CVE-2026-3419: Fastify's Missing End Anchor in "subtypeNameReg" Allows Malformed Content-Types to Pass Validation
(updated )
Fastify incorrectly accepts malformed Content-Type headers containing trailing characters after the subtype token, in violation of RFC 9110 §8.3.1. For example, a request sent with Content-Type: application/json garbage passes validation and is processed normally, rather than being rejected with 415 Unsupported Media Type.
When regex-based content-type parsers are in use (a documented Fastify feature), the malformed value is matched against registered parsers using the full string including the trailing garbage. This means a request with an invalid content-type may be routed to and processed by a parser it should never have reached.
References
- cna.openjsf.org/security-advisories.html
- github.com/advisories/GHSA-573f-x89g-hqp9
- github.com/fastify/fastify
- github.com/fastify/fastify/commit/67f6c9b32cb3623d3c9470cc17ed830dd2f083d7
- github.com/fastify/fastify/security/advisories/GHSA-573f-x89g-hqp9
- httpwg.org/specs/rfc9110.html
- nvd.nist.gov/vuln/detail/CVE-2026-3419
- www.cve.org/CVERecord?id=CVE-2026-3419
Code Behaviors & Features
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