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[ CVE-2026-107722 · Published 8 Oct 2026 ]

fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion

CVSS 9.8 EPSS <1%Fix available

[ Why it matters ]Priority signals

  • Public proof-of-concept exploit exists (CISA SSVC)
  • Critical severity: CVSS 9.8

[ Description ]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.

[ Fixes ]Fixes and patches

  • fast-jwt 6.3.0Fixed version · GitHub Advisories · npm package fast-jwt: upgrade to 6.3.0 or later (affected >= 6.2.0, <= 6.2.4)

[ Packages ]Affected open-source packages

[ References ]References