Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M
CVE-2026-107722fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion
Critical 9.8
In plain English
A critical-severity security flaw in fast-jwt from nearform. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 29% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
fast-jwt provides fast JSON Web Token (JWT) implementation. From 6.2.0 until 6.3.0, fast-jwt can misclassify RSA public-key text as an HMAC secret when the key has non-whitespace content before its PEM header. In src/crypto.js, performDetectPublicKeyAlgorithms trims whitespace but publicKeyPemMatcher remains start-anchored, so comments, control characters, zero-width characters, or wrapper text can prevent PEM detection and reach the HMAC fallback. An attacker who knows the public key bytes can sign arbitrary HS256 claims with that public material when HS256 is inferred or allowed, resulting in authentication or authorization bypass. An asymmetric-only algorithm allowlist prevents the attack. This issue is fixed in version 6.3.0.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | fast-jwt | 6.3.0 | GHSA-ww5h-9m49-7xx4 |
Affected products (as reported)
Weakness: CWE-347 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://github.com/nearform/fast-jwt/security/advisories/GHSA-ww5h-9m49-7xx4 x_refsource_CONFIRM
- https://github.com/nearform/fast-jwt/pull/632 x_refsource_MISC
- https://github.com/nearform/fast-jwt/commit/d96bbc6c5336055a6dbfa318fdbab01197264cfa x_refsource_MISC
- https://github.com/nearform/fast-jwt/releases/tag/v6.3.0 x_refsource_MISC