TryGhost ActivityPub up to 1.2.5 Undo privileges management

Summaryinfo

A vulnerability, which was classified as problematic, was found in TryGhost ActivityPub up to 1.2.5. The affected element is the function Undo of the component Undo Handler. Executing a manipulation can lead to privileges management. The identification of this vulnerability is CVE-2026-63115. The attack may be launched remotely. There is no exploit available. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in TryGhost ActivityPub up to 1.2.5. It has been declared as problematic. This vulnerability affects the function Undo of the component Undo Handler. The manipulation with an unknown input leads to a privileges management vulnerability. The CWE definition for the vulnerability is CWE-269. The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor. As an impact it is known to affect integrity. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-63115 since 07/15/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1068.

Upgrading to version 1.2.6 eliminates this vulnerability.

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Productinfo

Vendor

• TryGhost

Name

• ActivityPub

Version

• 1.2.0
• 1.2.1
• 1.2.2
• 1.2.3
• 1.2.4
• 1.2.5

Website

• Product: https://github.com/TryGhost/ActivityPub/

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