CVE-2025-31133

A flaw was found in runc. This flaw exploits an issue with how masked paths are implementedin runc. When masking files, runc will bind-mount the container's /dev/null inode on top of the file. However, if an attacker can replace /dev/null with a symlink to some other procfs file, runc will instead bind-mount the symlink target read-write.

Red Hat considers this as an Important flaw since the impact is limited to local attack with minimal privileges in order to jeopardize the environment.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Potential mitigations for this issue include:

• Using user namespaces, with the host root user not mapped into the container's namespace. procfs file permissions are managed using Unix DAC and thus user namespaces stop a container process from being able to write to them.

• Not running as a root user in the container (this includes disabling setuid binaries with noNewPrivileges). As above, procfs file permissions are managed using Unix DAC and thus non-root users cannot write to them.

• Depending on the maskedPath configuration (the default configuratio nonly masks paths in /proc and /sys), using an AppArmor that blocks unexpectedwrites to any maskedPaths (as is the case with the defaultprofile used by Docker and Podman) will block attempts to exploit this issue. However, CVE-2025-52881 allows an attacker to bypass LSMlabels, and so this mitigation is not helpful when considered incombination with CVE-2025-52881.