{"id":"CVE-2026-63115","title":"ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts","summary":"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 …","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","cvssSource":"cna","cwe":["CWE-290","CWE-345","CWE-863"],"vendor":"TryGhost","product":"ActivityPub","affected":["ActivityPub >= 1.0.0, < 1.2.6"],"ssvc":{"exploitation":"none","automatable":"yes","technicalImpact":"partial","timestamp":"2026-11-02T17:12:41.067713Z"},"published":"2026-10-30","updated":"2026-11-02","sourceUpdated":"2026-11-02T18:09:53.602Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-63115","references":[{"url":"https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc","label":"https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc"},{"url":"https://github.com/TryGhost/ActivityPub/pull/1929","label":"https://github.com/TryGhost/ActivityPub/pull/1929"},{"url":"https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e","label":"https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e"},{"url":"https://github.com/TryGhost/ActivityPub/releases/tag/v1.2.6","label":"https://github.com/TryGhost/ActivityPub/releases/tag/v1.2.6"}],"tags":["cve.org"],"epss":0.00143,"epssPercentile":0.03644,"ingestedAt":"2026-11-05T19:30:42.766Z","slug":"CVE-2026-63115","body":"## Overview\n\nActivityPub 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.\n\n## Affected\n\n- `ActivityPub >= 1.0.0, < 1.2.6`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}