Is CVE-2026-35040 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-35040 being exploited in the wild.

How severe is CVE-2026-35040?

Medium severity, with a CVSS score of 5.3, published 9 April 2026.

Which products and versions are affected?

fast-jwt (npm) SEMVER:introduced=0,fixed=6.2.1.

Is there a fix for CVE-2026-35040?

Yes. fast-jwt (npm): upgrade to 6.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-35040 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-35040?

Check whether you run Nearform Fast-Jwt or fast-jwt (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-35040: fast-jwt stateful RegExp causes non-deterministic claim validation

CVE-2026-35040 · Severity: medium · CVSS 5.3 · Published 2026-04-09

fast-jwt is a JavaScript library for signing and verifying JSON Web Tokens (JWTs). When applications configure claim validation (audience, issuer, subject, JTI, or nonce) using regular expressions with global (/g) or sticky (/y) flags, the library intermittently rejects valid tokens in an alternating pattern—approximately 50% of verification attempts fail regardless of token validity. This causes authentication flapping, retry storms, and operational disruptions in production systems relying on JWT authentication.

Technologies: Nearform Fast-Jwt, fast-jwt (npm). Vendors: Nearform, npm.

Quick answers

Is CVE-2026-35040 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-35040 being exploited in the wild.
How severe is CVE-2026-35040?: Medium severity, with a CVSS score of 5.3, published 9 April 2026.
Which products and versions are affected?: fast-jwt (npm) SEMVER:introduced=0,fixed=6.2.1.
Is there a fix for CVE-2026-35040?: Yes. fast-jwt (npm): upgrade to 6.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-35040 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-35040?: Check whether you run Nearform Fast-Jwt or fast-jwt (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

fast-jwt (npm): upgrade to 6.2.1 or later
```
npm install fast-jwt@6.2.1
```

Technical details

The vulnerability exists in fast-jwt's validateClaimValues() function, which validates JWT claims using provided matchers. When a RegExp with /g or /y flags is passed as an allowed* option (allowedAud, allowedIss, allowedSub, allowedJti, allowedNonce), the library reuses the same RegExp object across multiple verification calls without resetting its lastIndex property. RegExp.prototype.test() mutates lastIndex when /g or /y flags are set, causing subsequent test() calls on the same object to alternate between matching and failing. An attacker or misconfigured application can exploit this to cause logical denial-of-service: valid, properly signed JWTs are intermittently rejected. The fix, released in version 6.2.1, wraps RegExp matchers to reset lastIndex = 0 before each test() call, restoring deterministic behavior. Applications are only affected if they explicitly use RegExp objects (not strings) with /g or /y flags in allowed* options—most applications using string patterns are unaffected.

Affected products

nearform fast-jwt <= 6.1.0

Identifiers

CVE-2026-35040
GHSA-3j8v-cgw4-2g6q

Timeline

2026-04-09: disclosed
2026-04-09: patched: Patched in version 6.2.1 (released same day as disclosure)

References

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