How severe is CVE-2021-29136?

CVE-2021-29136 has a CVSS score of 5.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-29136?

CVE-2021-29136 affects the following packages: github.com/opencontainers/umoci (Go). Ecosystems affected: Go.

How do I fix CVE-2021-29136?

Update github.com/opencontainers/umoci to 0.4.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29136 is resolved across your whole dependency graph.

How do I detect CVE-2021-29136 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/opencontainers/umoci, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-29136 if there is no patch (or I can't update yet)?

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.

Is CVE-2021-29136 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-29136 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-29136?

CVE-2021-29136 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 26th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-29136?

CVE-2021-29136 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-29136 published, and has it been updated?

CVE-2021-29136 was published on April 6, 2021 and was last updated on August 27, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2021-29136 — umoci

MEDIUMFix: opencontainers/umoci@d9efc31

CVE-2021-29136 is a medium-severity (CVSS 5.5) Improper Input Validation vulnerability in github.com/opencontainers/umoci. A fix is available for github.com/opencontainers/umoci — see the affected versions and patch details below.

Improper input validation in umoci

Also known asGHSA-9m95-8hx6-7p9vGO-2022-0815

Published

Apr 6, 2021

Updated

Aug 27, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs26th percentile — riskier than 26% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-29136 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐹github.com/opencontainers/umoci

Real-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

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".

Patches

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

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].

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/opencontainers/umoci`	all versions	0.4.7`go get github.com/opencontainers/umoci@v0.4.7`

Affected Products

2 products · 2 configurations

Application

umocilinuxfoundation

< 0.4.7

range

Application

singularitysylabs

< 3.7.3

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/opencontainers/umoci, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/opencontainers/umoci to 0.4.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29136 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.

Frequently Asked Questions

### 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 di