Vulnerability in Nearform Fast-Jwt
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
EPSS exploit prediction
EPSS: 0.006 (49.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 49th percentile — 49.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.006 · change: +0.002
Affected products
- Nearform Fast-Jwt — versions < 6.2.1
Weakness classification (CWE)
References
- github.com/nearform/fast-jwt/security/advisories/GHSA-3j8v-cgw4-2g6q (Exploit, Mitigation, Vendor Advisory)
- github.com/nearform/fast-jwt/pull/593 (Patch, Issue Tracking)
- github.com/nearform/fast-jwt/commit/18d25904e4617e8753526d1b3ab5a2cccdea726a (Patch)
- github.com/nearform/fast-jwt/releases/tag/v6.2.1 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-35040?
- CVE-2026-35040 is a medium-severity vulnerability in Nearform Fast-Jwt, classified under Expected Behavior Violation. CVSS score: 5.3/10. Published 2026-04-09.
- How severe is CVE-2026-35040?
- Medium severity. CVSS v3 base score is 5.3 out of 10.