Cis Docker 5.31

5.31 Ensure that the host's user namespaces are not shared (Manual)

Profile Applicability

• Level 1 - Docker - Linux

Description

You should not share the host's user namespaces with containers running on it.

Rationale

User namespaces ensure that a root process inside the container will be mapped to a non-root process outside the container. Sharing the user namespaces of the host with the container does not therefore isolate users on the host from users in the containers.

Impact

None

Audit Procedure

You should run the command below and ensure that it does not return any value for UsernsMode. If it returns a value of host, it means that the host user namespace is shared with its containers.

docker ps --quiet --all | xargs docker inspect --format '{{ .Id }}: UsernsMode={{ .HostConfig.UsernsMode }}'

Remediation

You should not share user namespaces between host and containers. For example, you should not run the command below:

docker run --rm -it --userns=host ubuntu bash

Default Value

By default, the host user namespace is shared with containers unless user namespace support is enabled.

References

1. https://docs.docker.com/engine/security/userns-remap/
2. https://docs.docker.com/engine/reference/commandline/run/#options

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