act Project Cache Poisoning Vulnerability Leads to Potential RCE

The act project, designed for local execution of GitHub Actions workflows, contains a critical vulnerability affecting versions prior to 0.2.86. The built-in actions/cache server, intended for local caching, inadvertently listens for connections on all network interfaces. This exposure allows any attacker capable of reaching the server, including those on the internet, to create caches with arbitrary keys and retrieve existing cache data. By predicting the cache keys used by local actions, an attacker can inject malicious content into the cache, paving the way for arbitrary remote code execution within the Docker container used by act. This vulnerability was addressed in version 0.2.86 of act. The CVSS v3.1 base score is 8.2, indicating a high severity threat.

Attack Chain

1. The attacker identifies a vulnerable act instance running a version prior to 0.2.86 with its cache server exposed on all interfaces.
2. The attacker probes the exposed act cache server to determine accessible endpoints and version information.
3. The attacker analyzes common GitHub Actions workflows and identifies predictable cache keys.
4. The attacker crafts a malicious cache archive containing payloads designed for remote code execution.
5. The attacker uploads the malicious cache archive to the vulnerable act instance using the predicted cache key.
6. A legitimate user triggers a local GitHub Actions workflow using act.
7. The act instance retrieves the attacker’s malicious cache archive instead of the expected legitimate cache.
8. The malicious payload within the cache is executed within the Docker container, leading to remote code execution on the host system running act.

Impact

Successful exploitation of this vulnerability allows an attacker to achieve arbitrary remote code execution on the host system running the vulnerable version of act. This can lead to complete system compromise, data theft, and further lateral movement within the network. The vulnerability affects any user running a version of act prior to 0.2.86 with the cache server exposed. While the number of directly affected users is unknown, the potential impact on development environments and CI/CD pipelines is significant.

Recommendation

• Upgrade to version 0.2.86 or later of the act project to remediate the vulnerability (CVE-2026-34042).
• Implement network access controls to restrict access to the act cache server to only trusted networks and hosts.
• Monitor network connections to the act cache server for unexpected or unauthorized access.
• Enable process monitoring on systems running act to detect potentially malicious processes spawned from Docker containers.