Docker: Race condition in docker cp allows bind mount redirection to host path

Summary

A race condition during docker cp mount setup allows a malicious container to redirect a bind mount target to an arbitrary host path, potentially overwriting host files or causing denial of service.

Details

When copying files into a container, the daemon sets up a temporary filesystem view by bind-mounting volumes into a private mount namespace. During this setup, the mount destination is created inside the container root and then a bind mount is attached using the container-relative path resolved to an absolute host path.

Between mountpoint creation and the mount() syscall, a process running inside the container can replace the destination (or a parent path component) with a symlink pointing to an arbitrary location on the host. The mount() syscall follows the symlink, causing the volume to be bind-mounted onto an arbitrary host path instead of the intended container path.

Impact

A malicious container can redirect a volume bind mount to an arbitrary host path. The impact depends on the volume content and mount options:

• If the volume is writable, arbitrary host files at the redirected path could be overwritten with the volume's contents.
• If the volume is read-only, the host path is masked by the mount for the duration of the operation, causing denial of service.
• In all cases the mount is temporary (torn down after the docker cp completes), but the effects of any writes persist.

Conditions for exploitation

• A container must have at least one volume mount.
• A process inside the container must be able to rapidly create and swap symlinks at the volume mount destination path.
• An operator must initiate a docker cp into that container, or call the PUT /containers/{id}/archive or HEAD /containers/{id}/archive API endpoints.

Not affected

• Containers that do not have volume mounts are not affected, as the race occurs during volume bind-mount setup.

Workarounds

• Only run containers from trusted images.
• Avoid using docker cp with untrusted running containers.
• Use authorization plugins to restrict access to the archive API endpoints (PUT /containers/{id}/archive, HEAD /containers/{id}/archive).