Description
Moby is the open source Linux container runtime and set of components used to build a variety of downstream container runtimes, including Docker CE, Mirantis Container Runtime (formerly Docker EE), and Docker Desktop. Moby allows for building container images using a set of build instructions (usually named and referred to as a "Dockerfile"), and a build context, which is not unlike the CWD in which the Dockerfile instructions are executed.
Containers may be built using a variety of tools and build backends available in the Moby ecosystem; in all cases, builds may not include files outside of the build context (such as using absolute or relative-parent paths). This is enforced through both checks in the build backends, and the containerization of the build process itself.
Versions of Git where CVE-2022-39253 is present and exploited by a malicious repository, when used in combination with Moby, are subject to an unexpected inclusion of arbitrary filesystem paths in the build context, without any visible warning to the user.
This issue was originally reported by Wenxiang Qian of Tencent Blade Team, and the root-cause analysis was performed by Cory Snider of Mirantis, with assistance from Bjorn Neergaard of the same. The issue was then reported to the Git project, and Taylor Blau led the process resolving the root issue in Git.
Impact
This vulnerability originates in Git, but can be used to violate assumptions that may have security implications for users of Moby and related components. Users may rely on the fact that a build context ensures that outside files cannot be referenced or incorporated using multiple enforcement mechanisms, or expect a warning if this does not hold true. A maliciously crafted Git repository exploiting CVE-2022-39253 can violate this assumption, and potentially include sensitive files that are subsequently uploaded to a container image repository, or disclosed by code inside the resulting container image.
As this issue cannot be triggered remotely, except by users who already have full control over the daemon through the API, and it requires exploiting a vulnerability in Git by convincing a user to build a maliciously crafted repository, the impact in Moby is considered low.
Patches
Moby 20.10.20, and Mirantis Container Runtime (formerly Docker Enterprise Edition) 20.10.14 will contain mitigations for CVE-2022-39253 when a Git clone is performed by Moby components (on either the daemon or API client side). However, as these mitigations only apply to certain scenarios (build of git+<protocol>://... URL contexts) and cannot protect against a malicious repository already on disk, users should update to a version of Git containing patches for CVE-2022-39253 on all their systems running both API clients and daemons.
Specifically, patches in Moby (including patches incorporated from BuildKit) protect against the following:
docker build with the legacy builder (e.g. DOCKER_BUILDKIT unset or set to 0) of a Git URL context. Note that depending on available API versions and the CLI version, the Git clone operation can take place on either the client or the daemon side. Both must be updated (or have Git updated) to fully protect this build method.docker build with the BuildKit builder (e.g. DOCKER_BUILDKIT=1) of a Git URL context.docker buildx build with BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 of a Git URL context.
Patches in BuildKit incorporated into Docker Compose protect against CVE-2022-39253 during Compose-driven builds of Git URL contexts.
Patches in Moby and related projects such as BuildKit, the Docker CLI, and Docker Compose cannot fully protect against CVE-2022-39253, as it may be triggered by a malicious repository already on disk that a unpatched Git client has interacted with (specifically, commands that check out submodules such as git clone --recursive, git submodule update, etc. may have already triggered the Git vulnerability).
Workarounds
While this behavior is unexpected and undesirable, and has resulted in this security advisory, users should keep in mind that building a container entails arbitrary code execution. Users should not build a repository/build context they do not trust, as containerization cannot protect against all possible attacks.
When building with BuildKit (e.g. docker buildx build or docker build with DOCKER_BUILDKIT=1), this issue cannot be exploited unless --build-arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 was also passed, as by default BuildKit will discard the .git directory of a Git URL context immediately after cloning and checking out the repository.
For more information
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