult,
                             singleImage,
                             dpi,

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ConvertOfficeController.java

@@ -8,6 +8,7 @@ import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 
 import org.apache.commons.io.FileUtils;
 import org.apache.commons.io.FilenameUtils;
@@ -67,7 +68,7 @@ public class ConvertOfficeController {
         if (extension == null || !isValidFileExtension(extension)) {
             throw new IllegalArgumentException("Invalid file extension");
         }
-        String extensionLower = extension.toLowerCase();
+        String extensionLower = extension.toLowerCase(Locale.ROOT);
 
         String baseName = FilenameUtils.getBaseName(originalFilename);
         if (baseName == null || baseName.isBlank()) {
@@ -141,7 +142,7 @@ public class ConvertOfficeController {
                                             p ->
                                                     p.getFileName()
                                                             .toString()
-                                                            .toLowerCase()
+                                                            .toLowerCase(Locale.ROOT)
                                                             .endsWith(".pdf"))
                                     .findFirst()
                                     .orElse(null);

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ExtractCSVController.java

@@ -7,6 +7,7 @@ import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -124,7 +125,7 @@ public class ExtractCSVController {
     }
 
     private String generateEntryName(String baseName, int pageNum, int tableIndex) {
-        return String.format("%s_p%d_t%d.csv", baseName, pageNum, tableIndex);
+        return String.format(Locale.ROOT, "%s_p%d_t%d.csv", baseName, pageNum, tableIndex);
     }
 
     private String getBaseName(String filename) {

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/BlankPageController.java

@@ -5,6 +5,7 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -68,7 +69,11 @@ public class BlankPageController {
         }
 
         double whitePixelPercentage = (whitePixels / (double) totalPixels) * 100;
-        log.info(String.format("Page has white pixel percent of %.2f%%", whitePixelPercentage));
+        log.info(
+                String.format(
+                        Locale.ROOT,
+                        "Page has white pixel percent of %.2f%%",
+                        whitePixelPercentage));
 
         return whitePixelPercentage >= whitePercent;
     }

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/CompressController.java

@@ -293,7 +293,7 @@ public class CompressController {
                         imageIdentity,
                         GeneralUtils.formatBytes(originalSize),
                         GeneralUtils.formatBytes(compressedSize),
-                        String.format("%.1f", reductionPercentage));
+                        String.format(Locale.ROOT, "%.1f", reductionPercentage));
             } else {
                 log.info(
                         "Image identity {}: Not suitable for compression, skipping", imageIdentity);
@@ -386,7 +386,7 @@ public class CompressController {
                     "Overall PDF compression: {} → {} (reduced by {}%)",
                     GeneralUtils.formatBytes(originalFileSize),
                     GeneralUtils.formatBytes(compressedFileSize),
-                    String.format("%.1f", overallReduction));
+                    String.format(Locale.ROOT, "%.1f", overallReduction));
             return newCompressedPDF;
         } catch (Exception e) {
             newCompressedPDF.close();
@@ -481,7 +481,7 @@ public class CompressController {
     private static String bytesToHexString(byte[] bytes) {
         StringBuilder sb = new StringBuilder();
         for (byte b : bytes) {
-            sb.append(String.format("%02x", b));
+            sb.append(String.format(Locale.ROOT, "%02x", b));
         }
         return sb.toString();
     }
@@ -538,7 +538,7 @@ public class CompressController {
                 "Total original image size: {}, compressed: {} (reduced by {}%)",
                 GeneralUtils.formatBytes(stats.totalOriginalBytes),
                 GeneralUtils.formatBytes(stats.totalCompressedBytes),
-                String.format("%.1f", overallImageReduction));
+                String.format(Locale.ROOT, "%.1f", overallImageReduction));
     }
 
     private static BufferedImage convertToGrayscale(BufferedImage image) {
@@ -848,6 +848,7 @@ public class CompressController {
         @Override
         public String toString() {
             return String.format(
+                    Locale.ROOT,
                     "%s_%s_%d_%dx%d_%s_%s_%d_%s_%s_%s",
                     pixelHash.substring(0, Math.min(8, pixelHash.length())),
                     colorSpace,
@@ -911,7 +912,7 @@ public class CompressController {
     // Increment optimization level if we need more compression
     private static int incrementOptimizeLevel(int currentLevel, long currentSize, long targetSize) {
         double currentRatio = currentSize / (double) targetSize;
-        log.info("Current compression ratio: {}", String.format("%.2f", currentRatio));
+        log.info("Current compression ratio: {}", String.format(Locale.ROOT, "%.2f", currentRatio));
 
         if (currentRatio > 2.0) {
             return Math.min(9, currentLevel + 3);
@@ -1184,7 +1185,7 @@ public class CompressController {
                     log.info(
                             "Post-Ghostscript file size: {} (reduced by {}%)",
                             GeneralUtils.formatBytes(postGsSize),
-                            String.format("%.1f", gsReduction));
+                            String.format(Locale.ROOT, "%.1f", gsReduction));
                 } else {
                     log.warn(
                             "Ghostscript compression failed with return code: {}",
@@ -1291,7 +1292,7 @@ public class CompressController {
                 log.info(
                         "Post-QPDF file size: {} (reduced by {}%)",
                         GeneralUtils.formatBytes(postQpdfSize),
-                        String.format("%.1f", qpdfReduction));
+                        String.format(Locale.ROOT, "%.1f", qpdfReduction));
 
