CVE-2021-41091

Workarounds

Limit access to the host to trusted users. Limit access to host volumes to trusted containers.

Credits

The Moby project would like to thank Joan Bruguera for responsibly disclosing this issue in accordance with the Moby security policy.

For more information

If you have any questions or comments about this advisory:

• Open an issue
• Email us at [email protected] if you think you’ve found a security bug

A bug was found in Moby (Docker Engine) where the data directory (typically /var/lib/docker) contained subdirectories with insufficiently restricted permissions, allowing otherwise unprivileged Linux users to traverse directory contents and execute programs. When containers included executable programs with extended permission bits (such as setuid), unprivileged Linux users could discover and execute those programs. When the UID of an unprivileged Linux user on the host collided with the file owner or group inside a container, the unprivileged Linux user on the host could discover, read, and modify those files.

A file, directory, or other resource is assigned permissions that allow broader access than intended. Typical impact: unauthorized read, modification, or execution of the resource.

CVE-2021-41091 has a CVSS score of 5.9 (Medium). The vector is requires local access, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (20.10.9); upgrading removes the vulnerable code path.

Kodem intelligence

This bug has been fixed in Moby (Docker Engine) 20.10.9. Users should update to this version as soon as possible. Running containers should be stopped and restarted for the permissions to be fixed.

1. What is CVE-2021-41091? CVE-2021-41091 is a medium-severity incorrect permission assignment for critical resource vulnerability in github.com/moby/moby (go), affecting versions < 20.10.9. It is fixed in 20.10.9. A file, directory, or other resource is assigned permissions that allow broader access than intended.
2. How severe is CVE-2021-41091? CVE-2021-41091 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2021-41091?
   • github.com/moby/moby (go) (versions < 20.10.9)
   • github.com/docker/docker (go) (versions < 20.10.9)
4. Is there a fix for CVE-2021-41091? Yes. CVE-2021-41091 is fixed in 20.10.9. Upgrade to this version or later.
5. Is CVE-2021-41091 exploitable, and should I be worried? Whether CVE-2021-41091 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-41091 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-41091?
   • Upgrade github.com/moby/moby to 20.10.9 or later
   • Upgrade github.com/docker/docker to 20.10.9 or later