CVE-2026-34076: Clerk backend SSRF in clerkFrontendApiProxy feature

CVE-2026-34076 · Severity: high · CVSS 7.4 · Published 2026-04-01

Clerk's authentication libraries include an optional proxy feature that forwards API requests between frontend and backend systems. An unauthenticated attacker can exploit path manipulation to redirect the proxy to leak secret API keys to attacker-controlled servers, compromising all authentication and authorization functions for affected applications.

Technologies: Clerk Backend, @clerk/backend (npm), @clerk/express (npm), @clerk/fastify (npm). Vendors: Clerk, npm.

Quick answers

Is CVE-2026-34076 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-34076 being exploited in the wild.
How severe is CVE-2026-34076?
High severity, with a CVSS score of 7.4, published 1 April 2026.
Which products and versions are affected?
Clerk Backend 3.0.0 through 3.2.2; @clerk/backend (npm) SEMVER:introduced=3.0.0,fixed=3.2.3; @clerk/express (npm) SEMVER:introduced=2.0.0,fixed=2.0.7; @clerk/fastify (npm) SEMVER:introduced=3.1.0,fixed=3.1.5; and 1 more.
Is there a fix for CVE-2026-34076?
Yes. @clerk/backend (npm): upgrade to 3.2.3 or later; @clerk/express (npm): upgrade to 2.0.7 or later; @clerk/fastify (npm): upgrade to 3.1.5 or later; @clerk/hono (npm): upgrade to 0.1.5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-34076 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-34076?
Check whether you run Clerk Backend, @clerk/backend (npm), @clerk/express (npm) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • @clerk/backend (npm): upgrade to 3.2.3 or later
    npm install @clerk/backend@3.2.3
  • @clerk/express (npm): upgrade to 2.0.7 or later
    npm install @clerk/express@2.0.7
  • @clerk/fastify (npm): upgrade to 3.1.5 or later
    npm install @clerk/fastify@3.1.5
  • @clerk/hono (npm): upgrade to 0.1.5 or later
    npm install @clerk/hono@0.1.5

Technical details

The clerkFrontendApiProxy function in Clerk's backend SDKs is vulnerable to Server-Side Request Forgery (CWE-918) due to insufficient validation of request paths. An unauthenticated attacker can craft a malicious request path containing double slashes that causes the proxy to forward requests to an attacker-controlled server, leaking the application's Clerk-Secret-Key header. The vulnerability is triggered when the frontendApiProxy feature is explicitly enabled in middleware configuration—it is disabled by default. Attack vector is network-based with high complexity due to path manipulation requirements. An attacker who captures the secret key can forge authentication tokens and impersonate any user in the application.

Affected products

  • Clerk backend 3.0.0 through 3.2.2
  • Clerk express 2.0.0 through 2.0.6
  • Clerk hono 0.1.0 through 0.1.4
  • Clerk fastify 3.1.0 through 3.1.4

Identifiers

  • CVE-2026-34076
  • GHSA-gjxx-92w9-8v8f

Timeline

  • 2026-03-27: disclosed: Advisory published; discovered during internal code audit
  • 2026-03-27: patched: Patches released: @clerk/backend 3.2.3, @clerk/express 2.0.7, @clerk/hono 0.1.5, @clerk/fastify 3.1.5

References

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