             } catch (IOException e) {
                 if (returnCode != null && returnCode.getRc() != 3) {

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/OCRController.java

@@ -9,6 +9,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -336,7 +337,9 @@ public class OCRController {
                             };
 
                     File pageOutputPath =
-                            new File(tempOutputDir, String.format("page_%d.pdf", pageNum));
+                            new File(
+                                    tempOutputDir,
+                                    String.format(Locale.ROOT, "page_%d.pdf", pageNum));
 
                     if (shouldOcr) {
                         // Convert page to image
@@ -359,7 +362,9 @@ public class OCRController {
                                     pageNum + 1, renderDpi, e);
                         }
                         File imagePath =
-                                new File(tempImagesDir, String.format("page_%d.png", pageNum));
+                                new File(
+                                        tempImagesDir,
+                                        String.format(Locale.ROOT, "page_%d.png", pageNum));
                         ImageIO.write(image, "png", imagePath);
 
                         // Build OCR command
@@ -367,7 +372,9 @@ public class OCRController {
                         command.add("tesseract");
                         command.add(imagePath.toString());
                         command.add(
-                                new File(tempOutputDir, String.format("page_%d", pageNum))
+                                new File(
+                                                tempOutputDir,
+                                                String.format(Locale.ROOT, "page_%d", pageNum))
                                         .toString());
                         command.add("-l");
                         command.add(String.join("+", selectedLanguages));

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/PrintFileController.java

@@ -9,6 +9,7 @@ import java.awt.print.PrinterJob;
 import java.io.IOException;
 import java.nio.file.Paths;
 import java.util.Arrays;
+import java.util.Locale;
 
 import javax.imageio.ImageIO;
 import javax.print.PrintService;
@@ -57,11 +58,14 @@ public class PrintFileController {
         try {
             // Find matching printer
             PrintService[] services = PrintServiceLookup.lookupPrintServices(null, null);
+            String normalizedPrinterName = printerName.toLowerCase(Locale.ROOT);
             PrintService selectedService =
                     Arrays.stream(services)
                             .filter(
                                     service ->
-                                            service.getName().toLowerCase().contains(printerName))
+                                            service.getName()
+                                                    .toLowerCase(Locale.ROOT)
+                                                    .contains(normalizedPrinterName))
                             .findFirst()
                             .orElseThrow(
                                     () ->

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/StampController.java

@@ -11,6 +11,7 @@ import java.time.LocalDate;
 import java.time.LocalTime;
 import java.time.format.DateTimeFormatter;
 import java.util.List;
+import java.util.Locale;
 
 import javax.imageio.ImageIO;
 
@@ -117,7 +118,7 @@ public class StampController {
 
         String customColor = request.getCustomColor();
         float marginFactor =
-                switch (request.getCustomMargin().toLowerCase()) {
+                switch (request.getCustomMargin().toLowerCase(Locale.ROOT)) {
                     case "small" -> 0.02f;
                     case "medium" -> 0.035f;
                     case "large" -> 0.05f;

app/core/src/main/java/stirling/software/SPDF/controller/api/pipeline/PipelineDirectoryProcessor.java

@@ -19,6 +19,7 @@ import java.time.format.DateTimeFormatter;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.stream.Stream;
@@ -203,7 +204,7 @@ public class PipelineDirectoryProcessor {
                                                         ? filename.substring(
                                                                         filename.lastIndexOf(".")
                                                                                 + 1)
-                                                                .toLowerCase()
+                                                                .toLowerCase(Locale.ROOT)
                                                         : "";
 
                                         // Check against allowed extensions
@@ -213,7 +214,8 @@ public class PipelineDirectoryProcessor {
                                                                 extension.toLowerCase());
                                         if (!isAllowed) {
                                             log.info(
-                                                    "Skipping file with unsupported extension: {} ({})",
+                                                    "Skipping file with unsupported extension: {}"
+                                                            + " ({})",
                                                     filename,
                                                     extension);
                                         }
@@ -226,7 +228,8 @@ public class PipelineDirectoryProcessor {
                                                 fileMonitor.isFileReadyForProcessing(path);
                                         if (!isReady) {
                                             log.info(
-                                                    "File not ready for processing (locked/created last 5s): {}",
+                                                    "File not ready for processing (locked/created"
+                                                            + " last 5s): {}",
                                                     path);
                                         }
                                         return isReady;

app/core/src/main/java/stirling/software/SPDF/controller/api/pipeline/PipelineProcessor.java

