EUVD-2026-20898
GHSA-3j8v-cgw4-2g6q
CVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Lifecycle Timeline
4Description
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.
Analysis
fast-jwt before 6.2.1 fails to properly validate JWTs when RegExp modifiers with stateful behavior (/g for global matching and /y for sticky matching) are used in the allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options. This causes valid authentication tokens to be rejected in an alternating 50% failure pattern due to RegExp state persistence across verification calls, degrading availability of JWT-protected services without compromising token security itself. …
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