CVE-2021-21699 in Active Choices Plugin
Summary
by MITRE • 11/12/2021
Jenkins Active Choices Plugin 2.5.6 and earlier does not escape the parameter name of reactive parameters and dynamic reference parameters, resulting in a stored cross-site scripting (XSS) vulnerability exploitable by attackers with Job/Configure permission.
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Analysis
by VulDB Data Team • 11/16/2021
The vulnerability identified as CVE-2021-21699 affects the Jenkins Active Choices Plugin version 2.5.6 and earlier, representing a critical stored cross-site scripting flaw that undermines the security posture of Jenkins environments. This vulnerability specifically targets the plugin's handling of parameter names within reactive parameters and dynamic reference parameters, where insufficient output escaping creates opportunities for malicious actors to inject persistent script code into the system. The flaw exists because the plugin fails to properly sanitize user-supplied parameter names before rendering them in web interfaces, allowing attackers to craft malicious inputs that execute in the context of other users who view affected pages.
The technical nature of this vulnerability aligns with CWE-79, which describes improper neutralization of input during web page generation, specifically manifesting as a stored XSS condition. Attackers with minimal privileges including the Job/Configure permission can exploit this weakness by creating or modifying job configurations that contain malicious parameter names. When these parameters are rendered in the Jenkins web interface, the unescaped script code executes in the browser context of authenticated users, potentially enabling session hijacking, credential theft, or further exploitation of the Jenkins environment. The stored nature of this vulnerability means that malicious payloads persist in the system until explicitly removed, making the impact more severe and long-lasting than transient XSS flaws.
The operational impact of CVE-2021-21699 extends beyond simple script execution, as it can serve as a foothold for more sophisticated attacks within Jenkins environments. An attacker could leverage this vulnerability to establish persistent access through session manipulation, potentially escalating privileges or accessing sensitive build artifacts and configuration data. The vulnerability affects Jenkins installations where the Active Choices Plugin is deployed, which is common in continuous integration environments where parameterized builds are utilized. Given that the plugin is widely used for creating dynamic build parameters, the attack surface is significant across organizations that rely on Jenkins for automated build processes and deployment pipelines.
Mitigation strategies for CVE-2021-21699 should prioritize immediate plugin updates to version 2.5.7 or later, which contain the necessary output escaping fixes to prevent parameter name injection. Organizations should also implement comprehensive input validation for all user-supplied parameter names within Jenkins jobs, particularly those involving the Active Choices Plugin. Network segmentation and principle of least privilege should be enforced to limit access to Jenkins configurations, reducing the potential attack surface. Regular security audits of Jenkins plugins and configurations are essential, with particular attention to parameter handling within build systems. Additionally, implementing content security policies and browser-based protections can provide defense-in-depth measures against exploitation attempts, though these should complement rather than replace the core patching and configuration controls. The vulnerability demonstrates the importance of proper output escaping in web applications and aligns with ATT&CK technique T1059.001 for command and scripting interpreter usage, as malicious scripts could potentially be used to execute commands on compromised systems.