@@ -8,6 +8,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.zip.ZipEntry;
@@ -121,7 +122,9 @@ public class PipelineProcessor {
                     boolean hasInputFileType = false;
                     for (String extension : inputFileTypes) {
                         if ("ALL".equals(extension)
-                                || file.getFilename().toLowerCase().endsWith(extension)) {
+                                || file.getFilename()
+                                        .toLowerCase(Locale.ROOT)
+                                        .endsWith(extension)) {
                             hasInputFileType = true;
                             MultiValueMap<String, Object> body = new LinkedMultiValueMap<>();
                             body.add("fileInput", file);
@@ -159,7 +162,8 @@ public class PipelineProcessor {
                         String providedExtension = "no extension";
                         if (filename != null && filename.contains(".")) {
                             providedExtension =
-                                    filename.substring(filename.lastIndexOf(".")).toLowerCase();
+                                    filename.substring(filename.lastIndexOf("."))
+                                            .toLowerCase(Locale.ROOT);
                         }
 
                         logPrintStream.println(
@@ -187,7 +191,10 @@ public class PipelineProcessor {
                                             file ->
                                                     finalinputFileTypes.stream()
                                                             .anyMatch(
-                                                                    file.getFilename().toLowerCase()
+                                                                    file.getFilename()
+                                                                                    .toLowerCase(
+                                                                                            Locale
+                                                                                                    .ROOT)
                                                                             ::endsWith))
                                     .toList();
                 }
@@ -228,7 +235,7 @@ public class PipelineProcessor {
                                                 if (filename != null && filename.contains(".")) {
                                                     return filename.substring(
                                                                     filename.lastIndexOf("."))
-                                                            .toLowerCase();
+                                                            .toLowerCase(Locale.ROOT);
                                                 }
                                                 return "no extension";
                                             })

app/core/src/main/java/stirling/software/SPDF/controller/api/security/GetInfoOnPDF.java

@@ -410,12 +410,12 @@ public class GetInfoOnPDF {
         float widthInCm = widthInInches * 2.54f;
         float heightInCm = heightInInches * 2.54f;
 
-        dimensionInfo.put("Width (px)", String.format("%.2f", width));
-        dimensionInfo.put("Height (px)", String.format("%.2f", height));
-        dimensionInfo.put("Width (in)", String.format("%.2f", widthInInches));
-        dimensionInfo.put("Height (in)", String.format("%.2f", heightInInches));
-        dimensionInfo.put("Width (cm)", String.format("%.2f", widthInCm));
-        dimensionInfo.put("Height (cm)", String.format("%.2f", heightInCm));
+        dimensionInfo.put("Width (px)", String.format(Locale.ROOT, "%.2f", width));
+        dimensionInfo.put("Height (px)", String.format(Locale.ROOT, "%.2f", height));
+        dimensionInfo.put("Width (in)", String.format(Locale.ROOT, "%.2f", widthInInches));
+        dimensionInfo.put("Height (in)", String.format(Locale.ROOT, "%.2f", heightInInches));
+        dimensionInfo.put("Width (cm)", String.format(Locale.ROOT, "%.2f", widthInCm));
+        dimensionInfo.put("Height (cm)", String.format(Locale.ROOT, "%.2f", heightInCm));
     }
 
     private static ArrayNode exploreStructureTree(List<Object> nodes) {

app/core/src/main/java/stirling/software/SPDF/controller/web/MetricsController.java

@@ -361,7 +361,7 @@ public class MetricsController {
         long hours = duration.toHoursPart();
         long minutes = duration.toMinutesPart();
         long seconds = duration.toSecondsPart();
-        return String.format("%dd %dh %dm %ds", days, hours, minutes, seconds);
+        return String.format(Locale.ROOT, "%dd %dh %dm %ds", days, hours, minutes, seconds);
     }
 
     @Setter

app/core/src/main/java/stirling/software/SPDF/controller/web/UploadLimitService.java

