CVE-2026-44351
fast-jwt: Empty HMAC secret accepted via async key resolver - JWT auth bypass
- Published
- May 13, 2026
- Updated
- May 14, 2026
- Assigning CNA
- GitHub_M
- Evidence observed
- Sep 15, 2026
Primary CVSS
nvd · CVSS 3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NLow · next 30 days
- Percentile
- 15.0%
- Model date
- Sep 21, 2026
EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.
Summary
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.
Sources
1Proof-of-concept for CVE-2026-44351, an authentication bypass in fast-jwt <6.2.4 where an empty HMAC key lets attackers forge arbitrary JWTs accepted as valid.
Responsible use
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