CVE-2026-29000 is a critical vulnerability affecting the pac4j-jwt library, specifically in versions prior to 4.5.9, 5.7.9, and 6.3.3. The issue arises from improper verification of cryptographic signatures (CWE-347) in the JwtAuthenticator component when handling encrypted JSON Web Tokens (JWTs). The vulnerability allows remote attackers who possess the server's RSA public key to create a JWE-wrapped PlainJWT token with arbitrary claims, including subject and role, effectively bypassing signature verification. This means attackers can forge authentication tokens to impersonate any user, including those with administrative privileges, without needing any authentication or user interaction. The flaw stems from the JwtAuthenticator's failure to properly validate the cryptographic signature on encrypted JWTs, allowing attackers to exploit the public key to craft tokens that the system accepts as valid. The CVSS 4.0 vector indicates network attack vector, low attack complexity, no privileges or user interaction required, and high impact on confidentiality, integrity, and availability. Although no exploits have been reported in the wild yet, the vulnerability's nature and ease of exploitation make it highly dangerous. The affected versions span multiple major releases, indicating a long-standing issue that requires immediate patching.

The impact of CVE-2026-29000 is severe for organizations using the pac4j-jwt library for authentication in their applications. Successful exploitation allows attackers to bypass authentication controls entirely, granting unauthorized access to any user account, including those with administrative rights. This can lead to full system compromise, data breaches, unauthorized data modification, and disruption of services. Since the vulnerability affects JWT-based authentication, it undermines the trust model of token-based security, potentially exposing sensitive systems and data to attackers. Organizations relying on pac4j-jwt in critical infrastructure, web applications, or microservices environments are at high risk. The ease of exploitation without authentication or user interaction increases the likelihood of automated attacks and widespread abuse. The vulnerability could facilitate lateral movement, privilege escalation, and persistent access within compromised environments, severely impacting confidentiality, integrity, and availability.

Organizations should immediately upgrade pac4j-jwt to versions 4.5.9, 5.7.9, or 6.3.3 or later, where the vulnerability has been fixed. In addition to patching, organizations should audit their JWT authentication implementations to ensure proper cryptographic signature verification is enforced, especially when handling encrypted JWTs. Restrict access to the server's RSA public key to minimize exposure, as possession of this key enables exploitation. Implement additional layers of authentication and authorization checks beyond JWT validation to detect and prevent token forgery. Monitor authentication logs for unusual token usage patterns or unexpected administrative access. Employ runtime application self-protection (RASP) or web application firewalls (WAF) with rules to detect malformed or suspicious JWT tokens. Conduct thorough security testing and code reviews of JWT handling components to identify similar weaknesses. Finally, educate developers on secure JWT usage and cryptographic best practices to prevent recurrence of such vulnerabilities.