runc CLI BindMount Verification Flaw Exposes Host Escape (1.4.0rc.2)
CVE-2025-31133 Published on November 6, 2025
runc container escape via "masked path" abuse due to mount race conditions
runc is a CLI tool for spawning and running containers according to the OCI specification. In versions 1.2.7 and below, 1.3.0-rc.1 through 1.3.1, 1.4.0-rc.1 and 1.4.0-rc.2 files, runc would not perform sufficient verification that the source of the bind-mount (i.e., the container's /dev/null) was actually a real /dev/null inode when using the container's /dev/null to mask. This exposes two methods of attack: an arbitrary mount gadget, leading to host information disclosure, host denial of service, container escape, or a bypassing of maskedPaths. This issue is fixed in versions 1.2.8, 1.3.3 and 1.4.0-rc.3.
Weakness Types
What is a Symlink following Vulnerability?
The software, when opening a file or directory, does not sufficiently account for when the file is a symbolic link that resolves to a target outside of the intended control sphere. This could allow an attacker to cause the software to operate on unauthorized files. A software system that allows UNIX symbolic links (symlink) as part of paths whether in internal code or through user input can allow an attacker to spoof the symbolic link and traverse the file system to unintended locations or access arbitrary files. The symbolic link can permit an attacker to read/write/corrupt a file that they originally did not have permissions to access.
CVE-2025-31133 has been classified to as a Symlink following vulnerability or weakness.
Race Condition Enabling Link Following
The software checks the status of a file or directory before accessing it, which produces a race condition in which the file can be replaced with a link before the access is performed, causing the software to access the wrong file. While developers might expect that there is a very narrow time window between the time of check and time of use, there is still a race condition. An attacker could cause the software to slow down (e.g. with memory consumption), causing the time window to become larger. Alternately, in some situations, the attacker could win the race by performing a large number of attacks.
Products Associated with CVE-2025-31133
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Affected Versions
opencontainers runc:- Version < 1.2.8 is affected.
- Version >= 1.3.0-rc.1, < 1.3.3 is affected.
- Version >= 1.4.0-rc.1, <= 1.4.0-rc.3 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.