Clerk: SSRF in the opt-in clerkFrontendApiProxy feature may leak secret keys to unintended host

Summary

The clerkFrontendApiProxy function in @clerk/backend is vulnerable to Server-Side Request Forgery (SSRF). An unauthenticated attacker can craft a request path that causes the proxy to send the application's Clerk-Secret-Key to an attacker-controlled server.

Affected packages

Only applications that have opted into the frontendApiProxy feature are affected. This feature is not enabled by default. Users of @clerk/nextjs are not affected due to how the framework handles repeated / in request paths.

Package	Affected versions	Fixed version
@clerk/backend	>= 3.0.0, <= 3.2.2	3.2.3
@clerk/express	>= 2.0.0, <= 2.0.6	2.0.7
@clerk/hono	>= 0.1.0, <= 0.1.4	0.1.5
@clerk/fastify	>= 3.1.0, <= 3.1.4	3.1.5

Search your codebase for the frontendApiProxy option. If none of the patterns below appear in your code, you are not affected.

@clerk/express

app.use(clerkMiddleware({ frontendApiProxy: { enabled: true } }));

@clerk/hono

app.use('*', clerkMiddleware({ frontendApiProxy: { enabled: true } }));

@clerk/fastify

fastify.register(clerkPlugin, { frontendApiProxy: { enabled: true } });

@clerk/backend

import { clerkFrontendApiProxy } from '@clerk/backend/proxy';

A quick way to check across your entire project:

grep -r "frontendApiProxy\|clerkFrontendApiProxy" .

If there are no matches, you are not using this feature.

Recommended actions

Clerk's internal logs show no evidence of users utilizing the built-in proxy with the impacted versions. Despite that, if you are on an impacted version and use the built-in proxy we recommend upgrading and rotating your Clerk Secret Key immediately.