CVE-2026-3419
Fastify 5.7.2 – 5.8.1
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:NSummary
CVE-2026-3419 is a medium-severity Incorrect Regular Expression (CWE-185) vulnerability in Fastify Fastify. Its CVSS base score is 5.3 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 28th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) and AC-4 (Information Flow Enforcement) — see the control section below for these in your framework.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-10056
Vulnerability Data
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…
more
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(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Malformed Content-Type header bypass enables exploitation of public-facing Fastify web app via invalid parser routing.
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
Mitigating Controls (NIST 800-53 r5) AI
Directly enforces validation of Content-Type header syntax per RFC 9110, rejecting malformed values with trailing garbage before parser selection.
Enforces information flow rules that can require strict Content-Type conformance, blocking requests that would otherwise reach an unintended regex-based parser.
Boundary protection mechanisms (e.g., WAF rules) can inspect and drop RFC-noncompliant Content-Type headers before they reach the Fastify instance.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly require correct regex design, testing, and review to avoid improper matching.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development can detect regex-related flaws before release.
Secure development lifecycle includes validation of input-handling constructs such as regex.
Application security requirements can mandate correct regex design and testing.
Secure coding standards directly address proper construction and review of regular expressions.