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Vulnerability record · CVE-2026-35040 · published 9 April 2026

CVE-2026-35040: Nearform fast-jwt vulnerability

Nearform · Fast Jwt

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.

5.3 CVSS 3.1 Medium EPSS 0.63% · top 52.0% CWE-440 · CWE-440CWE-697 · CWE-697
5.3CVSS 3.1 base score
0.63%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References, 1 tagged exploit
17 Jun 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/nearform/fast-jwt/commit/18d25904e4617e8753526d1b3ab5a2cccdea726a Patch
https://github.com/nearform/fast-jwt/pull/593 Issue TrackingPatch
https://github.com/nearform/fast-jwt/releases/tag/v6.2.1 ProductRelease Notes
https://github.com/nearform/fast-jwt/security/advisories/GHSA-3j8v-cgw4-2g6q ExploitMitigationVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.1CVE-2026-35039Nearform fast-jwt insufficient verification of data authenticity vulnerabilityfast-jwt provides fast JSON Web Token (JWT) implementation. From 0.0.1 to before 6.2.0, setting up a custom cacheKeyBuilder method which does not pro…EPSS 0.22%9.1CVE-2026-34950Nearform fast-jwt broken cryptographic algorithm vulnerabilityfast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ an…EPSS 0.27%7.5CVE-2026-35042Nearform fast-jwt insufficient verification of data authenticity vulnerabilityfast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, fast-jwt does not validate the crit (Critical) Header Parameter def…EPSS 0.20%6.5CVE-2026-35041Nearform fast-jwt inefficient regular expression (redos) vulnerabilityfast-jwt provides fast JSON Web Token (JWT) implementation. From 5.0.0 to 6.2.0, a denial-of-service condition exists in fast-jwt when the allowedAud…EPSS 0.43%5.9CVE-2023-48223Nearform fast-jwt improper input validation vulnerabilityfast-jwt provides fast JSON Web Token (JWT) implementation. Prior to version 3.3.2, the fast-jwt library does not properly prevent JWT algorithm conf…EPSS 0.68%

Source: NIST National Vulnerability Database (record CVE-2026-35040), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.