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CVE-2025-52881

CVE-2025-52881: runc container escape and denial of service due to arbitrary write gadgets and procfs write redirects

Impact

This attack is primarily a more sophisticated version of CVE-2019-19921, which was a flaw which allowed an attacker to trick runc into writing the LSM process labels for a container process into a dummy tmpfs file and thus not apply the correct LSM labels to the container process. The mitigation runc applied for CVE-2019-19921 was fairly limited and effectively only caused runc to verify that when runc writes LSM labels that those labels are actual procfs files.

Rather than using a fake tmpfs file for /proc/self/attr/<label>, an attacker could instead (through various means) make /proc/self/attr/<label> reference a real procfs file, but one that would still be a no-op (such as /proc/self/sched). This would have the same effect but would clear the "is a procfs file" check. Runc is aware that this kind of attack would be possible (even going so far as to discuss this publicly as "future work" at conferences), and runc is working on a far more comprehensive mitigation of this attack, but this security issue was disclosed before runc could complete this work.

In all known versions of runc, an attacker can trick runc into misdirecting writes to /proc to other procfs files through the use of a racing container with shared mounts (runc has also verified this attack is possible to exploit using a standard Dockerfile with docker buildx build as that also permits triggering parallel execution of containers with custom shared mounts configured). This redirect could be through symbolic links in a tmpfs or theoretically other methods such as regular bind-mounts.

Note that while /proc/self/attr/<label> was the example used above (which is LSM-specific), this issue affect all writes to /proc in runc and thus also affects sysctls (written to /proc/sys/...) and some other APIs.

Additional Impacts

While investigating this issue, runc discovered that another risk with these redirected writes is that they could be redirected to dangerous files such as rather than just no-op files like . For instance, the default AppArmor profile name in Docker is , which when written to would cause the host system to crash.

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CVE-2025-52881

CVE-2025-52881:

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/opencontainers/runc	go	<= 1.2.7	1.2.8
github.com/opencontainers/selinux	go	<= 1.12.0
github.com/opencontainers/runc	go	>= 1.3.0-rc.1, <= 1.3.2	1.3.3
github.com/opencontainers/runc	go	>= 1.4.0-rc.1, <= 1.4.0-rc.2	1.4.0-rc.3

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Root Cause Analysis

The vulnerability is a sophisticated race condition (TOCTOU) that allows an attacker to trick runc into writing to an arbitrary file within the procfs filesystem. This is achieved by manipulating mount points or symbolic links between the time runc checks a path and the time it writes to it.

The analysis of the patches reveals several key areas where runc performed unsafe operations on procfs using string-based paths.

Direct Procfs Writes: Functions like setProcAttr in libcontainer/apparmor/apparmor_linux.go and the now-removed writeSystemProperty in libcontainer/rootfs_linux.go directly constructed paths to files in /proc and wrote to them. An attacker could race these operations to redirect the write to a sensitive file, leading to denial of service (by writing to /proc/sysrq-trigger) or container escape (by writing to /proc/sys/kernel/core_pattern). The same pattern was present in the vendored go-selinux library in functions like SetExecLabel.
Racy Mount Setup: The mountToRootfs function in libcontainer/rootfs_linux.go was responsible for setting up mounts. Its logic was path-based and racy, creating the opportunity for an attacker to replace a directory with a symlink pointing into /proc between the path resolution/creation step and the actual mount operation. This is the foundational part of the exploit that allows the write gadgets to be aimed at sensitive procfs files.
Unsafe Procfs Reads: Functions like Stat in libcontainer/system/proc.go and setupUser in libcontainer/init_linux.go performed reads from procfs using constructed paths. While the immediate impact of redirecting a read is lower than a write, the patches systematically replaced all such unsafe path-based accesses with secure, file-descriptor-based operations to eliminate the entire class of vulnerability.

The patches introduce safe, handle-based APIs (, , etc.) and apply them to all identified vulnerable locations, ensuring that once a file handle is opened, subsequent operations are immune to symlink-based redirection attacks.

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Reasoning

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CVE-2025-52881: runc container escape and denial of service due to arbitrary write gadgets and procfs write redirects

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CVE-2025-52881: runc container escape and denial of service due to arbitrary write gadgets and procfs write redirects

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CVE-2025-52881: runc container escape and denial of service due to arbitrary write gadgets and procfs write redirects

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