CVE-2026-3419

Summary

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.

Affected Software

Weaknesses

• CWE-185: CWE-185: Incorrect Regular Expression

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: none
  • Automatable: yes
  • Technical Impact: partial

References

• https://github.com/fastify/fastify/security/advisories/GHSA-573f-x89g-hqp9
• https://github.com/fastify/fastify/commit/67f6c9b32cb3623d3c9470cc17ed830dd2f083d7
• https://httpwg.org/specs/rfc9110.html#field.content-type
• https://github.com/advisories/GHSA-573f-x89g-hqp9
• https://cna.openjsf.org/security-advisories.html
• https://www.cve.org/CVERecord?id=CVE-2026-3419

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