clerk · thiskevinwang · Sep 11, 2026
diff --git a/.changeset/strict-oauth-audience.md b/.changeset/strict-oauth-audience.md
@@ -0,0 +1,7 @@
+---
+'@clerk/backend': major
+---
+
+OAuth access token verification now requires a matching `aud` when a non-empty `audience` option is configured, for both opaque tokens and JWTs. Tokens with a missing, empty, malformed, or mismatched audience are rejected.
+
+Applications that configure `audience` must issue OAuth tokens with a matching audience before upgrading. Session JWT and M2M token verification behavior is unchanged.
diff --git a/packages/backend/src/jwt/assertions.ts b/packages/backend/src/jwt/assertions.ts
@@ -47,6 +47,23 @@ export const assertAudienceClaim = (aud?: unknown, audience?: unknown) => {
   }
 };
 
+export const assertOAuthAudienceClaim = (aud?: unknown, audience?: string | string[]) => {
+  if (![audience].flat().some(a => !!a)) {
+    return;
+  }
+
+  const hasAudience =
+    (typeof aud === 'string' && aud.length > 0) || (isArrayString(aud) && aud.every(a => a.length > 0));
+  if (!hasAudience) {
+    throw new TokenVerificationError({
+      reason: TokenVerificationErrorReason.TokenVerificationFailed,
+      message: `Invalid OAuth audience claim (aud) ${JSON.stringify(aud)}. Expected a non-empty string or a non-empty array of non-empty strings.`,
+    });
+  }
+
+  assertAudienceClaim(aud, audience);
+};
+
 export const assertHeaderType = (typ?: unknown, allowedTypes?: string | string[]) => {
   if (typeof typ === 'undefined' && typeof allowedTypes === 'undefined') {
     return;

diff --git a/packages/backend/src/jwt/verifyMachineJwt.ts b/packages/backend/src/jwt/verifyMachineJwt.ts
@@ -14,8 +14,10 @@ import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
 import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
 import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
 import { TokenType } from '../tokens/tokenTypes';
+import { assertOAuthAudienceClaim } from './assertions';
 
 export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
+  audience?: string | string[];
   jwtKey?: string;
   clockSkewInMs?: number;
 };
@@ -131,6 +133,21 @@ export async function verifyOAuthJwt(
     return { data: undefined, tokenType: TokenType.OAuthToken, errors: [result.error] };
   }
 
+  try {
+    assertOAuthAudienceClaim(result.payload.aud, options.audience);
+  } catch (error) {
+    return {
+      data: undefined,
+      tokenType: TokenType.OAuthToken,
+      errors: [
+        new MachineTokenVerificationError({
+          code: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+          message: (error as Error).message,
+        }),
+      ],
+    };
+  }
+
   return {
     data: IdPOAuthAccessToken.fromJwtPayload(result.payload, options.clockSkewInMs),
     tokenType: TokenType.OAuthToken,

diff --git a/packages/backend/src/tokens/__tests__/request.test.ts b/packages/backend/src/tokens/__tests__/request.test.ts
@@ -1590,6 +1590,68 @@ describe('tokens.authenticateRequest(options)', () => {
       });
     });
 
+    test.each(['oauth_token', 'any'] as const)(
+      'rejects an opaque OAuth audience mismatch when acceptsToken is %s',
+      async acceptsToken => {
+        server.use(
+          http.post(mockMachineAuthResponses.oauth_token.endpoint, () => {
+            return HttpResponse.json({
+              ...mockVerificationResults.oauth_token,
+              aud: 'https://other.example.com',
+            });
+          }),
+        );
+
+        const request = mockRequest({ authorization: `Bearer ${mockTokens.oauth_token}` });
+        const requestState = await authenticateRequest(
+          request,
+          mockOptions({ acceptsToken, audience: 'https://resource.example.com' }),
+        );
+
+        expect(requestState).toBeMachineUnauthenticated({
+          tokenType: 'oauth_token',
+          reason: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+          message:
+            'Invalid JWT audience claim (aud) "https://other.example.com". Is not included in "["https://resource.example.com"]". (code=token-verification-failed, status=n/a)',
+        });
+        expect(requestState.toAuth()).toBeMachineUnauthenticatedToAuth({
+          tokenType: 'oauth_token',
+          isAuthenticated: false,
+        });
+      },
+    );
+
+    describe.each(['opaque', 'JWT'] as const)('%s OAuth token without aud', format => {
+      test.each(['oauth_token', 'any'] as const)(
+        'rejects a configured audience when acceptsToken is %s',
+        async acceptsToken => {
+          server.use(
+            http.post(mockMachineAuthResponses.oauth_token.endpoint, () => {
+              return HttpResponse.json(mockVerificationResults.oauth_token);
+            }),
+            http.get('https://api.clerk.test/v1/jwks', () => HttpResponse.json(mockJwks)),
+          );
+          const token = format === 'opaque' ? mockTokens.oauth_token : mockSignedOAuthAccessTokenJwt;
+          const request = mockRequest({ authorization: `Bearer ${token}` });
+          const requestState = await authenticateRequest(
+            request,
+            mockOptions({ acceptsToken, audience: 'https://resource.example.com' }),
+          );
+
+          expect(requestState).toBeMachineUnauthenticated({
+            tokenType: 'oauth_token',
+            reason: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+            message:
+              'Invalid OAuth audience claim (aud) undefined. Expected a non-empty string or a non-empty array of non-empty strings. (code=token-verification-failed, status=n/a)',
+          });
+          expect(requestState.toAuth()).toBeMachineUnauthenticatedToAuth({
+            tokenType: 'oauth_token',
+            isAuthenticated: false,
+          });
+        },
+      );
+    });
+
     test('accepts machine secret when verifying machine-to-machine token', async () => {
       server.use(
         http.post(mockMachineAuthResponses.m2m_token.endpoint, ({ request }) => {

