CVE-2021-21699: 
Java vulnerability analysis and mitigation

CVE-2021-21699 (SECURITY-2219) is a stored cross-site scripting (XSS) vulnerability discovered in Jenkins Active Choices Plugin versions 2.5.6 and earlier. The vulnerability was disclosed on November 12, 2021, affecting the uno-choice plugin component. The issue stems from the plugin's failure to properly escape parameter names of reactive parameters and dynamic reference parameters (Jenkins Advisory, OSS Security).

The vulnerability is classified as High severity and affects the parameter name handling functionality in the Active Choices Plugin. The core issue lies in insufficient validation and escaping of parameter names in reactive parameters and dynamic reference parameters, which can lead to stored XSS attacks. The vulnerability requires an attacker to have Job/Configure permission to be exploitable (Jenkins Advisory, FortiGuard).

Successful exploitation of this vulnerability could allow attackers to execute arbitrary script code in the security context of the target user's browser. This could potentially lead to system compromise through the execution of malicious scripts in the context of the affected application (FortiGuard).

The vulnerability has been fixed in Active Choices Plugin version 2.5.7, which properly escapes references to parameter names. Organizations are strongly recommended to upgrade to this version or later to address the security issue. The fix was released as part of the Jenkins security advisory on November 12, 2021 (Jenkins Advisory, OSS Security).