CVE-2026-76847 act 0.2.81 through 0.2.89 Missing Authorization in the Artifacts V4 Backend
Attackers can read, modify, or delete build artifacts of concurrent jobs due to missing authorization checks and hardcoded cryptographic keys in the act tool.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-76847?
CVE-2026-76847 is a high-severity missing authorization vulnerability in nektos act, published on 24 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
act starts an HTTP Artifacts V4 backend whenever a workflow uses actions/upload-artifact@v4 or actions/download-artifact@v4. The control-plane RPCs of that backend, including CreateArtifact, GetSignedArtifactURL, ListArtifacts, FinalizeArtifact and DeleteArtifact, accept a caller-supplied workflow_run_backend_id and never check that it belongs to the requester: validateRunIDV4 in pkg/artifacts/artifacts_v4.go parses the value and returns it with the comparison against the requesting task's run ID left commented out. The signed URLs the backend issues are authenticated by an HMAC whose key is hardcoded to the four bytes 0xba 0xdb 0xee 0xf0, identical in every build, computed over a concatenation of endpoint, expiry, artifact name and task ID with no length prefix or delimiter, so signatures are both forgeable and ambiguous between differing artifact name and task ID pairs. The --artifact-server-addr flag defaults to the host's outbound address rather than loopback, leaving the backend reachable from the surrounding network. Any client that can reach it may read, overwrite or delete the artifacts of a concurrently running job with no credentials, exposing build outputs such as secrets and deployment credentials and permitting their replacement before the owning job consumes them.
Which versions are affected by CVE-2026-76847?
How do I fix CVE-2026-76847?
No fix has been published for CVE-2026-76847 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-76847 being exploited?
There is no known exploitation of CVE-2026-76847 in the wild and no public exploit code.
Sources
Also known as GHSA-crfw-6q9f-w49f.