CVE-2024-23651: Moby BuildKit Race Condition Vulnerability

CVE-2024-23651 Overview

CVE-2024-23651 is a race condition vulnerability in BuildKit, a toolkit for converting source code to build artifacts. When two malicious build steps run in parallel and share the same cache mounts with subpaths, a race condition can occur that allows files from the host system to become accessible to the build container. This vulnerability represents a significant container escape risk that could lead to unauthorized access to sensitive host system data.

Critical Impact

A successful exploitation of this race condition vulnerability could allow attackers to break container isolation and access files on the host system, potentially exposing sensitive configuration files, credentials, or other confidential data stored outside the container boundary.

Affected Products

• Mobyproject BuildKit versions prior to v0.12.5
• Docker builds using vulnerable BuildKit versions with cache mounts
• CI/CD systems utilizing BuildKit for container image builds

Discovery Timeline

• 2024-01-31 - CVE-2024-23651 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-23651

Vulnerability Analysis

This vulnerability exploits a Time-of-Check Time-of-Use (TOCTOU) race condition in BuildKit's cache mount handling mechanism. BuildKit is designed to efficiently manage build caches through mount directives, allowing multiple build steps to share cached data for performance optimization. However, when multiple build steps execute concurrently and access the same cache mounts with subpaths using the --mount=type=cache,source=... option, improper synchronization creates a window where file access controls can be bypassed.

The vulnerability is particularly concerning because it undermines the fundamental security assumption that containers are isolated from the host file system. An attacker who can control or influence a Dockerfile being built could craft malicious build instructions that exploit this race condition to escape the container sandbox.

Root Cause

The root cause of CVE-2024-23651 lies in insufficient synchronization primitives protecting shared cache mount operations during parallel build step execution. When BuildKit processes concurrent build steps that reference the same cache mount paths, the lack of proper locking or atomic operations creates a race window. During this window, the file system state can become inconsistent, allowing one build step to access files that should be outside its permitted scope.

The CWE-362 (Race Condition) classification accurately describes this flaw, as the vulnerability arises from concurrent execution where the relative timing of events affects the correctness of the program's security boundaries.

Attack Vector

Exploitation requires an attacker to craft a malicious Dockerfile or BuildKit frontend that contains parallel build steps sharing cache mounts with carefully chosen subpaths. The attack vector is network-based since Dockerfiles can be distributed via container registries, Git repositories, or other network-accessible sources. However, the attack complexity is elevated because successful exploitation depends on winning the race condition, which requires precise timing.

The attack scenario involves executing parallel build steps that both utilize cache mounts pointing to overlapping paths. By manipulating the timing of file system operations within these parallel steps, an attacker can cause the build container to gain access to host files that should be protected by container isolation boundaries.

For detailed technical information about the exploitation mechanism, refer to the GitHub Security Advisory GHSA-m3r6-h7wv-7xxv.

Detection Methods for CVE-2024-23651

Indicators of Compromise

• Unusual file access patterns within build containers accessing paths outside expected cache directories
• Build logs showing parallel execution of steps with overlapping --mount=type=cache,source=... directives
• Unexpected reads of sensitive host system files such as /etc/passwd, /etc/shadow, or credential files during container builds
• Build containers exhibiting access to file system paths that should be outside their mount scope

Detection Strategies

• Monitor BuildKit build logs for Dockerfiles containing multiple parallel cache mount operations with subpath specifications
• Implement file integrity monitoring on the host system to detect unauthorized reads during container build operations
• Deploy runtime container security solutions that can detect anomalous file access patterns indicating container escape attempts
• Audit Dockerfiles in CI/CD pipelines for suspicious --mount=type=cache,source=... patterns with overlapping paths

Monitoring Recommendations

• Enable verbose logging for BuildKit operations to capture cache mount activities during builds
• Implement security scanning in CI/CD pipelines to identify Dockerfiles from untrusted sources containing cache mount directives
• Monitor for BuildKit processes exhibiting unusual file system access patterns that cross container boundaries
• Configure alerts for builds executing parallel steps with cache mounts to facilitate manual review

How to Mitigate CVE-2024-23651

Immediate Actions Required

• Upgrade BuildKit to version v0.12.5 or later immediately to address the race condition vulnerability
• Audit all Dockerfiles in use to identify those containing --mount=type=cache,source=... directives
• Avoid building Dockerfiles from untrusted sources until the environment is patched
• Review and restrict the use of BuildKit frontends from untrusted sources

Patch Information

The BuildKit maintainers have addressed this vulnerability in version v0.12.5. The fix implements proper synchronization for cache mount operations to prevent the race condition. Organizations should update their BuildKit installations as follows:

• Review the GitHub Pull Request #4604 for details on the fix implementation
• Download the patched version from the GitHub Release v0.12.5
• Consult the GitHub Security Advisory GHSA-m3r6-h7wv-7xxv for complete remediation guidance

Workarounds

• Avoid using BuildKit frontends from untrusted sources until patching is complete
• Do not build Dockerfiles from untrusted origins that contain cache mount directives with --mount=type=cache,source=... options
• Implement strict Dockerfile review processes to identify and reject builds containing suspicious cache mount configurations
• Consider running builds in isolated environments with minimal host file system exposure

# Verify BuildKit version and upgrade if necessary
buildctl --version

# If using Docker with BuildKit, ensure DOCKER_BUILDKIT is using patched version
docker buildx version

# Example: Update BuildKit via Docker Buildx
docker buildx create --use --driver docker-container --driver-opt image=moby/buildkit:v0.12.5