Cis Docker 5.20

5.20 Ensure mount propagation mode is not set to shared (Manual)

Profile Applicability

• Level 1 - Docker - Linux

Description

Mount propagation mode allows mounting volumes in shared, slave or private mode on a container. Do not use shared mount propagation mode unless explicitly needed.

Rationale

A shared mount is replicated at all mounts and changes made at any mount point are propagated to all other mount points.

Mounting a volume in shared mode does not restrict any other container from mounting and making changes to that volume.

As this is likely not a desirable option from a security standpoint, this feature should not be used unless explicitly required.

Impact

None.

Audit Procedure

docker ps --quiet --all | xargs docker inspect --format '{{ .Id }}: Propagation={{range $mnt := .Mounts}}{{json $mnt.Propagation}}{{end}}'

The above command returns the propagation mode for mounted volumes. The propagation mode should not be set to shared unless needed. The above command might throw errors if there are no mounts. In that case, this recommendation is not applicable.

Remediation

Do not mount volumes in shared mode propagation. For example, do not start a container as below:

docker run <Run arguments> --volume=/hostPath:/containerPath:shared <Container Image Name or ID> <Command>

Default Value

By default, the container mounts are private.

References

1. https://docs.docker.com/storage/bind-mounts/#configure-bind-propagation
2. https://docs.docker.com/engine/reference/run/#volume-shared-filesystems

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