@@ -65,6 +65,6 @@ public class UploadLimitService {
         if (bytes < 1024) return bytes + " B";
         int exp = (int) (Math.log(bytes) / Math.log(1024));
         String pre = "KMGTPE".charAt(exp - 1) + "B";
-        return String.format(Locale.US, "%.1f %s", bytes / Math.pow(1024, exp), pre);
+        return String.format(Locale.ROOT, "%.1f %s", bytes / Math.pow(1024, exp), pre);
     }
 }

app/core/src/main/java/stirling/software/SPDF/pdf/TextFinder.java

@@ -3,6 +3,7 @@ package stirling.software.SPDF.pdf;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -204,12 +205,14 @@ public class TextFinder extends PDFTextStripper {
             TextPosition pos = i < pageTextPositions.size() ? pageTextPositions.get(i) : null;
             debug.append(
                     String.format(
+                            Locale.ROOT,
                             "  [%d] '%c' (0x%02X) -> %s\n",
                             i,
                             c,
                             (int) c,
                             pos != null
-                                    ? String.format("(%.1f,%.1f)", pos.getX(), pos.getY())
+                                    ? String.format(
+                                            Locale.ROOT, "(%.1f,%.1f)", pos.getX(), pos.getY())
                                     : "null"));
         }
 

app/core/src/main/java/stirling/software/SPDF/service/ApiDocService.java

@@ -3,6 +3,7 @@ package stirling.software.SPDF.service;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.regex.Matcher;
 
@@ -88,7 +89,7 @@ public class ApiDocService {
                                 : RegexPatternUtils.getInstance().getApiDocInputTypePattern())
                         .matcher(description);
         while (matcher.find()) {
-            String type = matcher.group(1).toUpperCase();
+            String type = matcher.group(1).toUpperCase(Locale.ROOT);
             if (outputToFileTypes.containsKey(type)) {
                 return outputToFileTypes.get(type);
             }

app/core/src/main/java/stirling/software/SPDF/service/MetricsAggregatorService.java

@@ -1,6 +1,7 @@
 package stirling.software.SPDF.service;
 
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -70,7 +71,10 @@ public class MetricsAggregatorService {
 
                             String key =
                                     String.format(
-                                            "http_requests_%s_%s", method, uri.replace("/", "_"));
+                                            Locale.ROOT,
+                                            "http_requests_%s_%s",
+                                            method,
+                                            uri.replace("/", "_"));
                             double currentCount = counter.count();
                             double lastCount = lastSentMetrics.getOrDefault(key, 0.0);
                             double difference = currentCount - lastCount;

app/core/src/main/java/stirling/software/SPDF/service/SignatureService.java

@@ -7,6 +7,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.stream.Stream;
 
 import org.springframework.stereotype.Service;
@@ -92,7 +93,7 @@ public class SignatureService {
     }
 
     private boolean isImageFile(Path path) {
-        String fileName = path.getFileName().toString().toLowerCase();
+        String fileName = path.getFileName().toString().toLowerCase(Locale.ROOT);
         return fileName.endsWith(".jpg")
                 || fileName.endsWith(".jpeg")
                 || fileName.endsWith(".png")

app/core/src/main/java/stirling/software/SPDF/utils/text/TextEncodingHelper.java

@@ -1,6 +1,7 @@
 package stirling.software.SPDF.utils.text;
 
 import java.io.IOException;
+import java.util.Locale;
 
 import org.apache.pdfbox.pdmodel.font.PDFont;
 import org.apache.pdfbox.pdmodel.font.PDSimpleFont;
@@ -24,7 +25,8 @@ public class TextEncodingHelper {
             byte[] encoded = font.encode(text);
             if (encoded.length > 0) {
                 log.debug(
-                        "Text '{}' has good full-string encoding for font {} - permissively allowing",
+                        "Text '{}' has good full-string encoding for font {} - permissively"
+                                + " allowing",
                         text,
                         font.getName() != null ? font.getName() : "Unknown");
                 return true;
@@ -61,6 +63,7 @@ public class TextEncodingHelper {
         for (int i = 0; i < text.length(); ) {
             int codePoint = text.codePointAt(i);
             String charStr = new String(Character.toChars(codePoint));
+            String hex = String.format(Locale.ROOT, "%04X", codePoint); // U+%04X
             totalCodePoints++;
 
             try {
@@ -71,28 +74,23 @@ public class TextEncodingHelper {
 
                     if (charWidth >= 0) {
                         successfulCodePoints++;
-                        log.debug(
-                                "Code point '{}' (U+{}) encoded successfully",
-                                charStr,
-                                Integer.toHexString(codePoint).toUpperCase());
+                        log.debug("Code point '{}' (U+{}) encoded successfully", charStr, hex);
                     } else {
                         log.debug(
                                 "Code point '{}' (U+{}) has invalid width: {}",
                                 charStr,
-                                Integer.toHexString(codePoint).toUpperCase(),
+                                hex,
                                 charWidth);
                     }
                 } else {
                     log.debug(
-                            "Code point '{}' (U+{}) encoding failed - empty result",
-                            charStr,
-                            Integer.toHexString(codePoint).toUpperCase());
+                            "Code point '{}' (U+{}) encoding failed - empty result", charStr, hex);
                 }
             } catch (IOException | IllegalArgumentException e) {
                 log.debug(
                         "Code point '{}' (U+{}) validation failed: {}",
                         charStr,
-                        Integer.toHexString(codePoint).toUpperCase(),
+                        hex,
                         e.getMessage());
             }
 
