CVE-2021-21603 in Jenkins
Summary
by MITRE • 01/14/2021
Jenkins 2.274 and earlier, LTS 2.263.1 and earlier does not escape notification bar response contents, resulting in a cross-site scripting (XSS) vulnerability.
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Analysis
by VulDB Data Team • 02/14/2021
The vulnerability identified as CVE-2021-21603 represents a critical cross-site scripting flaw within Jenkins continuous integration platform versions 2.274 and earlier, including the LTS version 2.263.1 and earlier. This vulnerability resides in the notification bar functionality where response contents are not properly escaped before being rendered in web interfaces, creating a pathway for malicious actors to inject arbitrary JavaScript code into affected systems. The flaw specifically impacts the user interface components responsible for displaying notifications and alerts, making it particularly dangerous in environments where Jenkins serves as a central automation hub for development workflows.
The technical implementation of this vulnerability stems from inadequate input sanitization within Jenkins' notification handling mechanisms. When the system processes and displays response data from various plugins or automated processes, it fails to properly escape special characters that could be interpreted as HTML or JavaScript markup. This omission allows attackers to craft malicious payloads that, when processed by the notification bar, execute in the context of authenticated users' browsers. The vulnerability falls under CWE-79 which specifically addresses cross-site scripting flaws due to improper output encoding or escaping, and aligns with ATT&CK technique T1203 which involves obfuscation through encoding to avoid detection.
The operational impact of this vulnerability extends beyond simple script execution, as it can enable attackers to perform a range of malicious activities including credential theft, session hijacking, and privilege escalation within the Jenkins environment. Since Jenkins typically operates with elevated privileges and often contains sensitive build artifacts, credentials, and deployment configurations, successful exploitation could lead to complete compromise of the continuous integration pipeline. Attackers could potentially steal build credentials, access source code repositories, modify build processes, or even deploy malicious code to production environments through compromised Jenkins instances. The vulnerability is particularly concerning in enterprise environments where Jenkins serves as a critical component in DevOps workflows and where multiple users interact with the system.
Organizations should immediately implement mitigations including updating to Jenkins versions 2.275 or later for the main release and 2.263.2 or later for the LTS release, which contain the necessary patches to properly escape notification bar response contents. Additionally, administrators should review and restrict access permissions to Jenkins instances, implement proper network segmentation, and consider deploying web application firewalls to detect and block malicious payloads. Security monitoring should be enhanced to detect unusual notification patterns or JavaScript injection attempts, and regular security assessments should be conducted to identify similar vulnerabilities in plugins and custom integrations that may not have been updated. The patching process should be prioritized as a critical security measure, given the high potential for exploitation in environments where Jenkins is widely used for automated deployment processes.