Vulnerability · Published May 13, 2026 · Assigned by GitHub_M
CVE-2026-44351fast-jwt: Empty HMAC secret accepted via async key resolver - JWT auth bypass
Critical 9.1
In plain English
A critical-severity improper authentication in fast-jwt from nearform. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 16% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.
Official description
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | fast-jwt | 6.2.4 | GHSA-gmvf-9v4p-v8jc |
Affected products (as reported)
Weakness: CWE-287, CWE-326, CWE-1391 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
References
- https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc x_refsource_CONFIRM