A monitoring agent
The socket mount and --privileged together are root on the host with extra steps
docker run -d --name agent -v /var/run/docker.sock:/var/run/docker.sock --privileged myagent:1
Docker Command Explainer
Understand a command somebody handed you before you run it. Four things in particular give a container effective control of the host, and none of them looks alarming.
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Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
Host networking
-p stops doing anything, and loopback-only services become reachable
docker run -d --network host --name proxy nginx
An ordinary command
Every flag explained, and nothing that hands over the host
docker run -d --name web -p 8080:80 -v /srv:/data nginx:alpine
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Mounting the Docker socket read-only and calling it safe
It is a socket, not a file. Read-only does not stop a write, and a write is an API call that can start a privileged container.
Instead:Use a socket proxy that allows only the specific API calls needed.
Using --privileged to fix a permissions error
It grants every capability and disables the security profiles, when the problem is usually one capability or one device.
Instead:Find the specific need with --cap-add or --device, and start from --cap-drop=ALL.
Putting a docker flag after the image name
Everything after the image is passed to the container's process, so the flag never reaches Docker and appears to be ignored.
Instead:Put every docker flag before the image name.
Some flags are equivalent to root on the host
--privileged, --pid=host, --network host and a mount of the Docker socket all remove the boundary the container was supposed to provide.
Mounting the Docker socket is the quietest one
Anything that can talk to it can start a container with --privileged and the host's filesystem mounted, so the isolation is decorative. Adding :ro changes nothing, because the API is a socket rather than a file and read-only does not stop a write to it.
--privileged is every capability at once
Plus device access and no seccomp or AppArmor profile. A process inside can load kernel modules and mount the host filesystem. It is almost always a blunt fix for one missing capability, and finding out which is worth an afternoon.
--pid=host exposes other processes' environments
The container sees and can signal every process on the machine, and can read their /proc entries, which include environment variables and therefore other services' credentials.
--network host removes network isolation
-p stops doing anything, every port the process opens is open on the host, and it can reach services bound to 127.0.0.1 that were meant to be local only.
SYS_ADMIN is close to privileged on its own
It permits mount, and mount permits reaching the host filesystem. NET_ADMIN and SYS_PTRACE are also far broader than their names suggest. Dropping everything with --cap-drop=ALL and adding back what is needed is the shape to aim for.
Everything after the image is arguments to the container
Not to Docker. A flag placed after the image name is passed straight through to the process, which is the commonest reason a flag appears to be ignored.
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