Plane is an open-source project management tool. Prior to 1.4.0, the cycle-issues endpoint accepts issue UUIDs in the request body without validating that they belong to the caller's workspace. An authenticated user can add issues from any workspace to a cycle they control. If a victim issue is already assigned to a cycle, the operation removes it from the victim's cycle, causing a destructive cross-tenant write. This issue is fixed in 1.4.0.