Fastify incorrectly accepts malformed Content-Type headers containing trailing characters after the subtype token, in violation of RFC 9110 ยง8.3.1(https://httpwg.org/specs/rfc9110.html#field.content-type). 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.
Impact: An attacker can send requests with RFC-invalid Content-Type headers that bypass validity checks, reach content-type parser matching, and be processed by the server. Requests that should be rejected at the validation stage are instead handled as if the content-type were valid.
Workarounds: Deploy a WAF rule to protect against this
Fix:
The fix is available starting with v5.8.1.
Weakness
The product specifies a regular expression in a way that causes data to be improperly matched or compared.
Potential Mitigations
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/15.html
- https://cwe.mitre.org/data/definitions/6.html
- https://cwe.mitre.org/data/definitions/79.html
References
- https://cna.openjsf.org/security-advisories.html
- https://github.com/advisories/GHSA-573f-x89g-hqp9
- https://github.com/fastify/fastify/commit/67f6c9b32cb3623d3c9470cc17ed830dd2f083d7
- https://github.com/fastify/fastify/security/advisories/GHSA-573f-x89g-hqp9
- https://httpwg.org/specs/rfc9110.html#field.content-type
- https://www.cve.org/CVERecord?id=CVE-2026-3419