GHSA-9ccr-r5hg-74gf: GitHub Copilot CLI: Nested Bare Repository Can Execute Arbitrary Commands via core.fsmonitor
SummaryA security vulnerability has been identified in GitHub Copilot CLI where a malicious bare git repository nested inside a project directory can achieve arbitrary code execution when the agent performs git operations. By exploiting git's automatic bare repository discovery during directory traversal, an attacker can set `core.fsmonitor` or other executable config keys to run arbitrary commands without user awareness or approval.DetailsGit supports bare repositories — repositories without a working tree — which can be discovered automatically when git traverses the directory hierarchy looking for a `.git` directory. When git discovers a bare repository, it reads and applies its configuration, including keys that specify external commands to execute.The vulnerability arises because git's `core.fsmonitor` config key (and 15+ similar keys such as `core.hookspath`, `diff.external`, `merge.tool`, etc.) can specify arbitrary shell commands that git will execute as part of normal operations like `status`, `diff`, or `rev-parse`.Attack ScenarioAn attacker can exploit this by:Creating a bare git repository nested inside a seemingly normal project directory (e.g., `vendor/malicious.git/` or a deeply nested subdirectory)Configuring `core.fsmonitor` (or similar keys) in that bare repository to execute a malicious commandWhen GitHub Copilot CLI performs any git operation that traverses into or through that directory, git auto-discovers the bare repository, reads its config, and executes the attacker's commandThis can occur when:The agent navigates into a subdirectory containing the buried bare repoThe agent runs `git status`, `git diff`, or other routine git commandsThe agent uses tools like `grep` or `glob` that may trigger git operations in subdirectoriesPrior to the fix, the CLI had no protection against git auto-discovering bare repositories during directory traversal.ImpactAn attacker who can place a malicious bare repository inside a project — for example, through:A pull request adding a directory that contains a bare repositoryA compromised or malicious dependency that includes a bare repositoryA cloned repository that already contains nested bare repositories— could achieve arbitrary code execution on the user's workstation whenever GitHub Copilot CLI performs git operations in or near the malicious directory.Successful exploitation could lead to data exfiltration, credential theft, file modification, or further system compromise.Affected VersionsGitHub Copilot CLI versions prior to 1.0.42Remediation and MitigationFixThe fix sets `safe.bareRepository=explicit` via git's `GIT_CONFIG_COUNT` / `GIT_CONFIG_KEY_*` / `GIT_CONFIG_VALUE_*` environment variable mechanism, which has the highest precedence over all config file sources. This prevents git from automatically discovering and using bare repositories during directory traversal — only explicitly allowlisted bare repositories will be used.User Actions**Upgrade** GitHub Copilot CLI to **1.0.43** or later.**Exercise caution** when working in repositories that contain nested bare git repositories.**Review** project directories for unexpected bare repositories, especially in `vendor/`, `third_party/`, or deeply nested subdirectories.
CVSS v4.0: 8.5
CVSS v3.1: 7.8