# Cis Docker 5.31

> Cis Docker 5.31 is an Agent Skill for devops & security published in the CyberStrikeus/CyberStrike repository on GitHub: a SKILL.md instruction file an AI agent can load to do this task.

Ensure that the host's user namespaces are not shared

- Skill name: `cis-docker-5.31`
- Category: [DevOps & Security](https://www.openparable.com/skill/devops-security/1)
- Source: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Server_Software/Docker/CIS_Docker_Benchmark_v1.8.0/cis-docker-5.31/SKILL.md
- Repository: CyberStrikeus/CyberStrike (1,211 GitHub stars when collected)

## SKILL.md

# 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

## CIS Controls

| Controls Version | Control                                                                                                                                                                                                                                      | IG 1 | IG 2 | IG 3 |
| ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | 3.12 Segment Data Processing and Storage Based on Sensitivity<br>Segment data processing and storage based on the sensitivity of the data. Do not process sensitive data on enterprise assets intended for lower sensitivity data.           |      | ●    | ●    |
| v7               | 12 Boundary Defense<br>Boundary Defense                                                                                                                                                                                                      |      |      |      |
| v7               | 14.1 Segment the Network Based on Sensitivity<br>Segment the network based on the label or classification level of the information stored on the servers, locate all sensitive information on separated Virtual Local Area Networks (VLANs). |      | ●    | ●    |

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Source: https://www.openparable.com/skill/devops-security/cis-docker-5-31 (OpenParable). Cite the canonical URL above.