CVE-2026-76169 is a high-severity (CVSS 7.5) CWE-288 vulnerability in fastify. A fix is available for fastify — see the affected versions and patch details below.
fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- 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-2026-76169.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-76169 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,415 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
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.
fastifynpmDescription
Impact
Fastify routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the preHandler declared in its setNotFoundHandler(). When the request method has no route in the main router, a malformed request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected.
Patches
Patched in fastify 5.12.2. Malformed URLs are now routed through the configured onBadUrl and onMaxParamLength handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed.
Workarounds
Reject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global onRequest authentication hook does not mitigate this, because the malformed-URL path skips it.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify | ≥ 4.0.0&&< 5.12.2 | 5.12.2npm install fastify@5.12.2 |
Affected Products
fastifyfastifyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify to 5.12.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-76169 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.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This is an Important flaw in fastify that allows an unauthenticated attacker to bypass authentication hooks. By sending a specially crafted malformed URL, an attacker can access private fallback handlers and read their responses, thereby circumventing intended access controls in Red Hat OpenShift AI and Red Hat…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-76169 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-76169 in your dependencies?
Find it across npm, including transitive dependencies.