fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers (CVE-2026-76169) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-76169

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

Answer in brief

CVE-2026-76169 records a Unknown severity vulnerability in fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers. The current sources do not mark it as known exploited. The current feed maps fastify/fastify (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 4, 2026Updated Sep 4, 2026Source checked Sep 4, 2026First seen by HOL Sep 4, 2026Material review Sep 4, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps fastify/fastify (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
fastify/fastifygeneric	>=4.0.0 <5.12.2	5.12.2

Recommended response

Check inventory. Check lockfiles and deployed manifests for fastify/fastify.
Review the reported fix. Update fastify/fastify to 5.12.2 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 4, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 4, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 4, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.