The agent-to-controller security subsystem limits which files on the Jenkins controller can be accessed by agent processes.
Multiple vulnerabilities in the file path filtering implementation of Jenkins 2.318 and earlier, LTS 2.303.2 and earlier allow agent processes to read and write arbitrary files on the Jenkins controller file system, and obtain some information about Jenkins controller file systems.
SECURITY-2538 / CVE-2021-21692: The operations FilePath#renameTo and FilePath#moveAllChildrenTo only check read permission on the source path.
We expect that most of these vulnerabilities have been present since SECURITY-144 was addressed in the 2014-10-30 security advisory.
Jenkins 2.319, LTS 2.303.3 addresses these security vulnerabilities.
SECURITY-2538 / CVE-2021-21692: The operations FilePath#renameTo and FilePath#moveAllChildrenTo check both read and delete permissions on the source path.
As some common operations are now newly subject to access control, it is expected that plugins sending commands from agents to the controller may start failing. Additionally, the newly introduced path canonicalization means that instances using a custom builds directory (Java system property jenkins.model.Jenkins.buildsDir) or partitioning JENKINS_HOME using symbolic links may fail access control checks. See the documentation for how to customize the configuration in case of problems.
If you are unable to immediately upgrade to Jenkins 2.319, LTS 2.303.3, you can install the Remoting Security Workaround Plugin. It will prevent all agent-to-controller file access using FilePath APIs. Because it is more restrictive than Jenkins 2.319, LTS 2.303.3, more plugins are incompatible with it. Make sure to read the plugin documentation before installing it.