@@ -100,16 +98,20 @@ public class TextEncodingHelper {
         }
 
         double successRate =
-                totalCodePoints > 0 ? (double) successfulCodePoints / totalCodePoints : 0;
+                totalCodePoints > 0 ? (double) successfulCodePoints / totalCodePoints : 0.0;
+        String pct =
+                String.format(
+                        Locale.ROOT, "%.1f%%", successRate * 100); // Pre-formatting percentage!
+
         boolean isAcceptable = successRate >= 0.95;
 
         log.debug(
-                "Array validation for '{}': {}/{} code points successful ({:.1f}%) - {}",
+                "Array validation for '{}': {}/{} code points successful ({}) - {}",
                 text,
                 successfulCodePoints,
                 totalCodePoints,
-                successRate * 100,
-                isAcceptable ? "ALLOWING" : "rejecting");
+                pct,
+                (isAcceptable ? "ALLOWING" : "rejecting"));
 
         return isAcceptable;
     }

app/core/src/test/java/stirling/software/SPDF/pdf/TextFinderTest.java

@@ -4,6 +4,7 @@ import static org.junit.jupiter.api.Assertions.*;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Locale;
 
 import org.apache.pdfbox.pdmodel.PDDocument;
 import org.apache.pdfbox.pdmodel.PDPage;
@@ -49,13 +50,16 @@ class TextFinderTest {
                 expectedCount,
                 foundTexts.size(),
                 String.format(
-                        "Expected %d matches for search term '%s'", expectedCount, searchTerm));
+                        Locale.ROOT,
+                        "Expected %d matches for search term '%s'",
+                        expectedCount,
+                        searchTerm));
 
         if (expectedTexts != null) {
             for (String expectedText : expectedTexts) {
                 assertTrue(
                         foundTexts.stream().anyMatch(text -> text.getText().equals(expectedText)),
-                        String.format("Expected to find text: '%s'", expectedText));
+                        String.format(Locale.ROOT, "Expected to find text: '%s'", expectedText));
             }
         }
 
@@ -271,7 +275,10 @@ class TextFinderTest {
             // Each pattern should find at least one match in our test content
             assertFalse(
                     foundTexts.isEmpty(),
-                    String.format("Pattern '%s' should find at least one match", regexPattern));
+                    String.format(
+                            Locale.ROOT,
+                            "Pattern '%s' should find at least one match",
+                            regexPattern));
         }
 
         @Test

app/proprietary/src/main/java/stirling/software/proprietary/audit/AuditUtils.java

@@ -5,6 +5,7 @@ import java.time.Instant;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
@@ -307,8 +308,8 @@ public class AuditUtils {
 
         // For HTTP methods, infer based on controller and path
         if (httpMethod != null && path != null) {
-            String cls = controller.getSimpleName().toLowerCase();
-            String pkg = controller.getPackage().getName().toLowerCase();
+            String cls = controller.getSimpleName().toLowerCase(Locale.ROOT);
+            String pkg = controller.getPackage().getName().toLowerCase(Locale.ROOT);
 
             if ("GET".equals(httpMethod)) return AuditEventType.HTTP_REQUEST;
 
@@ -374,8 +375,8 @@ public class AuditUtils {
         }
 
         // Otherwise infer from controller and path
-        String cls = controller.getSimpleName().toLowerCase();
-        String pkg = controller.getPackage().getName().toLowerCase();
+        String cls = controller.getSimpleName().toLowerCase(Locale.ROOT);
+        String pkg = controller.getPackage().getName().toLowerCase(Locale.ROOT);
 
         if ("GET".equals(httpMethod)) return AuditEventType.HTTP_REQUEST;
 

app/proprietary/src/main/java/stirling/software/proprietary/security/CustomLogoutSuccessHandler.java

@@ -5,6 +5,7 @@ import java.security.cert.X509Certificate;
 import java.security.interfaces.RSAPrivateKey;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 
 import org.springframework.core.io.Resource;
 import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
@@ -145,7 +146,7 @@ public class CustomLogoutSuccessHandler extends SimpleUrlLogoutSuccessHandler {
         registrationId = oAuthToken.getAuthorizedClientRegistrationId();
 
         // Redirect based on OAuth2 provider
-        switch (registrationId.toLowerCase()) {
+        switch (registrationId.toLowerCase(Locale.ROOT)) {
             case "keycloak" -> {
                 KeycloakProvider keycloak = oauth.getClient().getKeycloak();
 

app/proprietary/src/main/java/stirling/software/proprietary/security/InitialSecuritySetup.java

