CVE-2026-18427: fastify fastify-static Path Traversal

Vulnerability summary

CVE-2026-18427 is a high (CVSS 7.5) Path Traversal vulnerability in fastify fastify-static. EPSS estimates a 0.66% chance of exploitation in the next 30 days, higher than 50.6% of scored vulnerabilities.

Timeline

1. 06-08-2026
   PublishedVulnerability first disclosed.
2. 06-08-2026
   Last modifiedVulnerability information updated.

Description

@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-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.
• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.