Despite the huge advantages that containers offer in application portability, acceleration of CI/CD pipelines and agility of deployment environments, the biggest concern has always been about isolation. Since all the containers running on a host share the same underlying kernel, any malicious code breaking out of a container can compromise the entire host, and hence all the applications running on the host and potentially in the cluster.
That fear of container isolation failing to hold up turned out to be true yesterday when a vulnerability in runC was announced. runC is the key and most popular software component that most container engines rely on for spinning up containers on a host. The announced vulnerability allows an attacker to break out of the container isolation through a well-crafted attack (technical details of the vulnerability and the exploit are at https://seclists.org/oss-sec/2019/q1/119) and compromise the entire host. The vulnerability is particularly nasty because it is not covered by the default AppArmor or SELinux kernel-enforced sandboxing policies.