CVE-2026-76169
Received
Received - Intake
Fastify Prefix Bypass via Malformed URL in Not-Found Handler
Vulnerability report for CVE-2026-76169, including description, CVSS score, EPSS score, affected products, exploitability, helpful resources, and attack-flow context.
Publication date: 2026-09-04
Last updated on: 2026-09-04
Assigner: openjs
Description
Description
fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| fastify | fastify | From 4.0.0 (inc) to 5.12.2 (exc) |
| fastify | fastify | 5.12.2 |
Helpful Resources
Exploitability
CWE
KEV
| CWE ID | Description |
|---|---|
| CWE-288 | The product requires authentication, but the product has an alternate path or channel that does not require authentication. |
