Authentication bypass via inconsistent URL normalization
Published Apr 15, 2026 · Updated Apr 15, 2026
Authentication bypass in Fastify @fastify/express 4.0.4 and earlier allows remote attackers to access protected routes via crafted URLs. The enhanceRequest function decodes percent escapes but leaves duplicate slashes and semicolon delimiters intact, so Express skips path-scoped middleware after Fastify matches the normalized route. Exploitation requires ignoreDuplicateSlashes or useSemicolonDelimiter and exposes only routes protected by affected Express path middleware.
Summary
What happened
Authentication bypass in Fastify @fastify/express 4.0.4 and earlier allows remote attackers to access protected routes via crafted URLs. The enhanceRequest function decodes percent escapes but leaves duplicate slashes and semicolon delimiters intact, so Express skips path-scoped middleware after Fastify matches the normalized route. Exploitation requires ignoreDuplicateSlashes or useSemicolonDelimiter and exposes only routes protected by affected Express path middleware.
The record
- CVE
- CVE-2026-33808
- Published
- Apr 15, 2026
- Updated
- Apr 15, 2026
- Vendor
- Fastify
- Product
- @fastify/express
- Classifications
- CWE-436, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 15, 2026CVE publishedPublication date reported by the CVE source.
- Apr 15, 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=0 <4.0.5
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.
Public exploit references
- Fastify advisory crafted-path reproductionproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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