nektos act up to 0.2.89 HTTP Artifacts V4 Backend artifacts_v4.go validateRunIDV4 workflow_run_backend_id privileges management

Summaryinfo

A vulnerability labeled as critical has been found in nektos act up to 0.2.89. Affected by this issue is the function validateRunIDV4 of the file pkg/artifacts/artifacts_v4.go of the component HTTP Artifacts V4 Backend. Such manipulation of the argument workflow_run_backend_id leads to privileges management. This vulnerability is referenced as CVE-2026-76847. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability, which was classified as critical, has been found in nektos act up to 0.2.89. Affected by this issue is the function validateRunIDV4 of the file pkg/artifacts/artifacts_v4.go of the component HTTP Artifacts V4 Backend. The manipulation of the argument workflow_run_backend_id with an unknown input leads to a privileges management vulnerability. Using CWE to declare the problem leads to CWE-269. The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor. Impacted is confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-76847 since 08/19/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 08/24/2026). This vulnerability is assigned to T1068 by the MITRE ATT&CK project.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Vendor

• nektos

Name

• act

Version

• 0.2.0
• 0.2.1
• 0.2.2
• 0.2.3
• 0.2.4
• 0.2.5
• 0.2.6
• 0.2.7
• 0.2.8
• 0.2.9
• 0.2.10
• 0.2.11
• 0.2.12
• 0.2.13
• 0.2.14
• 0.2.15
• 0.2.16
• 0.2.17
• 0.2.18
• 0.2.19
• 0.2.20
• 0.2.21
• 0.2.22
• 0.2.23
• 0.2.24
• 0.2.25
• 0.2.26
• 0.2.27
• 0.2.28
• 0.2.29
• 0.2.30
• 0.2.31
• 0.2.32
• 0.2.33
• 0.2.34
• 0.2.35
• 0.2.36
• 0.2.37
• 0.2.38
• 0.2.39
• 0.2.40
• 0.2.41
• 0.2.42
• 0.2.43
• 0.2.44
• 0.2.45
• 0.2.46
• 0.2.47
• 0.2.48
• 0.2.49
• 0.2.50
• 0.2.51
• 0.2.52
• 0.2.53
• 0.2.54
• 0.2.55
• 0.2.56
• 0.2.57
• 0.2.58
• 0.2.59
• 0.2.60
• 0.2.61
• 0.2.62
• 0.2.63
• 0.2.64
• 0.2.65
• 0.2.66
• 0.2.67
• 0.2.68
• 0.2.69
• 0.2.70
• 0.2.71
• 0.2.72
• 0.2.73
• 0.2.74
• 0.2.75
• 0.2.76
• 0.2.77
• 0.2.78
• 0.2.79
• 0.2.80
• 0.2.81
• 0.2.82
• 0.2.83
• 0.2.84
• 0.2.85
• 0.2.86
• 0.2.87
• 0.2.88
• 0.2.89

CPE 2.3info

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