diff --git a/packages/backend/src/tokens/__tests__/verify.test.ts b/packages/backend/src/tokens/__tests__/verify.test.ts
@@ -2,6 +2,7 @@ import { http, HttpResponse } from 'msw';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
 import type { APIKey, IdPOAuthAccessToken, M2MToken } from '../../api';
+import { MachineTokenVerificationError, MachineTokenVerificationErrorCode } from '../../errors';
 import {
   createJwt,
   mockJwks,
@@ -32,7 +33,10 @@ async function createSignedOAuthJwt(
   return data!;
 }
 
-async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
+async function createSignedM2MJwt(
+  payload: Record<string, unknown> = mockM2MJwtPayload,
+  cat: string | undefined = JWT_CATEGORY_M2M_TOKEN,
+) {
   const { data } = await signJwt(payload, signingJwks, {
     algorithm: 'RS256',
     header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
@@ -69,6 +73,19 @@ describe('tokens.verify(token, options)', () => {
     expect(data).toEqual(mockJwtPayload);
   });
 
+  it('continues accepting session JWTs without aud when audience is configured', async () => {
+    server.use(http.get('https://api.clerk.test/v1/jwks', () => HttpResponse.json(mockJwks)));
+
+    const result = await verifyToken(mockJwt, {
+      apiUrl: 'https://api.clerk.test',
+      secretKey: 'a-valid-key',
+      audience: 'https://resource.example.com',
+    });
+
+    expect(result.data).toEqual(mockJwtPayload);
+    expect(result.errors).toBeUndefined();
+  });
+
   it('verifies the token by fetching the JWKs from Backend API when secretKey is provided', async () => {
     server.use(
       http.get(
@@ -257,6 +274,101 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
     expect(data.scopes).toEqual(['read:foo', 'write:bar']);
   });
 
