runc through 1.0-rc6 as used in Docker before 18.09.2 and other products allows attackers to overwrite the host runc binary (and consequently obtain host root access) by leveraging the ability to execute a command as root within one of these types of containers: (1) a new container with an attacker-controlled image or (2) an existing container to which the attacker previously had write access that can be attached with docker exec. This occurs because of file-descriptor mishandling related to /proc/self/exe.