CVE-2026-76169

fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers

Published Sep 4, 2026 CVSS 3.1 · 7.5 High EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-p68q-wchp-6fh7 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
History
Type Values Removed Values Added
CPEs cpe:2.3:a:fastify:fastify:*:*:*:*:*:node.js:*:*
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Fastify
Fastify fastify
Vendors & Products Fastify
Fastify fastify
Type Values Removed Values Added
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.
Title fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
Weaknesses CWE-288
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
openjs
Published
Updated
Sep 4, 2026, 4:20 PM
Reserved
Aug 19, 2026, 9:33 AM
Status
Analyzed
Published
Sep 4, 2026, 10:17 AM
Modified
Sep 15, 2026, 8:07 PM
Severity
Important
Public date
Sep 4, 2026, 9:51 AM