EUVD-2026-291012

fkie_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.

GHSA-3j8v-cgw4-2g6q — fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)

Affected: npm: fast-jwt

## Impact

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 - /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, leading to: - Intermittent user authentication failures - Potential retry storms in applications - Operational monitoring alerts

## Affected Configurations

### This vulnerability ONLY affects applications that:

- Use RegExp objects (not strings) in the allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options - Use stateful RegExp modifiers such a /g or /y

Example: allowedAud: /abc/g ← IMPACTED Example: allowedAud: "/abc/" ← SAFE

### Not Affected

- Applications using string patterns for audience validation (most common) - Applications using RegExp patterns without stateful modifiers

### Assessment Guide To determine if you're affected:

Check if allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options use RegExp objects (/pattern/ or new RegExp()) If yes, review the pattern for stateful modifiers like /g, /y If…

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EUVD-2026-291012

fkie_cve-2026-35040

GHSA-3j8v-cgw4-2g6q — fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)