Authentication bypass via unprefixed non-string middleware paths
Published Jun 30, 2026 · Updated Jun 30, 2026
Improper authorization in @fastify/express 4.0.6 and earlier allows remote attackers to bypass path-scoped middleware. Inside prefixed plugin scopes, the use() path-rewrite logic prefixes only string mount paths, leaving arrays and regular expressions unmatched against the actual prefixed request URL. Exploitation requires an application to protect a prefixed route with non-string path-scoped middleware; successful requests skip controls such as authentication, authorization, rate limiting, or auditing.
Summary
What happened
Improper authorization in @fastify/express 4.0.6 and earlier allows remote attackers to bypass path-scoped middleware. Inside prefixed plugin scopes, the use() path-rewrite logic prefixes only string mount paths, leaving arrays and regular expressions unmatched against the actual prefixed request URL. Exploitation requires an application to protect a prefixed route with non-string path-scoped middleware; successful requests skip controls such as authentication, authorization, rate limiting, or auditing.
The record
- CVE
- CVE-2026-6556
- Published
- Jun 30, 2026
- Updated
- Jun 30, 2026
- Vendor
- Fastify
- Product
- @fastify/express
- Classifications
- CWE-285, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 30, 2026CVE publishedPublication date reported by the CVE source.
- Jun 30, 2026Record updatedLatest update available in the CVE record.
Exploitability
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Is a vulnerable build present?
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- Affected versionversion=0 <4.0.7
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Public exploit references
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Technologies
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Your stack
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