For this week's TTP Tuesday, we're providing two Docker container escapes and a Docker container privilege escalation. These TTPs demonstrate how to escape from a Docker container using a cgroup controller
release_agent, and to elevate privilege within a container by abusing
hostPID permission. If a container is vulnerable, an attacker may be able to spawn a root shell on the host system.
Cgroups are a Linux kernel feature used to create hierarchical process groups. Linux kernel subsystems use these to modify the process behavior of processes groups, such as limiting the resources allocated to those processes.
A cgroup becomes empty when there are no running processes within it and when it has no child cgroups. When this happens, the cgroup's
releaseagent is invoked and performs whatever action is stored in the
By manipulating a cgroup's
release_agent file, it's possible to perform a container escape to the host system.
Note that this is not a vulnerability in Docker code, but rather a common security misconfiguration of Docker containers.
Is my Docker container vulnerable to cgroup controller escapes? chain on each Docker container in your environment to test if you are vulnerable.
The chain tests two Docker cgroup controller escapes that will start a privileged Pneuma agent on the Docker host, and a privilege escalation that requires the
hostPid flag enabled.
The recommended remediation is to not run containers in privileged mode.
Check out the TTP Is my Docker container vulnerable to cgroup controller escape? on the Prelude chains website.
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