CVE-2026-34076 — @clerk/backend
HIGHCVE-2026-34076 is a high-severity (CVSS 7.4) Server-Side Request Forgery (SSRF) vulnerability in @clerk/backend. A fix is available for @clerk/backend — see the affected versions and patch details below.
Clerk JavaScript: SSRF in the opt-in clerkFrontendApiProxy feature may leak secret keys to unintended host
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34076.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34076 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@clerk/backendnpm@clerk/expressnpm@clerk/hononpmDescription
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.
- Upgrade to the patched version of
@clerk/backend(and@clerk/express,@clerk/hono, etc.) - 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 under API Keys. You should deploy your application with the updated key before revoking the existing key.
- 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @clerk/backend | ≥ 3.0.0&&< 3.2.3 | 3.2.3npm install @clerk/backend@3.2.3 |
| 📦npm | @clerk/express | ≥ 2.0.0&&< 2.0.7 | 2.0.7npm install @clerk/express@2.0.7 |
| 📦npm | @clerk/hono | ≥ 0.1.0&&< 0.1.5 | 0.1.5npm install @clerk/hono@0.1.5 |
| 📦npm | @clerk/fastify | ≥ 3.1.0&&< 3.1.5 | 3.1.5npm install @clerk/fastify@3.1.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @clerk/backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @clerk/backend to 3.2.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34076 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34076 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34076. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-34076 in your dependencies?
O3 Security finds CVE-2026-34076 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.