UNIX Symbolic Link (Symlink) Following

Summary

runc is a CLI tool for spawning and running containers according to the OCI specification. In versions prior to 1.3.6, 1.4.0-rc.1 prior to 1.4.3, 1.5.0-rc.1, and 1.5.0-rc.2, when setting up the container rootfs, setupPtmx and setupDevSymlinks call os.Remove and os.Symlink with a filepath.Join string which allow an image with /dev as a symlink to trick runc into deleting files called ptmx on the host or creating a hardcoded set of symlinks with specific names and targets in an arbitrary pre-existing host directory. This issue is not exploitable under Docker, because Docker creates a top-level read-only layer that masks any malicious /devsymlink present in the container image unlike some other Linux container tooling, whose higher-level runtimes built on runc remain exposed to exploitation via a malicious image. This issue has been fixed in versions 1.3.6, 1.4.3 and 1.5.0.