Virtual machines are generally considered secure. At least, secure enough to power highly multi-tenant, large-scale public clouds, where a single physical machine can host a large number of virtual instances belonging to different customers. Containers have many advantages over virtual machines: they boot faster, have less performance overhead, and use less resources. However, those advantages also stem from the fact that containers share the kernel of their host, instead of abstracting a new independent environment. This sharing has significant security implications, as kernel exploits can now lead to host-wide escalations. We will show techniques to harden Linux Containers; including kernel capabilities, mandatory access control, hardened kernels, user namespaces, and more, and discuss the remaining attack surface.