fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery

[ Why it matters ]Priority signals

• Public proof-of-concept exploit exists (CISA SSVC)

[ Description ]Official description

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.2.4, fast-jwt can classify raw serialized public JWK or JWKS JSON as an HMAC secret because src/crypto.js performDetectPublicKeyAlgorithms treats non-PEM strings as symmetric key material. If HS256 is explicitly allowed or inferred, an attacker who knows the exact serialized public-key bytes can use those bytes as an HMAC key and create a token containing arbitrary claims that createVerifier accepts. Serialization ordering or whitespace differences can prevent exploitation, and applications using supported PEM keys with an asymmetric-only algorithm allowlist are not affected. 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 = 6.2.4)

[ Packages ]Affected open-source packages

• fast-jwt npmAffected = 6.2.4 · fixed in 6.3.0 · GHSA-g3jj-5cmm-3hxx

[ References ]References

• github.com/nearform/fast-jwt/commit/10f9591349199ed2ab9fa1748ce92cdca697f6cf
• github.com/nearform/fast-jwt/pull/636
• github.com/nearform/fast-jwt/releases/tag/v6.3.0
• github.com/nearform/fast-jwt/security/advisories/GHSA-g3jj-5cmm-3hxx