Package Instance

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.

runc container escape with malicious config due to /dev/console mount and related races
### Impact ###
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). 

In runc version 1.0.0-rc3 and later, 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 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). 

The reason that the attacker can gain write access to these files is because the `/dev/console` bind-mount happens before `maskedPaths` and `readonlyPaths` are applied.

#### Additional Findings ####
While investigating this issue, runc discovered some other theoretical issues that may or may not be exploitable, as well as taking the opportunity to fix some fairly well-known issues related to consoles.

##### Issue 1: Problematic Usage of `os.Create` #####
Go provides an `os.Create` function for creating files, which older code in runc (dating back to the original `libcontainer` from the early 2010s) had a tendency to use fairly liberally. `os.Create` implies `O_CREAT|O_TRUNC` but by design it does not apply `O_NOFOLLOW` nor `O_EXCL`, meaning if the target is swapped with a malicious symlink runc can be tricked into truncating host files (which can lead to denial of service attacks, among other concerns). 

Runc conducted an audit of all `os.Create` usages in runc and found some suspicious usages related to device inodes, but based on runc's testing these were not exploitable in practice. Runc now has custom code lints to block any `os.Create` usage in runc, and plan to do a further audit of any other plain `os.*` operation usage throughout runc after this advisory becomes public. 

CVE-2024-45310 was a similar attack but without the `O_TRUNC` component (which resulted in a "Low" severity) -- a similar attack being exploitable would've been much more severe.

##### Issue 2: Malicious `/dev/pts/$n` Inode Attacks (`TIOCGPTPEER`) #####
The (very) classic API for constructing consoles involves first opening `/dev/ptmx` for reading and writing. This allocates a new pseudo-terminal and the returned file descriptor is the "master" end (which is used by higher-level runtimes to do I/O with the container). 

Traditionally, in order to get the "slave" end, you do `ioctl(ptm, TIOCGPTN)` to get the pseudo-terminal number and then open the file in `/dev/pts/` with the corresponding base-10 decimal number of the number returned by `TIOCGPTN`. The naive way of doing this is vulnerable to very basic race attacks where `/dev/pts/$n` is replaced with a different pseudo-terminal or other malicious file. 

In order to provide a mechanism to mitigate this risk, Aleksa Sarai (@cyphar from SUSE) implemented `TIOCGPTPEER` back in 2017 to provide a race-free way of doing the last `TIOCGPTN` step by opening the peer end of the pseudo-terminal directly. However, at the time it was believed to be too impractical to implement this protection in runc due to its no-monitor-process architecture (unlike runtimes like LXC which made use of `TIOCGPTPEER` almost immediately). While working on this advisory, runc found a way to make `TIOCGPTN` usage on pre-4.13 kernels still safe against race attacks and so have implemented both `TIOCGPTPEER` support as well as safe `TIOCGPTN` support as a fallback. 

Another possible target of attack would be replacing `/dev/ptmx` or `/dev/pts/ptmx` with a different inode and tricking runc into trying to operate on it. This is very similar to the core issue in CVE-2025-31133 and had a similar solution. 

Runc's analysis was that while this attack appears to be potentially problematic in theory, it seems unlikely to actually be exploitable due to how consoles are treated (runc tries to do several pseudo-terminal-specific `ioctl`s and will error out if they fail -- which happens for most other file types). In principle you could imagine a DoS attack using a disconnected NFS handle but it seems impractical to exploit. However, runc felt it prudent to include a solution (and this also provides a safe mechanism to get the source mount for the `/dev/console` bind-mount issue at the beginning of this advisory).

### 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")
 * ff94f9991bd3 ("*: switch to safer securejoin.Reopen")
 * 531ef794e4ec ("console: use TIOCGPTPEER when allocating peer PTY")
 * 398955bccb7f ("console: add fallback for pre-TIOCGPTPEER kernels")
 * 9be1dbf4ac67 ("console: avoid trivial symlink attacks for /dev/console")
 * de87203e625c ("console: verify /dev/pts/ptmx before use")
 * 01de9d65dc72 ("rootfs: avoid using os.Create for new device inodes")
 * aee7d3fe355d ("ci: add lint to forbid the usage of os.Create")

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.

[CVE-2025-31133]: https://github.com/opencontainers/runc/security/advisories/GHSA-9493-h29p-rfm2
[CVE-2025-52565]: https://github.com/opencontainers/runc/security/advisories/GHSA-qw9x-cqr3-wc7r
[CVE-2025-52881]: https://github.com/opencontainers/runc/security/advisories/GHSA-cgrx-mc8f-2prm
[RELEASES.md]: 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. 

An attacker would still be able to bind-mount host paths into the container but if the host uids and gids mapped into the container do not overlap with ordinary users on the host (which is the generally recommended configuration) then the attacker would likely not be able to read or write to most sensitive host files (depending on the Unix DAC permissions of the host files). Note that this is still technically more privilege than an unprivileged user on the host -- because the bind-mount is done by a privileged process, the attacker would be able to get access to directories whose parents may have denied search access (i.e., they may be able to access paths inside a `chmod 700` directory that would normally block them from resolving subpaths). 

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.
 * The default `containers-selinux` SELinux policy mitigates this issue, as (unlike CVE-2025-31133) the `/dev/console` bind-mount does not get relabeled and so the container process cannot write to the bind-mounted procfs file by default.

   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.

