Moby is an open-source project created by Docker to enable software containerization. A bug was found in Moby (Docker Engine) versions prior to 20.10.18 where supplementary groups are not set up properly. If an attacker has direct access to a container and manipulates their supplementary group access, they may be able to use supplementary group access to bypass primary group restrictions in some cases, potentially gaining access to sensitive information or gaining the ability to execute code in that container. This bug is fixed in Moby (Docker Engine) 20.10.18. Running containers should be stopped and restarted for the permissions to be fixed. For users unable to upgrade, this problem can be worked around by not using the `"USER $USERNAME"` Dockerfile instruction. Instead by calling `ENTRYPOINT ["su", "-", "user"]` the supplementary groups will be set up properly.
CWE-863 - Incorrect Authorization
Authorization is a security mechanism performed by an application to grant or deny access to the requested resources by verifying the privileges of the user. When an application lacks effective authorization mechanisms, it enables unauthorized users to gain unintended privileges and illegitimate access to resources. Such a vulnerability may result in exposure of sensitive information, denial of service, arbitrary code execution, and complete system takeover.