CVE-2026-34042 is a high-severity (CVSS 8.2) CWE-862 vulnerability in github.com/nektos/act. A fix is available for github.com/nektos/act — see the affected versions and patch details below.
act: actions/cache server allows malicious cache injection
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34042.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34042 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/nektos/actReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/nektos/act | all versions | 0.2.86go get github.com/nektos/act@v0.2.86 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nektos/act, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/nektos/act to 0.2.86 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34042 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34042 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34042. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-34042 in your dependencies?
O3 Security finds CVE-2026-34042 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.