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. 1. Upgrade to the patched version of "@clerk/backend" (and "@clerk/express", "@clerk/hono", etc.) 2. Rotate your Clerk Secret Key after upgrading - if an attacker exploited this vulnerability, they may have captured your key. Rotate it in the "Clerk Dashboard" (https://dashboard.clerk.com) under API Keys. You should deploy your application with the updated key before revoking the existing key. 3. Audit access logs for requests to your proxy endpoint ("/__clerk/" by default) containing double slashes in the path. Credit Discovered during an internal code audit.