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Vulnerability · Published Oct 30, 2026 · Assigned by GitHub_M

CVE-2026-63115ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts

Medium 5.3

In plain English

A medium-severity authentication bypass by spoofing in ActivityPub from TryGhost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 4% of all known vulnerabilities).

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.1%top 96.3% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected products (as reported)

VendorProductVersions
TryGhost ActivityPub >= 1.0.0, < 1.2.6

Weakness: CWE-290, CWE-345, CWE-863 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

References