The vulnerability is located in pac4j-jwt’s JwtAuthenticator component in versions prior to 4.5.9, 5.7.9, and 6.3.3. An attacker who has the server’s RSA public key can create a JSON Web Encryption (JWE) wrapped PlainJWT containing arbitrary subject and role claims. Because the token bypasses the signature verification step, the attacker can authenticate as any user, including administrators, thereby gaining unauthorized access to protected resources.

The affected product is pac4j pac4j-jwt. Systems running any version of pac4j-jwt earlier than 4.5.9, 5.7.9, or 6.3.3 are impacted. No other products or vendors are listed as affected.

The CVSS base score of 9.3 marks this flaw as critical. The EPSS score of < 1% indicates a very low but non-zero probability that exploit code for this vulnerability is circulating in the wild, and it is not currently listed in the CISA KEV catalog. Exploitation requires remote access to the application and possession of the RSA public key, which an attacker can obtain if it is improperly exposed. Given the high potential impact and the fact that the malicious token can be crafted and sent over any network connection, administrators should treat this as an urgent patching issue.