CVE-2026-41579

### Impact 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. Please note that this issue is not exploitable under Docker because [it creates a top-level `ro` layer that masks any malicious `/dev` symlink present in the container image](https://github.com/moby/moby/blob/docker-v29.4.1/daemon/initlayer/setup_unix.go) (this is also done without mounting the lower layers so there is no opportunity for the malicious `/dev` symlink to trick it into resolving to some other path). Unfortunately, Podman and containerd^† do not do this and so users using those higher-level runtimes with runc can be exploited via a malicious image. This issue mirrors a [somewhat similar issue in crun](https://github.com/containers/crun/security/advisories/GHSA-7vwr-4279-7gq5), which was also published recently. ###### † Actually, at the time the issue was analysed, [containerd had dead code that implemented this feature](https://github.com/containerd/containerd/blob/v2.2.3/pkg/rootfs/init_linux.go) but the implementation contained several security issues that would arguably have made it more exploitable than in runc. Luckily, the code appears to have never been used (at least since 2017) and [the code has since been removed](https://github.com/containerd/containerd/issues/13238). #### Mitigating Factors #### There are a few mitigating factors about this issue which reduce the impact for most users quite significantly, and is the reason why we decided to release the fix publicly without an embargo. While the deletion of `ptmx` seems like a significant issue, in practice it is quite limited. Notably, `devpts` does not permit you to unlink `/dev/pts/ptmx` regardl