diff --git a/src/lib/app.ts b/src/lib/app.ts
index 6ae1716e37..36f2f2c19e 100644
--- a/src/lib/app.ts
+++ b/src/lib/app.ts
@@ -118,12 +118,16 @@ export default async function getApp(
         }
         case IAuthType.ENTERPRISE: {
             app.use(baseUriPath, apiTokenMiddleware(config, services));
-            config.authentication.customAuthHandler(app, config, services);
+            if (config.authentication.customAuthHandler) {
+                config.authentication.customAuthHandler(app, config, services);
+            }
             break;
         }
         case IAuthType.HOSTED: {
             app.use(baseUriPath, apiTokenMiddleware(config, services));
-            config.authentication.customAuthHandler(app, config, services);
+            if (config.authentication.customAuthHandler) {
+                config.authentication.customAuthHandler(app, config, services);
+            }
             break;
         }
         case IAuthType.DEMO: {
@@ -138,7 +142,9 @@ export default async function getApp(
         }
         case IAuthType.CUSTOM: {
             app.use(baseUriPath, apiTokenMiddleware(config, services));
-            config.authentication.customAuthHandler(app, config, services);
+            if (config.authentication.customAuthHandler) {
+                config.authentication.customAuthHandler(app, config, services);
+            }
             break;
         }
         case IAuthType.NONE: {
diff --git a/src/lib/default-custom-auth-deny-all.ts b/src/lib/default-custom-auth-deny-all.ts
index 2fdbb736b7..8e29bc42c6 100644
--- a/src/lib/default-custom-auth-deny-all.ts
+++ b/src/lib/default-custom-auth-deny-all.ts
@@ -1,11 +1,11 @@
+import { Express } from 'express';
 import { IUnleashConfig } from './types/option';
 
 const customAuthWarning =
     'You have to configure a custom authentication middleware. Read https://docs.getunleash.io/docs/reference/deploy/configuring-unleash for more details';
 
 export function defaultCustomAuthDenyAll(
-    // eslint-disable-next-line @typescript-eslint/explicit-module-boundary-types
-    app: any,
+    app: Express,
     config: IUnleashConfig,
 ): void {
     const logger = config.getLogger('src/lib/app/customAuthHandler');
diff --git a/src/lib/server-impl.ts b/src/lib/server-impl.ts
index 5e273dedd4..dd3fe82ec9 100644
--- a/src/lib/server-impl.ts
+++ b/src/lib/server-impl.ts
@@ -18,6 +18,7 @@ import {
     IUnleashOptions,
     IUnleashServices,
     RoleName,
+    CustomAuthHandler,
 } from './types';
 
 import User, { IUser } from './types/user';
@@ -212,4 +213,5 @@ export type {
     IAuthRequest,
     IApiRequest,
     SimpleAuthSettings,
+    CustomAuthHandler,
 };
diff --git a/src/lib/types/option.ts b/src/lib/types/option.ts
index eccb651d7d..887c88b429 100644
--- a/src/lib/types/option.ts
+++ b/src/lib/types/option.ts
@@ -1,8 +1,10 @@
+import { Express } from 'express';
 import EventEmitter from 'events';
 import { LogLevel, LogProvider } from '../logger';
 import { ILegacyApiTokenCreate } from './models/api-token';
 import { IFlagResolver, IExperimentalOptions, IFlags } from './experimental';
 import SMTPTransport from 'nodemailer/lib/smtp-transport';
+import { IUnleashServices } from './services';
 
 export interface ISSLOption {
     rejectUnauthorized: boolean;
@@ -53,10 +55,16 @@ export enum IAuthType {
     NONE = 'none',
 }
 
+export type CustomAuthHandler = (
+    app: Express,
+    config: Partial<IUnleashConfig>,
+    services?: IUnleashServices,
+) => void;
+
 export interface IAuthOption {
     enableApiToken: boolean;
     type: IAuthType;
-    customAuthHandler?: Function;
+    customAuthHandler?: CustomAuthHandler;
     createAdminUser?: boolean;
     initialAdminUser?: {
         username: string;
diff --git a/src/test/fixtures/fake-user-store.ts b/src/test/fixtures/fake-user-store.ts
index 9831044163..bd78c34de0 100644
--- a/src/test/fixtures/fake-user-store.ts
+++ b/src/test/fixtures/fake-user-store.ts
@@ -47,7 +47,6 @@ class UserStoreMock implements IUserStore {
     }
 
     async insert(user: User): Promise<User> {
-        // eslint-disable-next-line no-param-reassign
         user.id = this.idSeq;
         this.idSeq += 1;
         this.data.push(user);
@@ -55,7 +54,6 @@ class UserStoreMock implements IUserStore {
     }
 
     async update(id: number, user: User): Promise<User> {
-        // eslint-disable-next-line no-param-reassign
         this.data = this.data.map((o) => {
             if (o.id === id) return { ...o, name: user.name };
             return o;