CVE-2025-52881

runc is a CLI tool for spawning and running containers according to the OCI specification. In versions 1.2.7, 1.3.2 and 1.4.0-rc.2, an attacker can trick runc into misdirecting writes to /proc to other procfs files through the use of a racing container with shared mounts (we have 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. While similar, the mitigation applied for the related CVE, CVE-2019-19921, was fairly limited and effectively only caused runc to verify that when LSM labels are written they are actually procfs files. This issue is fixed in versions 1.2.8, 1.3.3, and 1.4.0-rc.3.

runc is a CLI tool for spawning and running containers according to the OCI specification. In versions 1.2.7, 1.3.2 and 1.4.0-rc.2, an attacker can trick runc into misdirecting writes to /proc to other procfs files through the use of a racing container with shared mounts (we have 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. While similar, the mitigation applied for the related CVE, CVE-2019-19921, was fairly limited and effectively only caused runc to verify that when LSM labels are written they are actually procfs files. This issue is fixed in versions 1.2.8, 1.3.3, and 1.4.0-rc.3.

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 /proc/sysrq-trigger rather than just no-op files like /proc/self/sched. For instance, the default AppArmor profile name in Docker is docker-default, which when written to /proc/sysrq-trigger would cause the host system to crash.

When this was discovered, runc conducted an audit of other write operations within runc and found several possible areas where runc could be used as a semi-arbitrary write gadget when combined with the above race attacks. The most concerning attack scenario was the configuration of sysctls. Because the contents of the sysctl are free-form text, an attacker could use a misdirected write to write to /proc/sys/kernel/core_pattern and break out of the container (as described in CVE-2025-31133, kernel upcalls are not namespaced and so coredump helpers will run with complete root privileges on the host). Even if the attacker cannot configure custom sysctls, a valid sysctl string (when redirected to /proc/sysrq-trigger) can easily cause the machine to hang.

Note that the fact that this attack allows you to disable LSM labels makes it a very useful attack to combine with CVE-2025-31133 (as one of the only mitigations available to most users for that issue is AppArmor, and this attack would let you bypass that). However, the misdirected write issue above means that you could also achieve most of the same goals without needing to chain together attacks.

Patches

This advisory is being published as part of a set of three advisories:

• CVE-2025-31133
• CVE-2025-52881
• CVE-2025-52565

The patches fixing this issue have accordingly been combined into a single patchset. The following patches from that patchset resolve the issues in this advisory:

• db19bbed5348 ("internal/sys: add VerifyInode helper")
• 6fc191449109 ("internal: move utils.MkdirAllInRoot to internal/pathrs")
• ff94f9991bd3 ("*: switch to safer securejoin.Reopen")
• 44a0fcf685db ("go.mod: update to…

runc is a CLI tool for spawning and running containers according to the OCI specification.

Security Fix(es):

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.(CVE-2025-31133)

runc is a CLI tool for spawning and running containers according to the OCI specification. Versions 1.0.0-rc3 through 1.2.7, 1.3.0-rc.1 through 1.3.2, and 1.4.0-rc.1 through 1.4.0-rc.2, due to insufficient checks when bind-mounting /dev/pts/$n to /dev/console inside the container, an attacker can trick runc into bind-mounting paths which would normally be made read-only or be masked onto a path that the attacker can write to. This attack is very similar in concept and application to CVE-2025-31133, except that it attacks a similar vulnerability in a different target (namely, the bind-mount of /dev/pts/$n to /dev/console as configured for all containers that allocate a console). This happens after pivot_root(2), so this cannot be used to write to host files directly -- however, as with CVE-2025-31133, this can load to denial of service of the host or a container breakout by providing the attacker with a writable copy of /proc/sysrq-trigger or /proc/sys/kernel/core_pattern (respectively). This issue is fixed in versions 1.2.8, 1.3.3 and 1.4.0-rc.3.(CVE-2025-52565)

runc is a CLI tool for spawning and running containers according to the OCI specification. In versions 1.2.7, 1.3.2 and 1.4.0-rc.2, an attacker can trick runc into misdirecting writes to /proc to other procfs files through the use of a racing container with shared mounts (we have 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. While similar, the mitigation applied for the related CVE, CVE-2019-19921, was fairly limited and effectively only caused runc to verify that when LSM labels are written they are actually procfs files. This issue is fixed in versions 1.2.8, 1.3.3, and 1.4.0-rc.3.(CVE-2025-52881)

The runC tool is a lightweight, portable implementation of the Open Container Format (OCF) that provides container runtime.