@@ -142,7 +142,7 @@ public class InitialSecuritySetup {
             if (internalApiUserOpt.isPresent()) {
                 User internalApiUser = internalApiUserOpt.get();
                 // move to team internal API user
-                if (!internalApiUser.getTeam().getName().equals(TeamService.INTERNAL_TEAM_NAME)) {
+                if (!TeamService.INTERNAL_TEAM_NAME.equals(internalApiUser.getTeam().getName())) {
                     log.info(
                             "Moving internal API user to team: {}", TeamService.INTERNAL_TEAM_NAME);
                     Team internalTeam = teamService.getOrCreateInternalTeam();

app/proprietary/src/main/java/stirling/software/proprietary/security/config/AccountWebController.java

@@ -7,6 +7,7 @@ import java.time.temporal.ChronoUnit;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 
@@ -89,7 +90,8 @@ public class AccountWebController {
                 if (oauth.isSettingsValid()) {
                     String firstChar = String.valueOf(oauth.getProvider().charAt(0));
                     String clientName =
-                            oauth.getProvider().replaceFirst(firstChar, firstChar.toUpperCase());
+                            oauth.getProvider()
+                                    .replaceFirst(firstChar, firstChar.toUpperCase(Locale.ROOT));
                     providerList.put(OAUTH_2_AUTHORIZATION + oauth.getProvider(), clientName);
                 }
 

app/proprietary/src/main/java/stirling/software/proprietary/security/configuration/DatabaseConfig.java

@@ -1,5 +1,7 @@
 package stirling.software.proprietary.security.configuration;
 
+import java.util.Locale;
+
 import javax.sql.DataSource;
 
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -132,7 +134,7 @@ public class DatabaseConfig {
     private String getDriverClassName(String driverName) throws UnsupportedProviderException {
         try {
             ApplicationProperties.Driver driver =
-                    ApplicationProperties.Driver.valueOf(driverName.toUpperCase());
+                    ApplicationProperties.Driver.valueOf(driverName.toUpperCase(Locale.ROOT));
 
             return switch (driver) {
                 case H2 -> {

app/proprietary/src/main/java/stirling/software/proprietary/security/configuration/ee/KeygenLicenseVerifier.java

@@ -5,6 +5,7 @@ import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
 import java.net.http.HttpResponse;
 import java.util.Base64;
+import java.util.Locale;
 
 import org.bouncycastle.crypto.params.Ed25519PublicKeyParameters;
 import org.bouncycastle.crypto.signers.Ed25519Signer;
@@ -185,7 +186,7 @@ public class KeygenLicenseVerifier {
             byte[] signatureBytes = Base64.getDecoder().decode(encodedSignature);
 
             // Create the signing data format - prefix with "license/"
-            String signingData = String.format("license/%s", encryptedData);
+            String signingData = String.format(Locale.ROOT, "license/%s", encryptedData);
             byte[] signingDataBytes = signingData.getBytes();
 
             log.info("Signing data length: {} bytes", signingDataBytes.length);
@@ -335,7 +336,7 @@ public class KeygenLicenseVerifier {
                             .decode(encodedSignature.replace('-', '+').replace('_', '/'));
 
             // For ED25519_SIGN format, the signing data is "key/" + encodedPayload
-            String signingData = String.format("key/%s", encodedPayload);
+            String signingData = String.format(Locale.ROOT, "key/%s", encodedPayload);
             byte[] dataBytes = signingData.getBytes();
 
             byte[] publicKeyBytes = Hex.decode(PUBLIC_KEY);
@@ -510,8 +511,10 @@ public class KeygenLicenseVerifier {
             String licenseKey, String machineFingerprint, LicenseContext context) throws Exception {
         String requestBody =
                 String.format(
+                        Locale.ROOT,
                         "{\"meta\":{\"key\":\"%s\",\"scope\":{\"fingerprint\":\"%s\"}}}",
-                        licenseKey, machineFingerprint);
+                        licenseKey,
+                        machineFingerprint);
         HttpRequest request =
                 HttpRequest.newBuilder()
                         .uri(

app/proprietary/src/main/java/stirling/software/proprietary/security/controller/api/AdminSettingsController.java

@@ -4,6 +4,7 @@ import java.io.IOException;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -185,6 +186,7 @@ public class AdminSettingsController {
 
             return ResponseEntity.ok(
                     String.format(
+                            Locale.ROOT,
                             "Successfully updated %d setting(s). Changes will take effect on"
                                     + " application restart.",
                             updatedCount));
@@ -297,9 +299,11 @@ public class AdminSettingsController {
             String escapedSectionName = HtmlUtils.htmlEscape(sectionName);
             return ResponseEntity.ok(
                     String.format(
+                            Locale.ROOT,
                             "Successfully updated %d setting(s) in section '%s'. Changes will take"
                                     + " effect on application restart.",
-                            updatedCount, escapedSectionName));
+                            updatedCount,
+                            escapedSectionName));
 
