Buildkit GRPC SecurityMode privilege check: Build-time container breakout (CVE-2024-23653)
Tags: containersecurity, vulnerabilityinsights, kubernetes, docker
Published: 2024-02-02 02:00:51

Snyk has discovered a vulnerability in all versions of Docker Buildkit `<= v0.12.4`, as used by the Docker engine. The exploitation of this issue can result in container escape to the underlying host OS when building an image using a malicious Dockerfile or upstream image (i.e, when using `FROM`). This issue has been assigned CVE-2024-23653.


How does CVE-2024-23653 work?
-----------------------------


The vulnerability occurs due to a missing privilege check in a GRPC endpoint when called using a custom syntax format. Docker allows for the specification of a custom input format parser in place of the normal Dockerfile syntax by specifying the line `# syntax=` at the top of the input file. This line defines another Docker image, which will be used to parse the input into an intermediate representation. The parsing Docker image has access to a GRPC server with multiple endpoints to enable the creation and submission of the intermediate representation. One of these endpoints (`Container.Start`), which allows for the execution of build-time ephemeral containers, does not validate the `StartRequest.SecurityMode` argument against the expectations of the Buildkit configuration. This argument defines the privileges of the resultant container, enabling the parsing Docker image to launch a elevated privilege container during build time.


**In a successful attack, the now-executing privileged container can use its elevated privileges (for example, full Linux capabilities) to escape from the container and achieve full host root command execution.**


How to mitigate CVE-2024-23653
------------------------------


Buildkit mitigated this vulnerability by ensuring that the `StartRequest.SecurityMode` argument was checked against the security expectations of the Buildkit daemon to only allow elevated privilege container creation via this API when it is explicitly permitted.


**Synk recommends taking immediate action following the** [**Buildkit advisory**](https://github.com/moby/buildkit/security/advisories/GHSA-wr6v-9f75-vh2g) **to mitigate this security vulnerability.** The Buildkit v0.12.5 release includes patches for this issue. Technologies that bundle Buildkit (such as Docker Engine and Docker Desktop) should also be updated to their respective patched versions.


Evaluating risk
---------------


This vulnerability depends on a malicious container running or building within a vulnerable infrastructure. Due to the nature of this vulnerability, detection is difficult and requires runtime for the most accurate detection. As our existing Snyk products don’t operate on the application runtime, we built two tools to make the detection of this vulnerability feasible.


### Runtime detection


The [new Helios team](/news/snyk-acquires-runtime-data-pioneer-helios/) at Snyk has built an an [eBPF-based](https://ebpf.io/) runtime detection tool for this vulnerability, which can be found at [leaky-vessels-dynamic-detector](https://www.github.com/snyk/leaky-vessels-dynamic-detector), released under the Apache-2.0 license. eBPF is a commonly used instrumentation mechanism built into many modern Linux kernels.


**This tool can identify a running container attempting to exercise this vulnerability on underlying infrastructure, placing the underlying host at risk. Note that this tool cannot prevent the exploitation of this vulnerability, only warn of exposure.**


While updating your container infrastructure is our strong recommendation, you may be able to evaluate your risk or exposure using this tooling when updating is not immediately possible. Organizations should check with their container infrastructure provider to understand whether its infrastructure has been patched.


#### Installation and usage


To install and use our static detection tool, please do the following:


1. Clone the repository at <https://github.com/snyk/leaky-vessels-dynamic-detector>
2. Build the Go binary with `go build`.
3. Run the detector in your CI environment with: `sudo ./ebpf-detector`


For additional details, please check the [README.md](https://github.com/snyk/leaky-vessels-dynamic-detector/blob/main/README.md) file.


### Static analysis


In case you prefer not to run an external binary on your host or CI/CD pipeline, we’ve also built a static analysis detector, found at [https://github.com/snyk/leaky-vessels-static-detector](https://www.github.com/snyk/leaky-vessels-static-detector), that analyzes Dockerfiles and flags potential exploit attempts.


This tool can also examine images by parsing the `FROM` instruction or directly querying the local Docker daemon or public container registries like Dockerhub or GCR. Although the image history doesn’t include all the instructions a full Dockerfile would, it can show the `WORKDIR` and `ONBUILD` instructions of the parent image, which can indicate potentially nefarious images.


This is useful to protect against cases where a 3rd party base image has been compromised and tries to exploit the `run` or `build` commands of the child image. The static approach has a higher false positive (noise) rate, especially for exploits using the `--mount flag`, but can find images worthy of further investigation.


The Snyk team has performed ad hoc checks of Dockerfiles from public registries based on the images we see being used most frequently. While not exhaustive, our research did not find evidence suggesting that these vulnerabilities have been exploited. Snyk recommends that you continue monitoring your own environment and check your containers until patches are made available and deployed.


As always, using well-maintained parent images from trusted sources and staying up-to-date with the latest versions — including clearing out build caches and verifying the provenance of parent images — is a good best practice.


#### Installation and usage


To install and use our static detection tool, please do the following:


1. Clone the repository at [https://github.com/snyk/leaky-vessels-static-detector](https://www.github.com/snyk/leaky-vessels-static-detector)
2. Build the Go binary with `go build`.
3. Run the detector with: `./static-detector dockerfile -f [PATH_TO_DOCKERFILE].`
4. For additional execution options, please check the [README.md](https://github.com/snyk/leaky-vessels-static-detector/blob/main/README.md) file.


CVE-2024-23653 summary and advice
---------------------------------


This container breakout vulnerability is severe and has the potential to cause damage to any underlying host infrastructure that is building containers.


**Snyk recommends you update any instances of Buildkit to version v0.12.5 or later, as well as any software that depends on Buildkit.**


Snyk has also disclosed several other Docker vulnerabilities in our research, assigned CVEs: [CVE-21626](/blog/cve-2024-21626-runc-process-cwd-container-breakout), [CVE-2024-23651](/blog/cve-2024-23651-docker-buildkit-mount-cache-race), [CVE-2024-23652](/blog/cve-2024-23652-buildkit-build-time-container-teardown-arbitrary-delete). You can also read the [summary of our findings](/blog/leaky-vessels-docker-runc-container-breakout-vulnerabilities) across all the CVEs.


*This article is provided for informational purposes only. Snyk is not responsible for any errors or omissions, or for the results obtained from the use of this information.*