fix(backend): Clear __clerk_synced when page is protected via middleware

.changeset/witty-hotels-add.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': patch
+---
+
+Ensure `__clerk_synced` is removed from cross-origin return-back urls

packages/backend/src/__tests__/createRedirect.test.ts

@@ -99,6 +99,28 @@ describe('redirect(redirectAdapter)', () => {
       );
     });
 
+    it('removes __clerk_synced when cross-origin redirect', () => {
+      const returnBackUrl = 'https://current.url:3000/path?__clerk_synced=true&q=1#hash';
+      const encodedUrl = 'https%3A%2F%2Fcurrent.url%3A3000%2Fpath%3Fq%3D1%23hash';
+      const signUpUrl = 'https://lcl.dev/sign-up';
+
+      const redirectAdapterSpy = vi.fn().mockImplementation(_url => 'redirectAdapterValue');
+      const { redirectToSignUp } = createRedirect({
+        baseUrl: 'http://www.clerk.com',
+        devBrowserToken: 'deadbeef',
+        redirectAdapter: redirectAdapterSpy,
+        publishableKey: 'pk_test_Y2xlcmsubGNsLmRldiQ',
+        sessionStatus: 'active',
+        signUpUrl,
+      });
+
+      const result = redirectToSignUp({ returnBackUrl });
+      expect(result).toBe('redirectAdapterValue');
+      expect(redirectAdapterSpy).toHaveBeenCalledWith(
+        `${signUpUrl}?redirect_url=${encodedUrl}&__clerk_db_jwt=deadbeef`,
+      );
+    });
+
     it('returns path based url with development (kima) publishableKey (with staging Clerk) but without signUpUrl to redirectToSignUp', () => {
       const redirectAdapterSpy = vi.fn().mockImplementation(_url => 'redirectAdapterValue');
       const { redirectToSignUp } = createRedirect({
@@ -320,5 +342,25 @@ describe('redirect(redirectAdapter)', () => {
         `https://included.katydid-92.accounts.dev/sign-up/tasks?redirect_url=${encodedUrl}&__clerk_db_jwt=deadbeef`,
       );
     });
+
+    it('removes __clerk_synced when cross-origin redirect', () => {
+      const returnBackUrl = 'https://current.url:3000/path?__clerk_synced=true&q=1#hash';
+      const encodedUrl = 'https%3A%2F%2Fcurrent.url%3A3000%2Fpath%3Fq%3D1%23hash';
+
+      const redirectAdapterSpy = vi.fn().mockImplementation(_url => 'redirectAdapterValue');
+      const { redirectToSignUp } = createRedirect({
+        baseUrl: 'http://www.clerk.com',
+        devBrowserToken: 'deadbeef',
+        redirectAdapter: redirectAdapterSpy,
+        publishableKey: 'pk_test_Y2xlcmsubGNsLmRldiQ',
+        sessionStatus: 'pending',
+      });
+
+      const result = redirectToSignUp({ returnBackUrl });
+      expect(result).toBe('redirectAdapterValue');
+      expect(redirectAdapterSpy).toHaveBeenCalledWith(
+        `https://accounts.lcl.dev/sign-up/tasks?redirect_url=${encodedUrl}&__clerk_db_jwt=deadbeef`,
+      );
+    });
   });
 });

packages/backend/src/createRedirect.ts

@@ -17,12 +17,17 @@ const buildUrl = (
   const baseUrl = new URL(_baseUrl);
   const returnBackUrl = _returnBackUrl ? new URL(_returnBackUrl, baseUrl) : undefined;
   const res = new URL(_targetUrl, baseUrl);
+  const isCrossOriginRedirect = `${baseUrl.hostname}:${baseUrl.port}` !== `${res.hostname}:${res.port}`;
 
   if (returnBackUrl) {
+    if (isCrossOriginRedirect) {
+      returnBackUrl.searchParams.delete(constants.QueryParameters.ClerkSynced);
+    }
+
     res.searchParams.set('redirect_url', returnBackUrl.toString());
   }
   // For cross-origin redirects, we need to pass the dev browser token for URL session syncing
-  if (_devBrowserToken && baseUrl.hostname !== res.hostname) {
+  if (isCrossOriginRedirect && _devBrowserToken) {
     res.searchParams.set(constants.QueryParameters.DevBrowser, _devBrowserToken);
   }
   return res.toString();