CVE-2026-63115

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.

• CWE-290
• CWE-345
• CWE-863

• https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e
• 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

Published

Oct 30, 2026

Last Modified

Nov 2, 2026

Status

Awaiting Analysis

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