Several widely-used JSON Web Token (JWT) libraries, including node-jsonwebtoken, pyjwt, namshi/jose, php-jwt, and jsjwt, are affected by critical vulnerabilities that could allow attackers to bypass the verification step when using asymmetric keys (RS256, RS384, RS512, ES256, ES384, ES512).
The PHP JOSE Library is vulnerable to key confusion/algorithm substitution in the JWS component resulting in bypassing the signature verification via crafted tokens.
Critical vulnerabilities in JSON Web Token libraries.
It is possible for an attacker to bypass verification when "a token digitally signed with an asymetric key (RS/ES family) of algorithms but instead the attacker send a token digitally signed with a symmetric algorithm (HS* family)".