CVE-2026-18427

@fastify/static before version 10.1.3 contains an incomplete fix for a previous route guard bypass. The static file handler rejected only parent directory segments, but it did not canonicalize dot segments, duplicate slashes, encoded dots, or backslashes before route matching and before delegating to the send layer. As a result, an unauthenticated attacker could request a file protected by a route based guard using a non canonical path form that misses the guarded route yet resolves back onto the protected file, disclosing its contents. Applications that protect a subtree of the static root with a route based guard are affected, while applications relying on the allowedPath option are not. This is fixed in @fastify/static 10.1.3, which canonicalizes the pathname, including rejecting backslashes, on the path used for routing and serving.

• CWE-22
• CWE-436

• https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
• https://github.com/fastify/fastify-static/security/advisories/GHSA-423g-23ch-w7c6Mitigation, Vendor Advisory

Published

Aug 6, 2026

Last Modified

Sep 4, 2026

Status

Analyzed

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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