What is CVE-2021-29136?

CVE-2021-29136 is a medium-severity improper input validation vulnerability in github.com/opencontainers/umoci (go), affecting versions < 0.4.7. It is fixed in 0.4.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.

How severe is CVE-2021-29136?

CVE-2021-29136 has a CVSS score of 5.5 (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.

Which versions of github.com/opencontainers/umoci are affected by CVE-2021-29136?

github.com/opencontainers/umoci (go) versions < 0.4.7 is affected.

Is there a fix for CVE-2021-29136?

Yes. CVE-2021-29136 is fixed in 0.4.7. Upgrade to this version or later.

Is CVE-2021-29136 exploitable, and should I be worried?

Whether CVE-2021-29136 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.

What actually determines whether CVE-2021-29136 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.

How do I fix CVE-2021-29136?

Upgrade github.com/opencontainers/umoci to 0.4.7 or later.

CVE-2021-29136 in github.com/opencontainers/umoci

Q: How severe is CVE-2021-29136?

CVE-2021-29136 has a CVSS score of 5.5 (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.

Q: Which versions of github.com/opencontainers/umoci are affected by CVE-2021-29136?

github.com/opencontainers/umoci (go) versions < 0.4.7 is affected.

Q: Is there a fix for CVE-2021-29136?

Yes. CVE-2021-29136 is fixed in 0.4.7. Upgrade to this version or later.

Q: Is CVE-2021-29136 exploitable, and should I be worried?

Whether CVE-2021-29136 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.

Q: What actually determines whether CVE-2021-29136 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.

Q: How do I fix CVE-2021-29136?

Upgrade github.com/opencontainers/umoci to 0.4.7 or later.

Summary

Improper input validation in umoci

Workarounds

Note that if you use umoci as an unprivileged user (using the --rootless
flag) then umoci will not be able to overwrite any files that your user
doesn't have access to. Other possible mitigations are to run umoci
under an LSM profile such as AppArmor or SELinux to restrict the level
of access it has outside of container image directories.

References

Credits

Thanks to Robin Peraglie from Cure53 for discovering and reporting this
vulnerability.

For more information

If you have any questions or comments about this advisory

Open an issue in https://github.com/opencontainers/umoci.
Email us at [email protected].

Impact

umoci 0.4.6 and earlier can be tricked into modifying host files by
creating a malicious layer that has a symlink with the name "." (or
"/"). Because umoci deletes inodes if they change types, this results in
the rootfs directory being replaced with an attacker-controlled symlink.
Subsequent image layers will then be applied on top of the target of the
symlink (which could be any directory on the host filesystem the user
running umoci has access to).

While umoci does have defences against symlink-based attacks, they are
all implemented by resolving things relative to the rootfs directory --
if the rootfs itself is a symlink, umoci resolves it first.

This vulnerability affects both "umoci unpack" and "umoci raw unpack".

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2021-29136 has a CVSS score of 5.5 (Medium). The vector is requires local access, low 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 (0.4.7); upgrading removes the vulnerable code path.

Affected versions

github.com/opencontainers/umoci (< 0.4.7)

Security releases

github.com/opencontainers/umoci → 0.4.7 (go)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

This issue has been patched in umoci 0.4.7, see the references section
for the specific commit which fixed this vulnerability.

Frequently Asked Questions

What is CVE-2021-29136? CVE-2021-29136 is a medium-severity improper input validation vulnerability in github.com/opencontainers/umoci (go), affecting versions < 0.4.7. It is fixed in 0.4.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
How severe is CVE-2021-29136? CVE-2021-29136 has a CVSS score of 5.5 (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.
Which versions of github.com/opencontainers/umoci are affected by CVE-2021-29136? github.com/opencontainers/umoci (go) versions < 0.4.7 is affected.
Is there a fix for CVE-2021-29136? Yes. CVE-2021-29136 is fixed in 0.4.7. Upgrade to this version or later.
Is CVE-2021-29136 exploitable, and should I be worried? Whether CVE-2021-29136 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
What actually determines whether CVE-2021-29136 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.
How do I fix CVE-2021-29136? Upgrade github.com/opencontainers/umoci to 0.4.7 or later.