Bug 2445295 (CVE-2026-3419)

Summary: CVE-2026-3419 fastify: Fastify: Bypass of Content-Type validation via malformed Content-Type headers
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: bbrownin, jkoehler, lphiri, sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in fastify. This vulnerability allows a remote attacker to bypass standard validity checks by sending specially crafted HTTP requests with malformed Content-Type headers. As a result, the server may process requests that should have been rejected, potentially leading to unexpected behavior or misinterpretation of data.
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Description OSIDB Bzimport 2026-03-06 19:01:39 UTC 
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.