What’s Vulnerable › Vulnerabilities

CVE-2026-35040: fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-04-09 · updated 2026-10-26
• Weakness: CWE-440

Description

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.1, using certain modifiers on RegExp objects in the allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options in verify functions can cause certain unintended behaviours. This is because some modifiers are stateful and will cause failures in every second verification attempt regardless of the validity of the token provided. Such modifiers are /g (global matching) and /y (sticky matching). This does NOT allow invalid tokens to be accepted, only for valid tokens to be improperly rejected in some configurations. Instead it causes 50% of valid authentication requests to fail in an alternating pattern. This vulnerability is fixed in 6.2.1.

Affected products

• nearform fast-jwt: before 6.2.1 — fixed in 6.2.1

How to fix

Upgrade to a fixed version: nearform fast-jwt 6.2.1. Check the vendor advisory for mitigations.

References

• https://github.com/nearform/fast-jwt/commit/18d25904e4617e8753526d1b3ab5a2cccdea726a
• https://github.com/nearform/fast-jwt/pull/593
• https://github.com/nearform/fast-jwt/releases/tag/v6.2.1
• https://github.com/nearform/fast-jwt/security/advisories/GHSA-3j8v-cgw4-2g6q

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