GHSA-hg5r-vq93-9fv6: Gitea Actions Artifacts V4 signed URL HMAC ambiguity allows cross-repository artifact read and cross-task upload-state write

Summary Gitea Actions Artifacts V4 signed upload/download URLs can be rewritten to access a different running task and repository context while preserving the original HMAC signature. An attacker with permission to run a Gitea Actions job can turn a signed URL for an attacker-controlled artifact into a URL that reads artifacts from another task context, or writes attacker-controlled data into another task's artifact upload staging context, including in a private repository. This is one vulnerability with two exploit paths: - DownloadArtifact: cross-task/cross-repository artifact read, giving C:H. - UploadArtifact: cross-task artifact staging write and metadata mutation, giving I:H. Details The root cause is that the V4 artifact signed URL signature is built from raw concatenated fields without delimiters or length-prefixing: func (r *artifactV4Routes) buildSignature(endpoint, expires, artifactName string, taskID, artifactID int64) []byte { mac := hmac.New(sha256.New, setting.GetGeneralTokenSigningSecret()) mac.Write([]byte(endpoint)) mac.Write([]byte(expires)) mac.Write([]byte(artifactName)) _, _ = fmt.Fprint(mac, taskID) _, _ = fmt.Fprint(mac, artifactID) return mac.Sum(nil) } Affected code: routers/api/actions/artifactsv4.go:164-171. Because artifactName, taskID, and artifactID are concatenated without boundaries, two different URL tuples can produce the same HMAC input. For example: signed tuple: artifactName = "artifact-795-153" taskID = 48 artifactID = <attacker artifact id> forged tuple: artifactName = "artifact-795-1" taskID = 53 artifactID = 48<attacker artifact id> The final HMAC input suffix is identical: artifact-795-15348<attacker artifact id> The attacker does not need to know the target artifact's database artifactID. The forged URL's artifactID only needs to carry digits that preserve the original HMAC input. After verification, the actual target artifact is looked up by target task/run/attempt and artifactName, not by the signed artifact