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** ```ts app.use(clerkMiddleware({ frontendApiProxy: { enabled: true } })); ``` **@clerk/hono** ```ts app.use('*', clerkMiddleware({ frontendApiProxy: { enabled: true } })); ``` **@clerk/fastify** ```ts fastify.register(clerkPlugin, { frontendApiProxy: { enabled: true } }); ``` **@clerk/backend** ```ts import { clerkFrontendApiProxy } from '@clerk/backend/proxy'; ``` A quick way to check across your entire project: ```sh 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.

Source published

27 Mar 2026 19:58:19 UTC

Source modified

6 Apr 2026 17:04:07 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Publisher rating · HIGH