Security-control bypass via incomplete header schema normalization
Published Sep 4, 2026 · Updated Sep 4, 2026
Header validation bypass in Fastify before 5.12.2 allows remote attackers to bypass conditional security checks with crafted headers. Header-schema normalization lowercases top-level property and required names but leaves trigger and dependent names inside dependencies unchanged, so they do not match Node.js request headers. Exposure requires a route that uses canonical-case dependency names to guard a privileged branch, allowing that branch to run without its required header.
Summary
What happened
Header validation bypass in Fastify before 5.12.2 allows remote attackers to bypass conditional security checks with crafted headers. Header-schema normalization lowercases top-level property and required names but leaves trigger and dependent names inside dependencies unchanged, so they do not match Node.js request headers. Exposure requires a route that uses canonical-case dependency names to guard a privileged branch, allowing that branch to run without its required header.
The record
- CVE
- CVE-2026-84428
- Published
- Sep 4, 2026
- Updated
- Sep 4, 2026
- Vendor
- Fastify
- Product
- Fastify
- Classifications
- CWE-178, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 4, 2026CVE publishedPublication date reported by the CVE source.
- Sep 4, 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 <5.12.2
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
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Technologies
Your stack
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Your stack
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