Security Fix(es):

• runc: container escape via 'masked path' abuse due to mount race conditions (CVE-2025-31133)

• runc: container escape with malicious config due to /dev/console mount and related races (CVE-2025-52565)

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The podman tool manages pods, container images, and containers. It is part of the libpod library, which is for applications that use container pods. Container pods is a concept in Kubernetes.

Security Fix(es):

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The container-tools module contains tools for working with containers, notably podman, buildah, skopeo, and runc.

Security Fix(es):

• runc: container escape via 'masked path' abuse due to mount race conditions (CVE-2025-31133)

• runc: container escape with malicious config due to /dev/console mount and related races (CVE-2025-52565)

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The podman tool manages pods, container images, and containers. It is part of the libpod library, which is for applications that use container pods. Container pods is a concept in Kubernetes.

Security Fix(es):

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The buildah package provides a tool for facilitating building OCI container images. Among other things, buildah enables you to: Create a working container, either from scratch or using an image as a starting point; Create an image, either from a working container or using the instructions in a Dockerfile; Build both Docker and OCI images.

Security Fix(es):

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

• golang: archive/tar: Unbounded allocation when parsing GNU sparse map (CVE-2025-58183)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The buildah package provides a tool for facilitating building OCI container images. Among other things, buildah enables you to: Create a working container, either from scratch or using an image as a starting point; Create an image, either from a working container or using the instructions in a Dockerfile; Build both Docker and OCI images.

Security Fix(es):

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

• golang: archive/tar: Unbounded allocation when parsing GNU sparse map (CVE-2025-58183)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The container-tools module contains tools for working with containers, notably podman, buildah, skopeo, and runc.

Security Fix(es):

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

runc is a CLI tool for spawning and running containers according to the OCI specification. In versions 1.2.7, 1.3.2 and 1.4.0-rc.2, an attacker can trick runc into misdirecting writes to /proc to other procfs files through the use of a racing container with shared mounts (we have 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. While similar, the mitigation applied for the related CVE, CVE-2019-19921, was fairly limited and effectively only caused runc to verify that when LSM labels are written they are actually procfs files. This issue is fixed in versions 1.2.8, 1.3.3, and 1.4.0-rc.3.

The runC tool is a lightweight, portable implementation of the Open Container Format (OCF) that provides container runtime.

Security Fix(es):

• runc: container escape via 'masked path' abuse due to mount race conditions (CVE-2025-31133)
• runc: container escape with malicious config due to /dev/console mount and related races (CVE-2025-52565)
• runc: container escape and denial of service due to arbitrary write gadgets and procfs write redirects (CVE-2025-52881)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The runC tool is a lightweight, portable implementation of the Open Container Format (OCF) that provides container runtime.

Security Fix(es):

• runc: container escape via 'masked path' abuse due to mount race conditions (CVE-2025-31133)
• runc: container escape with malicious config due to /dev/console mount and related races (CVE-2025-52565)
• runc: container escape and denial of service due to arbitrary write gadgets and procfs write redirects (CVE-2025-52881)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The podman tool manages pods, container images, and containers. It is part of the libpod library, which is for applications that use container pods. Container pods is a concept in Kubernetes.

Security Fix(es):

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

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The container-tools module contains tools for working with containers, notably podman, buildah, skopeo, and runc.

Security Fix(es):

• runc: container escape via 'masked path' abuse due to mount race conditions (CVE-2025-31133)
• runc: container escape with malicious config due to /dev/console mount and related races (CVE-2025-52565)
• runc: container escape and denial of service due to arbitrary write gadgets and procfs write redirects (CVE-2025-52881)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.