CVE-2026-3419: fastify Incorrect Regular Expression

Vulnerability summary

CVE-2026-3419 is a medium (CVSS 5.3) Incorrect Regular Expression vulnerability in fastify. EPSS estimates a 0.50% chance of exploitation in the next 30 days, higher than 41.2% of scored vulnerabilities.

Timeline

1. 06-03-2026
   PublishedVulnerability first disclosed.
2. 09-03-2026
   Last modifiedVulnerability information updated.

Description

Fastify incorrectly accepts malformed `Content-Type` headers containing trailing characters after the subtype token, in violation of RFC 9110 §8.3.1(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.

• CWE-185Incorrect Regular ExpressionThe product specifies a regular expression in a way that causes data to be improperly matched or compared.