CVE-2026-35042

fast-jwt accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, fast-jwt does not validate the crit (Critical) Header Parameter defined in RFC 7515 §4.1.11. When a JWS token contains a crit array listing extensions that fast-jwt does not understand, the library accepts the token instead of rejecting it. This violates the MUST requirement in the RFC.

Affected

nearform · fast-jwt

CVSS v3.1 GITHUB

7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

0.20% chance of exploitation in the next 30 days, 9th percentile

Published

2026-04-06

Updated

2026-04-07

Proof-of-concept exploits (1)

• advisories/GHSA-hm7r-c7qw-ghp6

Security advisories

• nearform/fast-jwt/security/advisories/GHSA-hm7r-c7qw-ghp6

References

• CVE Record
• NVD entry