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
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
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
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is CVE-2026-34042 in your dependencies?
Find it across Go, including transitive dependencies.