ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts
Affects Ghost's ActivityPub service from 1.0.0 through 1.2.5, letting another account remove a follow or repost of its own. The CVSS score is 5.3 (medium) and no vendor rating is given. Fixed in 1.2.6.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Fix
- Fixed in 1.2.6
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The ActivityPub service for Ghost, which is a multi-tenant server built with Fedify, is affected by this flaw. A remote federated server can exploit the Undo handler to remove another account's follow relationship or repost, without that account's involvement, though no private data is exposed and content is not changed.
Summarized by anthropic/claude-haiku-5.5. Verify details against the original sources.
Read the original 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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- TryGhost ActivityPub
From 1.0.0 · fixed in 1.2.6
References 4
GitHub · TryGhost/ActivityPub
Questions about CVE-2026-63115
What does this mean?
The ActivityPub Undo handler does not check that the account sending an Undo request owns the follow or repost it names. A remote account can therefore remove a follow relationship or repost that belongs to a different account, without that account taking part.Source: github.com
Am I affected, and how do I check?
Ghost's ActivityPub service versions 1.0.0 through 1.2.5 are affected. To check, look at the Ghost version in use; versions below 1.2.6 include the bug.Source: github.com
What should I do about it?
Upgrade the ActivityPub service to 1.2.6, which contains the fix.Source: github.com
Is it being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data