| Summary | runc container escape via "masked path" abuse due to mount race conditions
### Impact ###
The OCI runtime specification has a `maskedPaths` feature that allows for files or directories to be "masked" by placing a mount on top of them to conceal their contents. This is primarily intended to protect against privileged users in non-user-namespaced from being able to write to files or access directories that would either provide sensitive information about the host to containers or allow containers to perform destructive or other privileged operations on the host (examples include `/proc/kcore`, `/proc/timer_list`, `/proc/acpi`, and `/proc/keys`).
`maskedPaths` can be used to either mask a directory or a file -- directories are masked using a new read-only `tmpfs` instance that is mounted on top of the masked path, while files are masked by bind-mounting the container's `/dev/null` on top of the masked path.
In all known versions of runc, when using the container's `/dev/null` to mask 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. While `/dev/null` is usually created by runc when doing container creation, it is possible for an attacker to create a `/dev/null` or modify the `/dev/null` inode created by runc through race conditions with other containers sharing 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 could lead to two separate issues:
#### Attack 1: Arbitrary Mount Gadget (leading to Host Information Disclosure, Host Denial of Service, or Container Escape) ####
By replacing `/dev/null` with a symlink to an attacker-controlled path, an attacker could cause runc to bind-mount an arbitrary source path to a path inside the container. This could lead to:
* **Host Denial of Service**: By bind-mounting files such as `/proc/sysrq-trigger`, the attacker can gain access to a read-write version of files which can be destructive to write to (`/proc/sysrq-trigger` would allow an attacker to trigger a kernel panic, shutting down the machine, or causing the machine to freeze without rebooting).
* **Container Escape**: By bind-mounting `/proc/sys/kernel/core_pattern`, the attacker can reconfigure a coredump helper -- as kernel upcalls are not namespaced, the configured binary (which could be a container binary or a host binary with a malicious command-line) will run with full privileges on the host system. Thus, the attacker can simply trigger a coredump and gain complete root privileges over the host.
Note that while `config.json` allows users to bind-mount arbitrary paths (and thus an attacker that can modify `config.json` arbitrarily could gain the same access as this exploit), because `maskedPaths` is applied by almost all higher-level container runtimes (and thus provides a guaranteed mount source) this flaw effectively allows any attacker that can spawn containers (with some degree of control over what kinds of containers are being spawned) to achieve the above goals.
#### Attack 2: Bypassing `maskedPaths` ####
While investigating Attack 1, runc discovered that the runc validation mechanism when bind-mounting `/dev/null` for `maskedPaths` would ignore `ENOENT` errors -- meaning that if an attacker deleted `/dev/null` before runc did the bind-mount, runc would silently skip applying `maskedPaths` for the container. (The original purpose of this `ENOENT`-ignore behaviour was to permit configurations where `maskedPaths` references non-existent files, but runc did not consider that the source path could also not exist in this kind of race-attack scenario.)
With `maskedPaths` rendered inoperative, an attacker would be able to access sensitive host information from files in `/proc` that would usually be masked (such as `/proc/kcore`). However, note that `/proc/sys` and `/proc/sysrq-trigger` are mounted read-only rather than being masked with files, so this attack variant will not allow the same breakout or host denial of service attacks as in Attack 1.
### 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")
* 8476df83b534 ("libct: add/use isDevNull, verifyDevNull")
* 1a30a8f3d921 ("libct: maskPaths: only ignore ENOENT on mount dest")
* 5d7b24240724 ("libct: maskPaths: don't rely on ENOTDIR for mount")
runc 1.2.8, 1.3.3, and 1.4.0-rc.3 have been released and all contain fixes for these issues. As per [runc's new release model](https://github.com/opencontainers/runc/blob/v1.4.0-rc.2/RELEASES.md), runc 1.1.x and earlier are no longer supported and thus have not been patched. https://github.com/opencontainers/runc/blob/v1.4.0-rc.2/RELEASES.md
### Mitigations ###
- Use containers with user namespaces (with the host root user not mapped into the container's user namespace). This will block most of the most serious aspects of these attacks, as the `procfs` files used for the container breakout use Unix DAC permissions and user namespaced users will not have access to the relevant files.
runc would also like to take this opportunity to re-iterate that runc **strongly** recommend all users use user namespaced containers. They have proven to be one of the best security hardening mechanisms against container breakouts, and the kernel applies additional restrictions to user namespaced containers above and beyond the user remapping functionality provided. With the advent of id-mapped mounts (Linux 5.12), there is very little reason to not use user namespaces for most applications. Note that using user namespaces to configure your container does not mean you have to enable unprivileged user namespace creation *inside* the container -- most container runtimes apply a seccomp-bpf profile which blocks `unshare(CLONE_NEWUSER)` inside containers regardless of whether the container itself uses user namespaces.
Rootless containers can provide even more protection if your configuration can use them -- by having runc itself be an unprivileged process, in general you would expect the impact scope of a runc bug to be less severe as it would only have the privileges afforded to the host user which spawned runc.
- For non-user namespaced containers, configure all containers you spawn to not permit processes to run with root privileges. In most cases this would require configuring the container to use a non-root user and enabling `noNewPrivileges` to disable any setuid or set-capability binaries. (Note that this is runc's general recommendation for a secure container setup -- it is very difficult, if not impossible, to run an untrusted program with root privileges safely.) If you need to use `ping` in your containers, there is a `net.ipv4.ping_group_range` sysctl that can be used to allow unprivileged users to ping without requiring setuid or set-capability binaries.
- Do not run untrusted container images from unknown or unverified sources.
- Depending on the configuration of `maskedPaths`, an AppArmor profile (such as the default one applied by higher level runtimes including Docker and Podman) can block write attempts to most of `/proc` and `/sys`. This means that even with a procfs file maliciously bind-mounted to a `maskedPaths` target, all of the targets of `maskedPaths` in the default configuration of runtimes such as Docker or Podman will still not permit write access to said files. However, if a container is configured with a `maskedPaths` that is not protected by AppArmor then the same attack can be carried out. Please note that CVE-2025-52881 allows an attacker to bypass LSM labels, and so this mitigation is not that helpful when considered in combination with CVE-2025-52881.
- Based on runc's analysis, SELinux policies have a limited effect when trying to protect against this attack. The reason is that the `/dev/null` bind-mount gets implicitly relabelled with `context=...` set to the container's SELinux context, and thus the container process will have access to the source of the bind-mount even if they otherwise wouldn't.
https://github.com/opencontainers/runc/security/advisories/GHSA-cgrx-mc8f-2prm
### Other Runtimes ###
As this vulnerability boils down to a fairly easy-to-make logic bug, runc has provided information to other OCI (crun, youki) and non-OCI (LXC) container runtimes about this vulnerability. Based on discussions with other runtimes, it seems that crun and youki may have similar security issues and will release a coordinated security release along with runc. LXC appears to also be vulnerable in some aspects, but [their security stance](https://linuxcontainers.org/lxc/security/) is (understandably) that non-user-namespaced containers are fundamentally insecure by design.
https://linuxcontainers.org/lxc/security/
### Credits ###
Thanks to Lei Wang (@ssst0n3 from Huawei) for finding and reporting the original vulnerability (Attack 1), and Li Fubang (@lifubang from acmcoder.com, CIIC) for discovering another attack vector (Attack 2) based on @ssst0n3's initial findings. |
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