fastify up to 5.8.0 Header Content-Type incorrect regex

Summaryinfo

A vulnerability has been found in fastify up to 5.8.0 and classified as problematic. This affects an unknown part of the component Header Handler. Performing a manipulation of the argument Content-Type results in incorrect regex. This vulnerability is reported as CVE-2026-3419. The attack is possible to be carried out remotely. No exploit exists. The affected component should be upgraded.

Detailsinfo

A vulnerability, which was classified as problematic, was found in fastify up to 5.8.0. This affects an unknown part of the component Header Handler. The manipulation of the argument Content-Type with an unknown input leads to a incorrect regex vulnerability. CWE is classifying the issue as CWE-185. The product specifies a regular expression in a way that causes data to be improperly matched or compared. This is going to have an impact on integrity. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-3419 since 03/01/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 5.8.1 eliminates this vulnerability. Applying the patch 67f6c9b32cb3623d3c9470cc17ed830dd2f083d7 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Name

• fastify

Version

• 5.0
• 5.1
• 5.2
• 5.3
• 5.4
• 5.5
• 5.6
• 5.7
• 5.8.0

License

• open-source

Website

• Product: https://github.com/fastify/fastify/

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