Bucket-Based Remote Cache Poisoning Vulnerability in Build Systems

A critical vulnerability allows contributors with pull request privileges to inject compromised artifacts from untrusted environments into trusted production environments. This issue arises in remote cache extensions for common build systems that use bucket-based remote caching, such as those relying on Amazon S3 or Google Cloud Storage. The vulnerability exploits the 'first-to-cache wins' principle, where artifacts from untrusted sources can poison the cache for trusted environments. This cache poisoning occurs during the artifact construction phase, before any security measures like encryption or checksum validation are applied, bypassing traditional security controls and leading to undetectable compromises in production.

Impact

Exploitation of this vulnerability allows for cache poisoning, where untrusted artifacts are introduced into the production environment. This can lead to unauthorized code execution, data exfiltration, lateral movement within the organization's infrastructure, installation of backdoors, compromise of the supply chain, theft of credentials, deployment of ransomware, competitive sabotage, and execution of time-delayed attacks.

Reproduction

The vulnerability can be reproduced by creating a pull request from an untrusted environment (such as a feature branch) and modifying the CI configuration to inject malicious code. Once the untrusted build completes, the poisoned artifact is uploaded to the shared cache, which is then accessed by the trusted environment, executing the compromised code with full privileges.

Remediation

There is no effective mitigation for this vulnerability in bucket-based remote cache solutions due to fundamental design limitations. However, if the solution allows it, cache writes from untrusted environments can be disabled, although this may significantly reduce the usefulness of the remote cache.