cis-docker-v170-5-13

name	cis-docker-v170-5.13
description	Ensure that the container's root filesystem is mounted as read only
category	cis-docker
version	1.7.0
author	cyberstrike-official
tags	["cis","docker","runtime","configuration","read-only"]
cis_id	5.13
cis_benchmark	CIS Docker Benchmark v1.7.0
tech_stack	["docker"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Docker Benchmark v1.7.0 - Control 5.13

Profile Applicability

• Level 1 - Docker - Linux

Description

The container's root filesystem should be treated as a 'golden image' by using Docker run's --read-only option. This prevents any writes to the container's root filesystem at container runtime and enforces the principle of immutable infrastructure.

Rationale

Enabling this option forces containers at runtime to explicitly define their data writing strategy to persist or not persist their data.

This also reduces security attack vectors since the container instance's filesystem cannot be tampered with or written to unless it has explicit read-write permissions on its filesystem folder and directories.

Impact

Enabling --read-only at container runtime may break some container OS packages if a data writing strategy is not defined. You should define what the container's data should and should not persist at runtime in order to decide which strategy to use.

Example:

• Enable use --tmpfs for temporary file writes to /tmp
• Use Docker shared data volumes for persistent data writes

Audit Procedure

You should run the following command on the docker host:

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

If the above command returns true, it means the container's root filesystem is mounted read-only.

If the above command returns false, it means the container's root filesystem is writeable.

Remediation

You should add a --read-only flag at a container's runtime to enforce the container's root filesystem being mounted as read only.

docker run <Run arguments> --read-only <Container Image Name or ID> <Command>

Enabling the --read-only option at a container's runtime should be used by administrators to force a container's executable processes to only write container data to explicit storage locations during its lifetime.

Examples of explicit storage locations during a container's runtime include, but are not limited to:

1. Using the --tmpfs option to mount a temporary file system for non-persistent data writes.

docker run --interactive -- --read-only --tmpfs  --tmpfs  centos /bin/bash