Security Flaw in fast-jwt Library by Nearform
CVE-2026-35042

7.5HIGH

Key Information:

Vendor

Nearform

Status
Fast-jwt
Vendor
CVE Published:
6 April 2026

What is CVE-2026-35042?

The fast-jwt library, which provides a fast implementation for JSON Web Tokens (JWT), contains a vulnerability in versions 6.1.0 and earlier. This flaw arises from the library's failure to correctly validate the 'crit' (Critical) header parameter as stipulated in RFC 7515 §4.1.11. When a JSON Web Signatures (JWS) token includes a 'crit' array with extensions that the fast-jwt library does not comprehend, the library mistakenly accepts the token rather than rejecting it. This behavior contravenes the requirements detailed in the RFC, potentially exposing applications utilizing this library to security risks associated with invalid token acceptance.

Affected Version(s)

fast-jwt <= 6.1.0

References

https://github.com/nearform/fast-jwt/security/advisories/...
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https://www.rfc-editor.org/rfc/rfc7515.html#section-4.1.11
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CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
High
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.