diff --git a/.changeset/tasty-poems-raise.md b/.changeset/tasty-poems-raise.md
new file mode 100644
index 0000000000..47556cb3ad
--- /dev/null
+++ b/.changeset/tasty-poems-raise.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': minor
+---
+
+Added validation to TokenFactory.issueToken that ensure any sub claim given is a valid entityRef. This will affect any custom resolver functions given to auth providers.
diff --git a/plugins/auth-backend/src/identity/TokenFactory.test.ts b/plugins/auth-backend/src/identity/TokenFactory.test.ts
index f5f06f1209..4b1c1804df 100644
--- a/plugins/auth-backend/src/identity/TokenFactory.test.ts
+++ b/plugins/auth-backend/src/identity/TokenFactory.test.ts
@@ -18,6 +18,7 @@ import { MemoryKeyStore } from './MemoryKeyStore';
 import { TokenFactory } from './TokenFactory';
 import { getVoidLogger } from '@backstage/backend-common';
 import { JWKS, JSONWebKey, JWT } from 'jose';
+import { stringifyEntityRef } from '@backstage/catalog-model';
 
 const logger = getVoidLogger();
 
@@ -28,6 +29,12 @@ function jwtKid(jwt: string): string {
   return header.kid;
 }
 
+const entityRef = stringifyEntityRef({
+  kind: 'User',
+  namespace: 'default',
+  name: 'JackFrost',
+});
+
 describe('TokenFactory', () => {
   it('should issue valid tokens signed by a listed key', async () => {
     const keyDurationSeconds = 5;
@@ -39,7 +46,7 @@ describe('TokenFactory', () => {
     });
 
     await expect(factory.listPublicKeys()).resolves.toEqual({ keys: [] });
-    const token = await factory.issueToken({ claims: { sub: 'foo' } });
+    const token = await factory.issueToken({ claims: { sub: entityRef } });
 
     const { keys } = await factory.listPublicKeys();
     const keyStore = JWKS.asKeyStore({
@@ -53,7 +60,7 @@ describe('TokenFactory', () => {
     expect(payload).toEqual({
       iss: 'my-issuer',
       aud: 'backstage',
-      sub: 'foo',
+      sub: entityRef,
       iat: expect.any(Number),
       exp: expect.any(Number),
     });
@@ -71,8 +78,12 @@ describe('TokenFactory', () => {
       logger,
     });
 
-    const token1 = await factory.issueToken({ claims: { sub: 'foo' } });
-    const token2 = await factory.issueToken({ claims: { sub: 'foo' } });
+    const token1 = await factory.issueToken({
+      claims: { sub: entityRef },
+    });
+    const token2 = await factory.issueToken({
+      claims: { sub: entityRef },
+    });
     expect(jwtKid(token1)).toBe(jwtKid(token2));
 
     await expect(factory.listPublicKeys()).resolves.toEqual({
@@ -89,7 +100,9 @@ describe('TokenFactory', () => {
       keys: [],
     });
 
-    const token3 = await factory.issueToken({ claims: { sub: 'foo' } });
+    const token3 = await factory.issueToken({
+      claims: { sub: entityRef },
+    });
     expect(jwtKid(token3)).not.toBe(jwtKid(token2));
 
     await expect(factory.listPublicKeys()).resolves.toEqual({
@@ -100,4 +113,20 @@ describe('TokenFactory', () => {
       ],
     });
   });
+
+  it('should throw an error with a non entityRef sub claim', async () => {
+    const keyDurationSeconds = 5;
+    const factory = new TokenFactory({
+      issuer: 'my-issuer',
+      keyStore: new MemoryKeyStore(),
+      keyDurationSeconds,
+      logger,
+    });
+
+    await expect(() => {
+      return factory.issueToken({
+        claims: { sub: 'UserId' },
+      });
+    }).rejects.toThrowError();
+  });
 });
diff --git a/plugins/auth-backend/src/identity/TokenFactory.ts b/plugins/auth-backend/src/identity/TokenFactory.ts
index 041a292d44..cb2a8b453d 100644
--- a/plugins/auth-backend/src/identity/TokenFactory.ts
+++ b/plugins/auth-backend/src/identity/TokenFactory.ts
@@ -19,6 +19,7 @@ import { JSONWebKey, JWK, JWS } from 'jose';
 import { Logger } from 'winston';
 import { v4 as uuid } from 'uuid';
 import { DateTime } from 'luxon';
+import { parseEntityRef } from '@backstage/catalog-model';
 
 const MS_IN_S = 1000;
 
@@ -72,6 +73,15 @@ export class TokenFactory implements TokenIssuer {
     const iat = Math.floor(Date.now() / MS_IN_S);
     const exp = iat + this.keyDurationSeconds;
 
+    // Validate that the subject claim is a valid EntityRef
+    try {
+      parseEntityRef(sub);
+    } catch (error) {
+      throw new Error(
+        '"sub" claim provided by the auth resolver is not a valid EntityRef.',
+      );
+    }
+
     this.logger.info(`Issuing token for ${sub}, with entities ${ent ?? []}`);
 
     return JWS.sign({ iss, sub, aud, iat, exp, ent }, key, {