CVE-2021-41091 is a medium-severity (CVSS 6.3) CWE-281 vulnerability in github.com/moby/moby. 2 public exploit references exist, so weaponization risk is real. A fix is available for github.com/moby/moby — see the affected versions and patch details below.
Moby (Docker Engine) Insufficiently restricted permissions on data directory
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-41091 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,333 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/moby/moby🐹github.com/docker/dockerReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Moby is an open-source project created by Docker to enable software containerization. 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. 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. For users unable to upgrade limit access to the host to trusted users. Limit access to host volumes to trusted containers.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/moby/moby | all versions | 20.10.9go get github.com/moby/moby@v20.10.9 |
| 🐹Go | github.com/docker/docker | all versions | 20.10.9go get github.com/docker/docker@v20.10.9 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/moby/moby, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/moby/moby to 20.10.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41091 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-41091 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-41091. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
In the OpenShift Container Platform (OCP), the Migration Toolkit for Virtualization (MTV) and the Red Hat Advanced Cluster Management for Kubernetes (RHACM) some components bundle github.com/moby/moby. However, successful exploitation requires direct access to the cluster hosts, which is not allowed by default and is…
| Product | Fixed in | Advisory |
|---|---|---|
| Migration Toolkit for Virtualization 2.2 | migration-toolkit-virtualization/mtv-controller-rhel8:2.2.0-39 | RHEA-2021:5066 |
Frequently Asked Questions
Is CVE-2021-41091 in your dependencies?
O3 Security finds CVE-2021-41091 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.