ActivityPub is a multi-tenant ActivityPub server for Ghost, built with Fedify. From 1.0.0 until 1.2.6, the "Undo" handler allows a remote federated server to cause the service to act on a "follow" or "repost" as though the activity came from a different account. This permits removal of an existing "follow" relationship or "repost" belonging to another account without that account's involvement. No private data is exposed and content is not modified; the affected relationships can be re-established. Services proxying to Ghost's hosted ActivityPub server have already been updated, while self-hosted services require an updated image and redeployment. This issue is fixed in version 1.2.6.