High severity · published Apr 6, 2026
CVE-2026-35042
fast-jwt accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)
7.5
CVSS
0.2%
EPSS
No
On KEV
Description
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 5% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FA
fast-jwt · nearform
<= 6.1.0 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-35042
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-35042
Exploit
https://github.com/nearform/fast-jwt/security/advisories/GHSA-hm7r-c7qw-ghp6
Technical Description
https://www.rfc-editor.org/rfc/rfc7515.html#section-4.1.11
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