Unauthenticated schema bypass via leading header space
Published Apr 15, 2026 · Updated Apr 15, 2026
Improper input validation in Fastify 5.3.2 through 5.8.4 allows remote attackers to bypass per-content-type body schemas. In lib/validation.js, getEssenceMediaType splits the raw Content-Type value before trimming it, while the parser trims first, so a leading space produces an empty validator lookup but a parsed body. The application must use schema.body.content; unauthenticated requests can then reach handlers with data that violates the configured schema.
Summary
What happened
Improper input validation in Fastify 5.3.2 through 5.8.4 allows remote attackers to bypass per-content-type body schemas. In lib/validation.js, getEssenceMediaType splits the raw Content-Type value before trimming it, while the parser trims first, so a leading space produces an empty validator lookup but a parsed body. The application must use schema.body.content; unauthenticated requests can then reach handlers with data that violates the configured schema.
The record
- CVE
- CVE-2026-33806
- Published
- Apr 15, 2026
- Updated
- Apr 15, 2026
- Vendor
- Fastify
- Product
- Fastify
- Classifications
- CWE-1287, 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=5.3.2 <5.8.5
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.
Weakness, pattern, technique
Public exploit references
- Leading-space Content-Type validation-bypass proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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