         } catch (IOException e) {
             log.error("Failed to save section settings to file: {}", e.getMessage(), e);
@@ -388,6 +392,7 @@ public class AdminSettingsController {
             String escapedKey = HtmlUtils.htmlEscape(key);
             return ResponseEntity.ok(
                     String.format(
+                            Locale.ROOT,
                             "Successfully updated setting '%s'. Changes will take effect on"
                                     + " application restart.",
                             escapedKey));
@@ -410,7 +415,7 @@ public class AdminSettingsController {
             return null;
         }
 
-        return switch (sectionName.toLowerCase()) {
+        return switch (sectionName.toLowerCase(Locale.ROOT)) {
             case "security" -> applicationProperties.getSecurity();
             case "system" -> applicationProperties.getSystem();
             case "ui" -> applicationProperties.getUi();
@@ -473,7 +478,7 @@ public class AdminSettingsController {
         }
 
         // Ensure first part is a valid section name
-        if (parts.length > 0 && !VALID_SECTION_NAMES.contains(parts[0].toLowerCase())) {
+        if (parts.length > 0 && !VALID_SECTION_NAMES.contains(parts[0].toLowerCase(Locale.ROOT))) {
             return false;
         }
 
@@ -578,8 +583,8 @@ public class AdminSettingsController {
 
     /** Check if a field name indicates sensitive data with full path context */
     private boolean isSensitiveFieldWithPath(String fieldName, String fullPath) {
-        String lowerField = fieldName.toLowerCase();
-        String lowerPath = fullPath.toLowerCase();
+        String lowerField = fieldName.toLowerCase(Locale.ROOT);
+        String lowerPath = fullPath.toLowerCase(Locale.ROOT);
 
         // Don't mask premium.key specifically
         if ("key".equals(lowerField) && "premium.key".equals(lowerPath)) {

app/proprietary/src/main/java/stirling/software/proprietary/security/model/User.java

@@ -3,6 +3,7 @@ package stirling.software.proprietary.security.model;
 import java.io.Serializable;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
@@ -84,7 +85,7 @@ public class User implements UserDetails, Serializable {
     }
 
     public void setAuthenticationType(AuthenticationType authenticationType) {
-        this.authenticationType = authenticationType.toString().toLowerCase();
+        this.authenticationType = authenticationType.toString().toLowerCase(Locale.ROOT);
     }
 
     public void addAuthorities(Set<Authority> authorities) {

app/proprietary/src/main/java/stirling/software/proprietary/security/oauth2/OAuth2Configuration.java

@@ -7,6 +7,7 @@ import static stirling.software.common.util.ValidationUtils.isStringEmpty;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Optional;
 import java.util.Set;
 
@@ -177,7 +178,7 @@ public class OAuth2Configuration {
 
         String name = oauth.getProvider();
         String firstChar = String.valueOf(name.charAt(0));
-        String clientName = name.replaceFirst(firstChar, firstChar.toUpperCase());
+        String clientName = name.replaceFirst(firstChar, firstChar.toUpperCase(Locale.ROOT));
 
         Provider oidcProvider =
                 new Provider(
@@ -187,7 +188,8 @@ public class OAuth2Configuration {
                         oauth.getClientId(),
                         oauth.getClientSecret(),
                         oauth.getScopes(),
-                        UsernameAttribute.valueOf(oauth.getUseAsUsername().toUpperCase()),
+                        UsernameAttribute.valueOf(
+                                oauth.getUseAsUsername().toUpperCase(Locale.ROOT)),
                         null,
                         null,
                         null);

app/proprietary/src/main/java/stirling/software/proprietary/security/service/CustomOAuth2UserService.java

@@ -1,5 +1,6 @@
 package stirling.software.proprietary.security.service;
 
+import java.util.Locale;
 import java.util.Optional;
 
 import org.springframework.security.authentication.LockedException;
@@ -44,7 +45,7 @@ public class CustomOAuth2UserService implements OAuth2UserService<OidcUserReques
             OidcUser user = delegate.loadUser(userRequest);
             OAUTH2 oauth2 = securityProperties.getOauth2();
             UsernameAttribute usernameAttribute =
-                    UsernameAttribute.valueOf(oauth2.getUseAsUsername().toUpperCase());
+                    UsernameAttribute.valueOf(oauth2.getUseAsUsername().toUpperCase(Locale.ROOT));
             String usernameAttributeKey = usernameAttribute.getName();
 
