fix(backend): Clock skew of 0 should not fall back

.changeset/wacky-dryers-hammer.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': patch
+---
+
+A clock skew of 0 will not fall back to the default value anymore.

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

@@ -217,4 +217,56 @@ describe('verifyJwt(jwt, options)', () => {
     expect(error?.message).toContain('Invalid JWT type');
     expect(error?.message).toContain('Expected "at+jwt, application/at+jwt"');
   });
+
+  it('rejects an expired JWT when clockSkewInMs is explicitly 0', async () => {
+    vi.setSystemTime(new Date((mockJwtPayload.exp + 1) * 1000));
+    const inputVerifyJwtOptions = {
+      key: mockJwks.keys[0],
+      issuer: mockJwtPayload.iss,
+      authorizedParties: ['https://accounts.inspired.puma-74.lcl.dev'],
+      clockSkewInMs: 0,
+    };
+    const { errors: [error] = [] } = await verifyJwt(mockJwt, inputVerifyJwtOptions);
+    expect(error).toBeDefined();
+    expect(error?.message).toContain('JWT is expired');
+  });
+
+  it('accepts a recently expired JWT within the default clock skew when clockSkewInMs is undefined', async () => {
+    vi.setSystemTime(new Date((mockJwtPayload.exp + 1) * 1000));
+    const inputVerifyJwtOptions = {
+      key: mockJwks.keys[0],
+      issuer: mockJwtPayload.iss,
+      authorizedParties: ['https://accounts.inspired.puma-74.lcl.dev'],
+    };
+    const { data } = await verifyJwt(mockJwt, inputVerifyJwtOptions);
+    expect(data).toEqual(mockJwtPayload);
+  });
+
+  it('falls back to the default clock skew when clockSkewInMs is NaN', async () => {
+    vi.setSystemTime(new Date((mockJwtPayload.exp + 1) * 1000));
+    const inputVerifyJwtOptions = {
+      key: mockJwks.keys[0],
+      issuer: mockJwtPayload.iss,
+      authorizedParties: ['https://accounts.inspired.puma-74.lcl.dev'],
+      clockSkewInMs: Number.NaN,
+    };
+    const { data } = await verifyJwt(mockJwt, inputVerifyJwtOptions);
+    expect(data).toEqual(mockJwtPayload);
+
+    vi.setSystemTime(new Date((mockJwtPayload.exp + 60) * 1000));
+    const { errors: [error] = [] } = await verifyJwt(mockJwt, inputVerifyJwtOptions);
+    expect(error?.message).toContain('JWT is expired');
+  });
+
+  it('falls back to the default clock skew when clockSkewInMs is Infinity', async () => {
+    vi.setSystemTime(new Date((mockJwtPayload.exp + 3600) * 1000));
+    const inputVerifyJwtOptions = {
+      key: mockJwks.keys[0],
+      issuer: mockJwtPayload.iss,
+      authorizedParties: ['https://accounts.inspired.puma-74.lcl.dev'],
+      clockSkewInMs: Number.POSITIVE_INFINITY,
+    };
+    const { errors: [error] = [] } = await verifyJwt(mockJwt, inputVerifyJwtOptions);
+    expect(error?.message).toContain('JWT is expired');
+  });
 });

packages/backend/src/jwt/verifyJwt.ts

@@ -131,7 +131,8 @@ export async function verifyJwt(
   options: VerifyJwtOptions,
 ): Promise<JwtReturnType<JwtPayload, TokenVerificationError>> {
   const { audience, authorizedParties, clockSkewInMs, key, headerType } = options;
-  const clockSkew = clockSkewInMs || DEFAULT_CLOCK_SKEW_IN_MS;
+  const clockSkew =
+    typeof clockSkewInMs === 'number' && Number.isFinite(clockSkewInMs) ? clockSkewInMs : DEFAULT_CLOCK_SKEW_IN_MS;
 
   const { data: decoded, errors } = decodeJwt(token);
   if (errors) {