GHSA-mxr3-8whj-j74r

Summary

Harden-Runner includes a policy option disable-sudo to prevent the GitHub Actions runner user from using sudo. This is implemented by removing the runner user from the sudoers file. However, this control can be bypassed as the runner user, being part of the docker group, can interact with the Docker daemon to launch privileged containers or access the host filesystem. This allows the attacker to regain root access or restore the sudoers file, effectively bypassing the restriction.

For an attacker to bypass this control, they would first need the ability to run their malicious code (e.g., by a supply chain attack similar to tj-actions or exploiting a Pwn Request vulnerability)) on the runner. This vulnerability has been fixed in Harden-Runner version v2.12.0.

Impact

An attacker with the ability to run their malicious code on a runner configured with disable-sudo: true can escalate privileges to root using Docker, defeating the intended security control.

Affected Configuration

• Harden-Runner configurations that use disable-sudo: true on GitHub-hosted runners or on ephemeral self-hosted VM-based runners. • This issue does not apply to Kubernetes-based Actions Runner Controller (ARC) Harden-Runner.

Mitigation / Fix

This vulnerability has been fixed in Harden-Runner version v2.12.0. Users should migrate to the stronger disable-sudo-and-containers policy. This setting: • Disables sudo access, • Removes access to dockerd and containerd sockets, • Uninstalls Docker from the runner entirely, preventing container-based privilege escalation paths.

Additional Improvements

• The disable-sudo option will be deprecated in the future, as it does not sufficiently restrict privilege escalation on its own. • Harden-Runner now includes detections to alert on attempts to evade the disable-sudo policy.

Credits

Reported by @loresuso and @darryk10. We would like to thank them for collaborating with us to mitigate the vulnerability.