CVE-2026-34042

act's built-in actions/cache server listens to connections on all interfaces and allows anyone who can connect to it, including someone anywhere on the internet, to create caches with arbitrary keys and retrieve all existing caches. If one can predict which cache keys will be used by local actions, one can create malicious caches containing whatever files one pleases, most likely allowing arbitrary remote code execution within the Docker container.

Discovery

Discovered while discussing forgejo/runner#294.

Proposed Mitigation

It was discussed to append a secret to ACTIONS_CACHE_URL to retain compatibility with GitHub's cache action and still allow authorization. Forgejo is considering also encoding which repo is currently being run in CI into the secret in the URL to prevent unrelated repos using the same (probably global) runner from seeing each other's caches.

The application does not perform an authorization check before performing a sensitive operation. Typical impact: unauthorized access to restricted functionality or data.

CVE-2026-34042 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.2.86); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34042? CVE-2026-34042 is a high-severity missing authorization vulnerability in github.com/nektos/act (go), affecting versions <= 0.2.85. It is fixed in 0.2.86. The application does not perform an authorization check before performing a sensitive operation.
2. How severe is CVE-2026-34042? CVE-2026-34042 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/nektos/act are affected by CVE-2026-34042? github.com/nektos/act (go) versions <= 0.2.85 is affected.
4. Is there a fix for CVE-2026-34042? Yes. CVE-2026-34042 is fixed in 0.2.86. Upgrade to this version or later.
5. Is CVE-2026-34042 exploitable, and should I be worried? Whether CVE-2026-34042 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-34042 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-34042? Upgrade github.com/nektos/act to 0.2.86 or later.