CVE-2021-21697 in Jenkins
Summary
by MITRE • 11/04/2021
Jenkins 2.318 and earlier, LTS 2.303.2 and earlier allows any agent to read and write the contents of any build directory stored in Jenkins with very few restrictions.
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Analysis
by VulDB Data Team • 11/09/2021
This vulnerability exists in Jenkins versions up to 2.318 and LTS versions up to 2.303.2 where the agent-to-controller communication mechanism fails to properly validate file access permissions. The flaw allows any connected agent to read and write build directory contents without adequate restrictions, creating a critical privilege escalation vector within the Jenkins environment. The vulnerability stems from insufficient input validation and access control enforcement in the build directory handling logic, which was designed to facilitate agent communication but inadvertently exposed sensitive build artifacts to unauthorized manipulation.
The technical implementation of this vulnerability exploits the trust relationship between Jenkins agents and the controller, where agents are granted elevated privileges to interact with build directories. Attackers can leverage this by connecting a malicious agent to the Jenkins controller and executing arbitrary file operations against any build directory. This includes reading sensitive configuration files, build logs, source code, and other artifacts that may contain credentials, secrets, or proprietary information. The flaw operates at the file system level, bypassing normal Jenkins access controls and permissions mechanisms that should normally restrict such operations.
The operational impact of CVE-2021-21697 is severe and multifaceted, potentially leading to complete system compromise and data exfiltration. An attacker with access to the Jenkins environment or the ability to establish an agent connection can extract sensitive build artifacts, modify build processes, and potentially inject malicious code into the build pipeline. This vulnerability directly violates the principle of least privilege and can result in credential theft, source code disclosure, and disruption of continuous integration processes. The attack surface is particularly concerning because Jenkins agents are often deployed in environments where they may have elevated privileges or be accessible to untrusted parties.
Mitigation strategies should focus on immediate version upgrades to Jenkins 2.319 or later for regular releases and 2.303.3 or later for LTS versions, which contain the necessary patches to address the access control flaw. Organizations should also implement network segmentation to restrict agent controller communication to trusted networks only, enforce strict agent authentication mechanisms, and monitor for unauthorized agent connections. The vulnerability aligns with CWE-284 Access Control Issues and maps to ATT&CK technique T1078 Valid Accounts for privilege escalation, while also representing a critical weakness in Jenkins' agent security model that requires comprehensive security review and remediation across all Jenkins installations.