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I'd like to set up a hardened Docker instance, mostly for running micro-services such as statically compiled golang applications. What I'm looking for is to protect the host OS from a rogue container and containers from one another. I tried to summarize the situation with the following scenario.

Scenario:

We have a server running a minimal OS such as alpine linux (busybox-based OS), with SElinux and grsec installed, activated and correctly configured.

On this server runs a Docker instance with 2 running containers, A and B and a volume V.

The container A contains a statically compiled application with no dependencies, networked to the public Internet (web app or public API). This application contains a HUGE bug something like arbitrary code execution/upload/full reverse shell the worse you can think of. This container is also networked to the volume V as upload destination, database, etc.

The host OS contains a flag that can only be read when root (enforced by SElinux).

The container B also contains a flag and an application but no connection to the outside world.

Attacker:

  • Human, knows about the huge bug in the application.
  • He wants to get the flags. The data in V isn't important.
  • He's not a spy agency but still a high-grade security specialist.
  • May have access to some zero-days we aren't aware of.

Assumptions:

  • The linux kernel has bugs but grsec is enough to cover that. Can't be an attack vector unless grsec is deactivated
  • Grsec and SElinux don't have bugs and aren't mis-configured.
  • A user root in the container is root outside the container (maybe some day this won't be true anymore...)
  • Docker is a real-world Docker. No known-bugs but has been affected by bugs in the past and it could happen again.
  • A log system for future investigation is already properly set up.

Goal:

  • Protect the flags. Probably not possible since we assume that Docker has bugs.
  • Reducing the attack surface.
  • Make the attacker's life difficult.
  • Set an alarm that trigger if the attacker tries to get the flags. Preferably way before he managed to get them.

Questions:

  • How realistic are my assumptions?
  • How would you achieve those goals?
  • How goods are my following suggestions?
  • Any general security advice for Docker?

My suggestions:

  • Configure SElinux such as no user can write on A, and no user can execute files on V.
  • Use an extremely minimal Docker image, without any userland. Something like:

    FROM scratch
    COPY app /
    ENTRYPOINT ["/app"]
    
  • De-escalate privilege before running the app. (Not sure what is the proper way to do that...)

  • A fake busybox user-land? Something that would trigger an alarm if we try to call /bin/sh, /bin/ls or anything like that.
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I'd refer you to the CIS Benchmarks for hardening guidelines. The current CIS Benchmark for Docker can be found here. These are an accepted industry standard for baseline hardening. They also offer guidelines for Linux et al, Web servers, DBs, etc.

  • That's a very interesting read, thanks ! But this benchmark doesn't provide any info about the lack of userland in a container being useful in security (image built from scrath vs from an OS image) and in general if minimalism can help (minimal OS, minimal image, etc.). But, that already answers 90+% of all real-life scenarios with Docker, so thank you for that. – Matthieu Aug 30 '16 at 17:48
  • Anytime :) and in general the smaller the attack surface, the more secure it is. That's not always 100% true but it can be a decent rule of thumb. – HashHazard Aug 30 '16 at 18:20
  • @Matthieu might be worth looking at the latest version of the guide (I updated the link in the answer), there's some new bits in there about container attack surface reduction. – Rоry McCune Oct 19 '16 at 9:55
  • "Do not install unnecessary packages in the container [...] You can even remove the package installer as a final hardening measure" That's exactly the kind of thing I was looking for. Thanks a lot! I was thinking about userland, but I didn't think about the package installer. That's a very good advice. – Matthieu Oct 19 '16 at 21:23

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