+  describe.each(['opaque', 'at+jwt', 'application/at+jwt'] as const)('%s OAuth token audience verification', format => {
+    const audience = 'https://resource.example.com';
+    const otherAudience = 'https://other.example.com';
+
+    beforeEach(() => {
+      vi.setSystemTime(new Date(mockOAuthAccessTokenJwtPayload.iat * 1000));
+    });
+
+    async function verifyWithAudience(aud: unknown, audience?: string | string[]) {
+      let token: string;
+      if (format === 'opaque') {
+        token = 'oat_8XOIucKvqHVr5tYP123456789abcdefghij';
+        server.use(
+          http.post('https://api.clerk.test/oauth_applications/access_tokens/verify', () => {
+            return HttpResponse.json({ ...mockVerificationResults.oauth_token, aud });
+          }),
+        );
+      } else {
+        server.use(http.get('https://api.clerk.test/v1/jwks', () => HttpResponse.json(mockJwks)));
+        token = await createSignedOAuthJwt({ ...mockOAuthAccessTokenJwtPayload, aud }, format);
+      }
+
+      return verifyMachineAuthToken(token, {
+        apiUrl: 'https://api.clerk.test',
+        secretKey: 'a-valid-key',
+        audience,
+      });
+    }
+
+    it.each([
+      { aud: audience, audience },
+      { aud: audience, audience: [otherAudience, audience] },
+      { aud: [otherAudience, audience], audience },
+      { aud: [otherAudience, audience], audience: [audience] },
+      { aud: undefined, audience: undefined },
+      { aud: undefined, audience: '' },
+      { aud: undefined, audience: [] },
+      { aud: '', audience: undefined },
+      { aud: [], audience: undefined },
+      { aud: otherAudience, audience: undefined },
+      { aud: otherAudience, audience: '' },
+      { aud: otherAudience, audience: [] },
+    ])('accepts aud=$aud with audience=$audience', async ({ aud, audience }) => {
+      const result = await verifyWithAudience(aud, audience);
+
+      expect(result.tokenType).toBe('oauth_token');
+      expect(result.data).toBeDefined();
+      expect((result.data as IdPOAuthAccessToken).aud).toEqual(aud);
+      expect(result.errors).toBeUndefined();
+    });
+
+    it.each([
+      { aud: otherAudience, audience },
+      { aud: otherAudience, audience: [audience] },
+      { aud: [otherAudience], audience },
+      { aud: [otherAudience], audience: [audience] },
+      { aud: `${audience}/other`, audience },
+      { aud: audience.toUpperCase(), audience },
+    ])('rejects aud=$aud with audience=$audience', async ({ aud, audience }) => {
+      const result = await verifyWithAudience(aud, audience);
+
+      expect(result.tokenType).toBe('oauth_token');
+      expect(result.data).toBeUndefined();
+      expect(result.errors).toHaveLength(1);
+      expect(result.errors![0]).toBeInstanceOf(MachineTokenVerificationError);
+      expect(result.errors![0]).toMatchObject({
+        code: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+        message: expect.stringContaining('Invalid JWT audience claim'),
+      });
+    });
+
+    it.each([undefined, null, '', [], [''], [audience, ''], 42, true, {}, [audience, 42]].map(aud => ({ aud })))(
+      'rejects missing, empty, or malformed aud=$aud when audience is configured',
+      async ({ aud }) => {
+        const result = await verifyWithAudience(aud, audience);
+
+        expect(result.tokenType).toBe('oauth_token');
+        expect(result.data).toBeUndefined();
+        expect(result.errors).toHaveLength(1);
+        expect(result.errors![0]).toBeInstanceOf(MachineTokenVerificationError);
+        expect(result.errors![0]).toMatchObject({
+          code: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+          message: expect.stringContaining('Invalid OAuth audience claim'),
+        });
+      },
+    );
+
+    it('rejects missing aud when audience is an array', async () => {
+      const result = await verifyWithAudience(undefined, [audience]);
+
+      expect(result.data).toBeUndefined();
+      expect(result.errors?.[0].code).toBe(MachineTokenVerificationErrorCode.TokenVerificationFailed);
+    });
+  });
+
   describe('handles API errors for API keys', () => {
     it('handles invalid token', async () => {
       const token = 'ak_invalid_token';
@@ -683,6 +795,21 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
       expect(data.scopes).toEqual(['mch_1xxxxx', 'mch_2xxxxx']);
     });
 
+    it('continues accepting M2M JWTs without aud when audience is configured', async () => {
+      server.use(http.get('https://api.clerk.test/v1/jwks', () => HttpResponse.json(mockJwks)));
+      const token = await createSignedM2MJwt({ ...mockM2MJwtPayload, aud: undefined });
+
+      const result = await verifyMachineAuthToken(token, {
+        apiUrl: 'https://api.clerk.test',
+        secretKey: 'a-valid-key',
+        audience: 'https://resource.example.com',
+      });
+
+      expect(result.tokenType).toBe('m2m_token');
+      expect(result.data).toBeDefined();
+      expect(result.errors).toBeUndefined();
+    });
+
     it('rejects M2M JWT with alg: none', async () => {
       server.use(
         http.get(

diff --git a/packages/backend/src/tokens/verify.ts b/packages/backend/src/tokens/verify.ts
@@ -11,6 +11,7 @@ import {
   TokenVerificationErrorReason,
 } from '../errors';
 import type { VerifyJwtOptions } from '../jwt';
+import { assertOAuthAudienceClaim } from '../jwt/assertions';
 import type { JwtReturnType, MachineTokenReturnType } from '../jwt/types';
 import { decodeJwt, verifyJwt } from '../jwt/verifyJwt';
 import { verifyM2MJwt, verifyOAuthJwt } from '../jwt/verifyMachineJwt';
@@ -228,8 +229,21 @@ async function verifyOAuthToken(
   try {
     const client = createBackendApiClient(options);
     const verifiedToken = await client.idPOAuthAccessToken.verify(accessToken);
+    assertOAuthAudienceClaim(verifiedToken.aud, options.audience);
     return { data: verifiedToken, tokenType: TokenType.OAuthToken, errors: undefined };
   } catch (err: any) {
+    if (err instanceof TokenVerificationError) {
+      return {
+        data: undefined,
+        tokenType: TokenType.OAuthToken,
+        errors: [
+          new MachineTokenVerificationError({
+            code: MachineTokenVerificationErrorCode.TokenVerificationFailed,
+            message: err.message,
+          }),
+        ],
+      };
+    }
     return handleClerkAPIError(TokenType.OAuthToken, err, 'OAuth token not found');
   }
 }