Skip to content

Improper input validation in umoci

Moderate severity GitHub Reviewed Published Apr 6, 2021 in opencontainers/umoci • Updated Jan 9, 2023

Package

gomod github.com/opencontainers/umoci (Go)

Affected versions

< 0.4.7

Patched versions

0.4.7

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

References

@cyphar cyphar published to opencontainers/umoci Apr 6, 2021
Reviewed May 20, 2021
Published to the GitHub Advisory Database Feb 15, 2022
Last updated Jan 9, 2023

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Local
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

EPSS score

0.047%
(18th percentile)

Weaknesses

CVE ID

CVE-2021-29136

GHSA ID

GHSA-9m95-8hx6-7p9v

Source code

No known source code
Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.