CVE-2026-35040

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.

• CWE-440
• CWE-697

• https://github.com/nearform/fast-jwt/commit/18d25904e4617e8753526d1b3ab5a2cccdea726aPatch
• https://github.com/nearform/fast-jwt/pull/593Issue Tracking, Patch
• https://github.com/nearform/fast-jwt/releases/tag/v6.2.1Product, Release Notes
• https://github.com/nearform/fast-jwt/security/advisories/GHSA-3j8v-cgw4-2g6qExploit, Mitigation, Vendor Advisory

Published

Apr 9, 2026

Last Modified

Jun 17, 2026

Status

Analyzed

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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