CVE-2021-21689 in Jenkins
Summary
by MITRE • 11/04/2021
FilePath#unzip and FilePath#untar were not subject to any agent-to-controller access control in Jenkins 2.318 and earlier, LTS 2.303.2 and earlier.
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Analysis
by VulDB Data Team • 11/09/2021
The vulnerability identified as CVE-2021-21689 represents a critical access control flaw in Jenkins continuous integration platform affecting versions up to 2.318 and LTS 2.303.2. This issue stems from insufficient authorization checks within the FilePath class methods, specifically the unzip and untar functionalities that handle archive extraction operations. The vulnerability exists because these methods do not properly validate whether the requesting agent has appropriate permissions to perform archive operations, creating a path for unauthorized access to sensitive system resources and potential privilege escalation scenarios.
The technical implementation of this flaw lies in the absence of proper access control mechanisms within Jenkins' core file handling components. When FilePath#unzip and FilePath#untar methods are invoked, they should verify that the calling agent possesses sufficient privileges to execute these operations, particularly when dealing with archive files that may contain malicious content or sensitive system files. The lack of such validation allows any authenticated user to potentially exploit these methods to extract archives to arbitrary locations on the system, bypassing normal access controls that should restrict such operations to authorized administrators or specific service accounts.
This vulnerability directly impacts Jenkins' security model by undermining the principle of least privilege and creating potential attack vectors for privilege escalation. An attacker who gains access to a Jenkins agent or user account with basic permissions could leverage this flaw to extract malicious archives containing shell scripts, binary payloads, or configuration files that could compromise the system. The operational impact extends beyond simple unauthorized file access, as these methods could be used to overwrite critical system files, inject malicious code, or establish persistent access points within the Jenkins infrastructure. The vulnerability particularly affects environments where Jenkins agents run with elevated privileges or where users have access to build jobs that might invoke these archive extraction methods.
From a cybersecurity perspective, this vulnerability aligns with CWE-284 Access Control Issues, specifically targeting improper access control in file system operations. It also maps to ATT&CK technique T1059 Command and Scripting Interpreter, as attackers could use this vulnerability to execute arbitrary code through extracted malicious payloads. The flaw represents a significant risk in continuous integration environments where Jenkins is often used to automate deployment processes and where agents frequently operate with elevated system privileges. Organizations using Jenkins in production environments should immediately apply the security patches released by the Jenkins team, which include proper access control validation for these methods. Additionally, implementing network segmentation, restricting agent access, and monitoring for unusual archive extraction activities can help mitigate the risk while waiting for patches to be deployed across all affected systems. The vulnerability underscores the importance of maintaining proper access controls in distributed systems and highlights the critical need for comprehensive security reviews of core system components that handle file operations and resource access.