32c3 Docker

This is a write-up of the docker challenge from the 32c3 CTF.

The Challenge

We were given ssh access is a box as the user eve, and we needed to read a flag(/home/adam/flag) that the user adam had read-only rights to.

adam had a binary(/home/adam/docker) which did:

/usr/bin/docker run -it --privileged=false -u 1337:65534 --cap-drop=ALL --net=host ubuntu /bin/bash

Which means that we had a bash shell inside a docker container running as adam but was using the same network stack as the host.

The Solution

As we were running as adam inside the container the logical step would be to break out of the container, and because the docker container was running with --net=host it was natural to think that it was a network related issue.

Thus the obvious choice is unix doman sockets, which are files that behaves as sockets but have some interesting features such as the ability to transfer file descriptors.

So digging though man pages we found something called abstract unix domain, which are unix domain sockets but not files. All this information can be found in man unix, along with information describing how to transfer file descriptors over unix domain sockets

After researching all this arcane magic we came up with: Which should be run outside of the docker as eve like:

./sendfd foobar /

And this should be run side of the docker as adam like:

./recvfd foobar /bin/bash

The twist

All of this was running inside a chroot, and adam's flag was a false flag, The real flag was at /flag and adam's was at /chroot/home/adam/flag.

But when your current working directory is already unreachable from the root you can break out of any chroot your are in. So this was not a problem.


This is not a bug in docker. Everything is working as intended. --net=host is harmful for your sandboxing.