Authentication bypass via undecoded middleware paths
Published Jan 19, 2026 · Updated Jan 19, 2026
Authentication bypass in Fastify @fastify/express before 4.0.3 allows remote authenticated users to access middleware-protected routes. The plugin passes the raw percent-encoded request URL to Express path matching, so /%61dmin misses /admin while Fastify decodes and dispatches the route. Exposure requires path-scoped Express middleware to enforce access controls for a Fastify route; bypassed middleware no longer constrains that request.
Summary
What happened
Authentication bypass in Fastify @fastify/express before 4.0.3 allows remote authenticated users to access middleware-protected routes. The plugin passes the raw percent-encoded request URL to Express path matching, so /%61dmin misses /admin while Fastify decodes and dispatches the route. Exposure requires path-scoped Express middleware to enforce access controls for a Fastify route; bypassed middleware no longer constrains that request.
The record
- CVE
- CVE-2026-22037
- Published
- Jan 19, 2026
- Updated
- Jan 19, 2026
- Vendor
- Fastify
- Product
- @fastify/express
- Classifications
- CWE-177, CWE-288, T1190
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Jan 19, 2026CVE publishedPublication date reported by the CVE source.
- Jan 19, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 4.0.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- GHSA-g6q3-96cp-5r5m encoded-path proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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