 * The default AppArmor policy used by Docker and Podman does not mitigate this issue (as access to `/dev/console`) is usually permitted. Users could create a custom profile that blocks access to `/dev/console`, but such a profile might break regular containers.

   Please note that CVE-2025-52881 allows an attacker to bypass LSM labels, and so the mitigation provided with a custom profile is not that helpful when considered in combination with CVE-2025-52881.

[CVE-2025-31133]: https://github.com/opencontainers/runc/security/advisories/GHSA-9493-h29p-rfm2
[CVE-2025-52881]: 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 co-ordinated security release along with runc. LXC appears to also be vulnerable in some aspects, but [their security stance][lxc-security] is (understandably) that non-user-namespaced containers are fundamentally insecure by design.

[lxc-security]: https://linuxcontainers.org/lxc/security/

### Credits ###

Thanks to Lei Wang (@ssst0n3 from Huawei) and Li Fubang (@lifubang from acmcoder.com, CIIC) for discovering and reporting the main `/dev/console` bind-mount vulnerability, as well as Aleksa Sarai (@cyphar from SUSE) for discovering Issues 1 and 2 and the original research into these classes of issues several years ago.

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 `/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 github.com/cyphar/filepath-securejoin@v0.5.0")
 * 77889b56db93 ("internal: add wrappers for securejoin.Proc*")
 * fdcc9d3cad2f ("apparmor: use safe procfs API for labels")
 * ff6fe1324663 ("utils: use safe procfs for /proc/self/fd loop code")
 * b3dd1bc562ed ("utils: remove unneeded EnsureProcHandle")
 * 77d217c7c377 ("init: write sysctls using safe procfs API")
 * 435cc81be6b7 ("init: use securejoin for /proc/self/setgroups")
 * d61fd29d854b ("libct/system: use securejoin for /proc/$pid/stat")
 * 4b37cd93f86e ("libct: align param type for mountCgroupV1/V2 functions")
 * d40b3439a961 ("rootfs: switch to fd-based handling of mountpoint targets")
 * ed6b1693b8b3 ("selinux: use safe procfs API for labels")
   - Please note that this patch includes a private patch for `github.com/opencontainers/selinux` that could not be made public through a public pull request (as it would necessarily disclose this embargoed security issue).

     The patch includes a complete copy of the forked code and a `replace` directive (as well as `go mod vendor` applied), which should still work with downstream build systems. If you cannot apply this patch, you can safely drop it -- some of the other patches in this series should block these kinds of racing mount attacks entirely.

     See https://github.com/opencontainers/selinux/pull/237 for the upstream patch.
 * 3f925525b44d ("rootfs: re-allow dangling symlinks in mount targets")
 * a41366e74080 ("openat2: improve resilience on busy systems")

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][RELEASES.md], runc 1.1.x and earlier are no longer supported and thus have not been patched.

[CVE-2025-31133]: https://github.com/opencontainers/runc/security/advisories/GHSA-9493-h29p-rfm2
[CVE-2025-52565]: https://github.com/opencontainers/runc/security/advisories/GHSA-qw9x-cqr3-wc7r
[CVE-2025-52881]: https://github.com/opencontainers/runc/security/advisories/GHSA-cgrx-mc8f-2prm
[RELEASES.md]: https://github.com/opencontainers/runc/blob/v1.4.0-rc.2/RELEASES.md

### Mitigations ###

 * Do not run untrusted container images from unknown or unverified sources.

 * For the basic no-op attack, this attack allows a container process to run with the same LSM labels as `runc`. For most AppArmor deployments this means it will be `unconfined`, and for SELinux it will likely be `container_runtime_t`. Runc has not conducted in-depth testing of the impact on SELinux -- it is possible that it provides some reasonable protection but it seems likely that an attacker could cause harm to systems even with such an SELinux setup.

 * For the more involved redirect and write gadget attacks, unfortunately most LSM profiles (including the standard container-selinux profiles) provide the container runtime access to sysctl files (including `/proc/sysrq-trigger`) and so LSMs likely do not provide much protection against these attacks.

 * Using rootless containers provides some protection against these kinds of bugs (privileged writes in runc being redirected) -- 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 this particular bug, the privilege escalation caused by the inadvertent write issue is entirely mitigated with rootless containers because the unprivileged user that the `runc` process is executing as cannot write to the aforementioned procfs files (even intentionally).

### 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 co-ordinated security release along with runc. LXC appears to use the host's `/proc` for all procfs operations, and so is likely not vulnerable to this issue (this is a trade-off -- runc uses the container's procfs to avoid CVE-2016-9962-style attacks).

[CVE-2016-9962]: https://seclists.org/fulldisclosure/2017/Jan/21

### Credits ###

Thanks to Li Fubang (@lifubang from acmcoder.com, CIIC) and Tõnis Tiigi (@tonistiigi from Docker) for both independently discovering this vulnerability, as well as Aleksa Sarai (@cyphar from SUSE) for the original research into this class of security issues and solutions.

Additional thanks go to Tõnis Tiigi for finding some very useful exploit templates for these kinds of race attacks using `docker buildx build`.

Information Exposure in RunC
RunC allowed additional container processes via 'runc exec' to be ptraced by the pid 1 of the container.  This allows the main processes of the container, if running as root, to gain access to file-descriptors of these new processes during the initialization and can lead to container escapes or modification of runC state before the process is fully placed inside the container.