             // todo: save user by OIDC ID instead of username

app/proprietary/src/main/java/stirling/software/proprietary/security/service/CustomUserDetailsService.java

@@ -1,5 +1,7 @@
 package stirling.software.proprietary.security.service;
 
+import java.util.Locale;
+
 import org.springframework.security.authentication.LockedException;
 import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.security.core.userdetails.UserDetailsService;
@@ -59,7 +61,7 @@ public class CustomUserDetailsService implements UserDetailsService {
         }
 
         AuthenticationType userAuthenticationType =
-                AuthenticationType.valueOf(authTypeStr.toUpperCase());
+                AuthenticationType.valueOf(authTypeStr.toUpperCase(Locale.ROOT));
         if (!user.hasPassword() && userAuthenticationType == AuthenticationType.WEB) {
             throw new IllegalArgumentException("Password must not be null");
         }

app/proprietary/src/main/java/stirling/software/proprietary/security/service/LoginAttemptService.java

@@ -1,5 +1,6 @@
 package stirling.software.proprietary.security.service;
 
+import java.util.Locale;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 
@@ -45,17 +46,19 @@ public class LoginAttemptService {
         if (!isBlockedEnabled || key == null || key.trim().isEmpty()) {
             return;
         }
-        attemptsCache.remove(key.toLowerCase());
+        String normalizedKey = key.toLowerCase(Locale.ROOT);
+        attemptsCache.remove(normalizedKey);
     }
 
     public void loginFailed(String key) {
         if (!isBlockedEnabled || key == null || key.trim().isEmpty()) {
             return;
         }
-        AttemptCounter attemptCounter = attemptsCache.get(key.toLowerCase());
+        String normalizedKey = key.toLowerCase(Locale.ROOT);
+        AttemptCounter attemptCounter = attemptsCache.get(normalizedKey);
         if (attemptCounter == null) {
             attemptCounter = new AttemptCounter();
-            attemptsCache.put(key.toLowerCase(), attemptCounter);
+            attemptsCache.put(normalizedKey, attemptCounter);
         } else {
             if (attemptCounter.shouldReset(ATTEMPT_INCREMENT_TIME)) {
                 attemptCounter.reset();
@@ -68,7 +71,8 @@ public class LoginAttemptService {
         if (!isBlockedEnabled || key == null || key.trim().isEmpty()) {
             return false;
         }
-        AttemptCounter attemptCounter = attemptsCache.get(key.toLowerCase());
+        String normalizedKey = key.toLowerCase(Locale.ROOT);
+        AttemptCounter attemptCounter = attemptsCache.get(normalizedKey);
         if (attemptCounter == null) {
             return false;
         }
@@ -80,7 +84,8 @@ public class LoginAttemptService {
             // Arbitrarily high number if tracking is disabled
             return Integer.MAX_VALUE;
         }
-        AttemptCounter attemptCounter = attemptsCache.get(key.toLowerCase());
+        String normalizedKey = key.toLowerCase(Locale.ROOT);
+        AttemptCounter attemptCounter = attemptsCache.get(normalizedKey);
         if (attemptCounter == null) {
             return MAX_ATTEMPT;
         }

app/proprietary/src/main/java/stirling/software/proprietary/security/service/UserService.java

@@ -5,6 +5,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.UUID;
@@ -118,6 +119,7 @@ public class UserService implements UserServiceInterface {
         return addApiKeyToUser(username);
     }
 
+    @Override
     public String getApiKeyForUser(String username) {
         User user =
                 findByUsernameIgnoreCase(username)
@@ -495,9 +497,10 @@ public class UserService implements UserServiceInterface {
                         .matches();
 
         List<String> notAllowedUserList = new ArrayList<>();
-        notAllowedUserList.add("ALL_USERS".toLowerCase());
+        notAllowedUserList.add("ALL_USERS".toLowerCase(Locale.ROOT));
         notAllowedUserList.add("anonymoususer");
-        boolean notAllowedUser = notAllowedUserList.contains(username.toLowerCase());
+        String normalizedUsername = username.toLowerCase(Locale.ROOT);
+        boolean notAllowedUser = notAllowedUserList.contains(normalizedUsername);
         return (isValidSimpleUsername || isValidEmail) && !notAllowedUser;
     }
 
@@ -545,6 +548,7 @@ public class UserService implements UserServiceInterface {
         }
     }
 
+    @Override
     public String getCurrentUsername() {
         Object principal = SecurityContextHolder.getContext().getAuthentication().getPrincipal();
 
@@ -601,6 +605,7 @@ public class UserService implements UserServiceInterface {
         }
     }
 
+    @Override
     public long getTotalUsersCount() {
         // Count all users in the database
         long userCount = userRepository.count();