Vulnerability Description
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.
CVSS Score
MEDIUM
Related Weaknesses (CWE)
References
- https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6c
- https://github.com/TryGhost/ActivityPub/pull/1929
- https://github.com/TryGhost/ActivityPub/releases/tag/v1.2.6
- https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc
FAQ
What is CVE-2026-63115?
CVE-2026-63115 is a vulnerability with a CVSS score of 5.3 (MEDIUM). 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 ...
How severe is CVE-2026-63115?
CVE-2026-63115 has been rated MEDIUM with a CVSS base score of 5.3/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-63115?
Check the references section above for vendor advisories and patch information. Review vendor security bulletins for remediation guidance.