fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-35040: 0.38% estimated exploitation activity in the next 30 days; percentile 32.38 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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:…

Published by the source

9 Apr 2026, 16:41 UTC

Last modified by the source

9 Apr 2026, 19:17 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3j8v-cgw4-2g6q

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-3j8v-cgw4-2g6q

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# Patch Next advisory brief: CVE-2026-35040

Advisory: fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)
IDs: CVE-2026-35040, GHSA-3j8v-cgw4-2g6q
Record: https://patchnext.com/advisories/cve-2026-35040 (revision 2, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-jwt 6.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected < 6.2.1; fixed in 6.2.1

## Suggested inspection (read-only)
- `npm ls fast-jwt --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fast-jwt.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-3j8v-cgw4-2g6q (OSV record GHSA-3j8v-cgw4-2g6q lists affected ranges: npm fast-jwt >=0 <6.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-3j8v-cgw4-2g6q (GitHub-reviewed advisory GHSA-3j8v-cgw4-2g6q lists affected ranges: npm fast-jwt >=0 <6.2.1)

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