A flaw was found in Buildah (and subsequently Podman Build) which allows containers to mount arbitrary locations on the host filesystem into build containers. A malicious Containerfile can use a dummy image with a symbolic link to the root filesystem as a mount source and cause the mount operation to mount the host root filesystem inside the RUN step. The commands inside the RUN step will then have read-write access to the host filesystem, allowing for full container escape at build time.
The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor.