Don't blindly trust Docker for the selfhosted stuff

It is my strong belief that you shouldn't go crazy with all-things-docker when deploying selfhosted services at home. Online forums, especially r/selfhosted, seem to foster an opinion that providing a Dockerfile or better yet a docker-compose.yml or even prebuilt public images on Docker Hub is an acceptable way to distribute software targeting the selfhosting crowd.

I agree it is very convenient to deploy complex multipart services via these tools. But the way many people appear to be doing that is a security nightmare! This is how we get to encounter Heartbleed in the wild four years after it should've been extinct.

There are many comprehensive writeups on Docker/Kubernetes security, I will highlight only a subset of problems below.

  • Shared libraries

    Running each service in its separate container results in having a separate set of shared libraries for each one of those services. It is convenient when you need to provide multiple incompatible dependencies at once, but that way the burden of tracking the state of all those dependencies lies on the user. Host OS can not tell you that one of the containers still ships a vulnerable version of some critical library - it's up to you to monitor and fix that.

  • Container rebuilding

    Fixing anything related to the container requires rebuilding the image. When you're using images from public registries you can not initiate image rebuild even when you know it's needed. Your best option is to contact the original uploader and to convince them to rebuild. That may take significant time during which the containers running that image remain vulnerable.

  • Images from untrusted sources

    In addition to the points above you put enormous amount of trust into people who provide the container you're running. In containerless scenario you're required to trust the vendor who provides the base OS and the developer who provides the custom applications you run upon that OS. When containers come into play, you must extend your trust to the maintainer of the container image, to the vendors who provide the base image that image is built upon, to all the developers who provide any piece of code included into that container. It does not even require malicious intent to introduce a vulnerability into the resulting container, simple incompetence of any of the parties involved may be just enough.

This is why containerizing any workload comes with a significant extra cost of designing and automating security maintenance procedures. It is easy to skip this step when you're a hobbyist - but that's just burying your head in the sand and waiting for some script kiddie or botnet to hijack your network.

Here is a rough overview of the required overhead:

  • You need to run only containers that are based on the images you've built yourself. This is the only way you can ensure swift rebuilding in case one of the base images provides a security update. This step may include running your own image registry and build service.
  • You need to audit every Dockerfile you intend to build. This can only be done manually. And you need to check all the base images in the chain up to either a FROM scratch stanza or to a base image from trusted list.
  • You need to maintain a list of trusted base images that come from vendors with good reputation in regards to handling security issues.
  • You need to blacklist any image that does not come either from a trusted list or from a Dockerfile you've audited yourself.
  • You need to setup automated image rebuilds and container rollouts: a) on schedule b) on any update in the base image dependency chain
  • You need to setup automated vulnerability monitoring for the images you're running. This will require a lot more effort than subscribing to RSS feed of your distro security announcements - as it would've been the case with containerless deployment.

Add that on top of usual container orchestration chores - and bare metal suddenly becomes attractive. Docker and Kubernetes are great tools that solve real world problems but using them in a secure manner requires continuous dedicated effort. For enterprise deployments the benefits of containerization usually outweigh the extra maintenance cost, but for hobbyist use I'm not so sure.