feat(backend): Introduce a handshake redirect loop detection mechanism

Author: dimkl

.changeset/sixty-mugs-clap.md

@@ -0,0 +1,5 @@
+---
+"@clerk/backend": patch
+---
+
+Retry handshake in case of handshake cookie collision in order to support multiple apps on same-level subdomains

integration/tests/sessions/root-subdomain-prod-instances.test.ts

@@ -311,7 +311,7 @@ test.describe('root and subdomain production apps @sessions', () => {
    * sub2.test.com <> clerk-instance-2
    *
    */
-  test.describe('multiple apps same domain for different production instances', () => {
+  test.describe('multiple apps different same-level subdomains  for different production instances', () => {
     const hosts = ['sub-1.multiple-apps-e2e.clerk.app', 'sub-2.multiple-apps-e2e.clerk.app'];
     let fakeUsers: FakeUser[];
     let server: Server;

packages/backend/src/constants.ts

@@ -19,6 +19,7 @@ const Cookies = {
   ClientUat: '__client_uat',
   Handshake: '__clerk_handshake',
   DevBrowser: '__clerk_db_jwt',
+  InfiniteRedirectionLoopCookie: '__clerk_redirection_loop',
 } as const;
 
 const QueryParameters = {

packages/backend/src/tokens/authenticateContext.ts

@@ -26,6 +26,7 @@ interface AuthenticateContextInterface extends AuthenticateRequestOptions {
   // handshake-related values
   devBrowserToken: string | undefined;
   handshakeToken: string | undefined;
+  handshakeRedirectLoopCounter: number;
   // url derived from headers
   clerkUrl: URL;
   // cookie or header session token
@@ -106,6 +107,7 @@ class AuthenticateContext {
     // Using getCookie since we don't suffix the handshake token cookie
     this.handshakeToken =
       this.getQueryParam(constants.QueryParameters.Handshake) || this.getCookie(constants.Cookies.Handshake);
+    this.handshakeRedirectLoopCounter = Number(this.getCookie(constants.Cookies.InfiniteRedirectionLoopCookie)) || 0;
   }
 
   private stripAuthorizationHeader(authValue: string | undefined | null): string | undefined {

packages/backend/src/tokens/request.ts

@@ -173,9 +173,17 @@ ${error.getFullMessage()}`,
     if (isRequestEligibleForHandshake(authenticateContext)) {
       // Right now the only usage of passing in different headers is for multi-domain sync, which redirects somewhere else.
       // In the future if we want to decorate the handshake redirect with additional headers per call we need to tweak this logic.
-      return handshake(authenticateContext, reason, message, headers ?? buildRedirectToHandshake());
+      const handshakeHeaders = headers ?? buildRedirectToHandshake();
+      // Introduce the mechanism to protect for infinite handshake redirect loops
+      // using a cookie and returning true if it's infinite redirect loop or false if we can
+      // proceed with triggering handshake.
+      const isRedirectLoop = setHandshakeInfiniteRedirectionLoopHeaders(handshakeHeaders);
+      if (isRedirectLoop) {
+        return signedOut(authenticateContext, reason, message);
+      }
+      return handshake(authenticateContext, reason, message, handshakeHeaders);
     }
-    return signedOut(authenticateContext, reason, message, new Headers());
+    return signedOut(authenticateContext, reason, message);
   }
 
   async function authenticateRequestWithTokenInHeader() {
@@ -193,6 +201,20 @@ ${error.getFullMessage()}`,
     }
   }
 
+  // We want to prevent infinite handshake redirection loops.
+  // We incrementally set a `__clerk_redirection_loop` cookie, and when it loops 3 times, we throw an error.
+  // We also utilize the `referer` header to skip the prefetch requests.
+  function setHandshakeInfiniteRedirectionLoopHeaders(headers: Headers): boolean {
+    if (authenticateContext.handshakeRedirectLoopCounter === 3) {
+      return true;
+    }
+
+    const newCounterValue = authenticateContext.handshakeRedirectLoopCounter + 1;
+    const cookieName = constants.Cookies.InfiniteRedirectionLoopCookie;
+    headers.append('Set-Cookie', `${cookieName}=${newCounterValue}; SameSite=Lax; HttpOnly; Max-Age=3`);
+    return false;
+  }
+
   function handleHandshakeTokenVerificationErrorInDevelopment(error: TokenVerificationError) {
     // In development, the handshake token is being transferred in the URL as a query parameter, so there is no
     // possibility of collision with a handshake token of another app running on the same local domain
@@ -223,12 +245,11 @@ ${error.getFullMessage()}`,
       error.reason === TokenVerificationErrorReason.JWKKidMismatch ||
       error.reason === TokenVerificationErrorReason.JWKFailedToResolve
     ) {
-      // Let the request go through and eventually retry another handshake
-      // TODO: set a cookie so break the infinite loop
+      // Let the request go through and eventually retry another handshake,
+      // only if needed - a handshake will be thrown if another rule matches
       return;
     }
-    // TODO: if N retries reached, return signedOut
-    const msg = `Clerk: Handshake token verification failed with "${error.reason}"`;
+    const msg = `Clerk: Handshake token verification failed with "${error.getFullMessage()}"`;
     return signedOut(authenticateContext, AuthErrorReason.UnexpectedError, msg);
   }
 
@@ -249,10 +270,15 @@ ${error.getFullMessage()}`,
       try {
         return await resolveHandshake();
       } catch (error) {
-        if (error instanceof TokenVerificationError) {
-          authenticateContext.instanceType === 'development'
-            ? handleHandshakeTokenVerificationErrorInDevelopment(error)
-            : handleHandshakeTokenVerificationErrorInProduction(error);
+        if (error instanceof TokenVerificationError && authenticateContext.instanceType === 'development') {
+          handleHandshakeTokenVerificationErrorInDevelopment(error);
+        }
+
+        if (error instanceof TokenVerificationError && authenticateContext.instanceType === 'production') {
+          const terminateEarly = handleHandshakeTokenVerificationErrorInProduction(error);
+          if (terminateEarly) {
+            return terminateEarly;
+          }
         }
       }
     }

packages/fastify/src/__tests__/__snapshots__/constants.test.ts.snap

@@ -8,6 +8,7 @@ exports[`constants from environment variables 1`] = `
     "ClientUat": "__client_uat",
     "DevBrowser": "__clerk_db_jwt",
     "Handshake": "__clerk_handshake",
+    "InfiniteRedirectionLoopCookie": "__clerk_redirection_loop",
     "Session": "__session",
   },
   "Headers": {