CVE-2021-21693: 
Java vulnerability analysis and mitigation

CVE-2021-21693 is a security vulnerability discovered in Jenkins 2.318 and earlier, and LTS 2.303.2 and earlier, related to temporary file creation permissions. The vulnerability was disclosed on November 4, 2021, as part of a larger security advisory addressing multiple vulnerabilities in Jenkins core (Jenkins Advisory).

The vulnerability stems from a flaw in the agent-to-controller security subsystem where permission to create temporary files is only checked after they have been created. This implementation weakness is part of the file path filtering system that controls which files on the Jenkins controller can be accessed by agent processes. The issue was tracked as SECURITY-2539 and was present since approximately 2014 when SECURITY-144 was addressed (Jenkins Advisory).

This vulnerability could potentially allow agent processes to bypass intended access controls during temporary file creation operations, potentially leading to unauthorized file operations on the Jenkins controller file system. The vulnerability was part of a larger set of critical file path filtering bypass issues that could allow agent processes to read and write arbitrary files on the Jenkins controller file system (Jenkins Advisory).

The vulnerability was fixed in Jenkins 2.319 and LTS 2.303.3, where permission to create files is now checked before they are created based on an artificial path. For users unable to immediately upgrade, installing the Remoting Security Workaround Plugin was recommended as a temporary solution, though this plugin is more restrictive and may cause incompatibility with some plugins (Jenkins Advisory).