CVE-2026-29000 is a critical vulnerability classified under CWE-347 (Improper Verification of Cryptographic Signature) affecting the pac4j-jwt library, a popular Java security framework component used for JWT authentication. The vulnerability arises from improper handling of encrypted JSON Web Tokens (JWTs) within the JwtAuthenticator module. Specifically, when processing encrypted JWTs, the library fails to properly verify cryptographic signatures if the attacker possesses the server's RSA public key. This allows the attacker to create a JWE-wrapped PlainJWT token containing arbitrary claims such as subject and roles. Because the signature verification is bypassed, the attacker can impersonate any user, including privileged administrators, effectively gaining unauthorized access. The flaw affects multiple major versions of pac4j-jwt prior to 4.5.9, 5.7.9, and 6.3.3, indicating a broad impact across different deployments. The vulnerability is remotely exploitable without requiring authentication or user interaction, increasing its risk profile. The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N) highlights network attack vector, low complexity, no privileges or user interaction needed, and high impact on confidentiality and integrity. No public exploit code or active exploitation has been reported yet, but the severity and ease of exploitation make it a critical threat to any organization relying on vulnerable pac4j-jwt versions for JWT authentication.

The impact of CVE-2026-29000 is severe for organizations worldwide using pac4j-jwt for authentication. Successful exploitation enables attackers to bypass authentication controls entirely, impersonating any user, including administrators, which can lead to full system compromise. This undermines the confidentiality and integrity of sensitive data and systems, potentially allowing unauthorized data access, privilege escalation, and lateral movement within networks. The vulnerability can disrupt availability indirectly by enabling attackers to manipulate or disable security controls. Given the widespread use of JWTs for securing web applications and APIs, this flaw threatens a broad range of sectors including finance, healthcare, government, and technology. Organizations relying on pac4j-jwt in their identity and access management infrastructure face significant risk of unauthorized access and data breaches if unpatched. The lack of required authentication or user interaction lowers the barrier for attackers, increasing the likelihood of exploitation once the vulnerability is known.

To mitigate CVE-2026-29000, organizations should immediately upgrade pac4j-jwt to versions 4.5.9, 5.7.9, or 6.3.3 or later where the vulnerability is patched. If upgrading is not immediately feasible, implement strict access controls to limit exposure of the server's RSA public key, as possession of this key is required for exploitation. Review and audit JWT handling and validation logic to ensure cryptographic signatures are properly verified, especially when processing encrypted tokens. Employ defense-in-depth by using additional authentication and authorization checks beyond JWT validation. Monitor authentication logs for anomalous token usage or unexpected privilege escalations. Consider deploying Web Application Firewalls (WAFs) with custom rules to detect and block malformed JWTs or suspicious authentication attempts. Educate development and security teams about the risks of improper JWT signature verification to prevent similar issues in future implementations.