Fastify up to 5.12.1 Not-Found Router information disclosure

Summaryinfo

A vulnerability identified as problematic has been detected in Fastify up to 5.12.1. Affected by this issue is some unknown functionality of the component Not-Found Router. The manipulation leads to information disclosure. This vulnerability is traded as CVE-2026-76169. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Fastify up to 5.12.1. This issue affects some unknown functionality of the component Not-Found Router. The manipulation with an unknown input leads to a information disclosure vulnerability. Using CWE to declare the problem leads to CWE-200. The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. Impacted is confidentiality. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-76169 since 08/19/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1592 for this issue.

Upgrading to version 5.12.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-70988). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• Fastify

Version

• 5.12.0
• 5.12.1

Website

• Product: https://github.com/fastify/fastify/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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