CVE-2026-107724: CWE-347: Improper Verification of Cryptographic Signature in nearform fast-jwt

The fast-jwt library version 6.2.4 improperly verifies cryptographic signatures due to incorrect classification of raw serialized public JWK or JWKS JSON as HMAC secret keys. Specifically, the function src/crypto.js performDetectPublicKeyAlgorithms treats non-PEM strings as symmetric key material. If the HS256 algorithm is allowed or inferred, an attacker who knows the exact serialized public key bytes can use those bytes as an HMAC key to forge tokens with arbitrary claims that pass verification. Variations in serialization ordering or whitespace can prevent exploitation. Applications using PEM keys with an asymmetric-only algorithm allowlist are not affected. The vulnerability is resolved in version 6.3.0.

An attacker with knowledge of the exact serialized public key bytes can forge JSON Web Tokens containing arbitrary claims that the token verifier accepts, potentially leading to unauthorized access or privilege escalation. The vulnerability affects confidentiality and integrity of JWT validation. There is no known exploitation in the wild as of the published date.

Upgrade fast-jwt to version 6.3.0 or later, where this issue is fixed. Applications using supported PEM keys with an asymmetric-only algorithm allowlist are not affected. No other mitigations are indicated.