February 12, 2019

Runc and CVE-2019-5736

This morning a container escape vulnerability in runc was announced. We wanted to provide some guidance to Kubernetes users to ensure everyone is safe and secure.

What Is Runc?

Very briefly, runc is the low-level tool which does the heavy lifting of spawning a Linux container. Other tools like Docker, Containerd, and CRI-O sit on top of runc to deal with things like data formatting and serialization, but runc is at the heart of all of these systems.

Kubernetes in turn sits on top of those tools, and so while no part of Kubernetes itself is vulnerable, most Kubernetes installations are using runc under the hood.

What Is The Vulnerability?

While full details are still embargoed to give people time to patch, the rough version is that when running a process as root (UID 0) inside a container, that process can exploit a bug in runc to gain root privileges on the host running the container. This then allows them unlimited access to the server as well as any other containers on that server.

If the process inside the container is either trusted (something you know is not hostile) or is not running as UID 0, then the vulnerability does not apply. It can also be prevented by SELinux, if an appropriate policy has been applied. RedHat Enterprise Linux and CentOS both include appropriate SELinux permissions with their packages and so are believed to be unaffected if SELinux is enabled.

The most common source of risk is attacker-controller container images, such as unvetted images from public repositories.

Read more at Kubernetes blog

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