The vulnerability lies in the Undo handler of the ActivityPub server used by Ghost. A federated server can craft an Undo message that tells the service to delete an existing follow or repost relationship as if it were coming from the target account. The effect is the removal of a social link that belongs to another user, without that user’s involvement or consent. This does not expose private data or alter content, but it can disrupt social relationships and user experience.

The issue affects the TryGhost:ActivityPub component in all versions between 1.0.0 and 1.2.6 inclusive. Both the hosted Ghost ActivityPub service and self‑hosted installations using older images are susceptible. The problem was corrected in release 1.2.6, which must be installed to eliminate the flaw.

The CVSS score of 5.3 indicates a moderate severity. The EPSS score is unknown, and the vulnerability is not listed in CISA's KEV catalog. Attackers can exploit the flaw by sending a forged Undo activity from any federated server that can communicate with the affected instance, so the vector is network‑based. Because the vulnerability requires the ability to sign or impersonate a remote server’s request, the exact exploitability depends on the trust model of the federation. Nonetheless, the potential to silently remove social links makes this a non‑trivial risk for platforms that rely on accurate follower or repost relationships.