fix(backend): Require configured JWT header type

packages/backend/src/jwt/__tests__/assertions.test.ts

@@ -107,10 +107,15 @@ describe('assertAudienceClaim(audience?, aud?)', () => {
 });
 
 describe('assertHeaderType(typ?, allowedTypes?)', () => {
-  it('does not throw error if type is missing', () => {
+  it('does not throw error if type is missing and allowed types are not configured', () => {
     expect(() => assertHeaderType(undefined)).not.toThrow();
-    expect(() => assertHeaderType(undefined, 'JWT')).not.toThrow();
-    expect(() => assertHeaderType(undefined, ['JWT', 'at+jwt'])).not.toThrow();
+  });
+
+  it('throws error if type is missing and allowed types are configured', () => {
+    expect(() => assertHeaderType(undefined, 'JWT')).toThrow(`Invalid JWT type undefined. Expected "JWT".`);
+    expect(() => assertHeaderType(undefined, ['JWT', 'at+jwt'])).toThrow(
+      `Invalid JWT type undefined. Expected "JWT, at+jwt".`,
+    );
   });
 
   it('does not throw error if type matches default allowed type (JWT)', () => {

packages/backend/src/jwt/__tests__/verifyJwt.test.ts

@@ -189,6 +189,22 @@ describe('verifyJwt(jwt, options)', () => {
     expect(error?.message).toContain('Expected "at+jwt"');
   });
 
+  it('rejects JWT with missing type when headerType is configured', async () => {
+    const jwtWithoutTyp = createJwt({
+      header: { typ: undefined },
+    });
+    const inputVerifyJwtOptions = {
+      key: mockJwks.keys[0],
+      issuer: mockJwtPayload.iss,
+      authorizedParties: ['https://accounts.inspired.puma-74.lcl.dev'],
+      headerType: 'at+jwt',
+    };
+    const { errors: [error] = [] } = await verifyJwt(jwtWithoutTyp, inputVerifyJwtOptions);
+    expect(error).toBeDefined();
+    expect(error?.message).toContain('Invalid JWT type undefined');
+    expect(error?.message).toContain('Expected "at+jwt"');
+  });
+
   it('rejects OAuth JWT when headerType does not match', async () => {
     const inputVerifyJwtOptions = {
       key: mockJwks.keys[0],

packages/backend/src/jwt/assertions.ts

@@ -47,12 +47,13 @@ export const assertAudienceClaim = (aud?: unknown, audience?: unknown) => {
   }
 };
 
-export const assertHeaderType = (typ?: unknown, allowedTypes: string | string[] = 'JWT') => {
-  if (typeof typ === 'undefined') {
+export const assertHeaderType = (typ?: unknown, allowedTypes?: string | string[]) => {
+  if (typeof typ === 'undefined' && typeof allowedTypes === 'undefined') {
     return;
   }
 
-  const allowed = Array.isArray(allowedTypes) ? allowedTypes : [allowedTypes];
+  const expectedTypes = allowedTypes ?? 'JWT';
+  const allowed = Array.isArray(expectedTypes) ? expectedTypes : [expectedTypes];
   if (!allowed.includes(typ as string)) {
     throw new TokenVerificationError({
       action: TokenVerificationErrorAction.EnsureClerkJWT,