# ActivityPub <= 9.3.1 - Unauthenticated Missing Authorization to Federated Comment Modification via Update and Undo Activities

- **ID:** WPSEC-2026-2043
- **Plugin:** ActivityPub (`activitypub`), https://wordpress.org/plugins/activitypub/
- **Affected versions:** all versions before 9.4.0
- **Fixed in:** 9.4.0 (Update to 9.4.0 or later.)
- **Severity:** Medium 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)
- **Weakness:** CWE-862
- **Usage among sites WPSec scans:** plugin Low, affected versions Low (as of 2026-10-30)
- **Attack surface analysis:** https://attacksurface.wpsec.com/plugin/activitypub
- **Fix released:** 2026-10-29
- **Published:** 2026-10-30
- **URL:** https://wpsec.com/vuln/WPSEC-2026-2043/

## Description

The ActivityPub plugin for WordPress is vulnerable to unauthorized modification of data via incoming Update and Undo activities in all versions up to, and including, 9.3.1, due to a missing ownership check for federated comments and received activities that have no remote actor recorded. This makes it possible for unauthenticated attackers who control a Fediverse account to overwrite the content of other users' federated comments, or undo their likes and reposts, when those items were received before the plugin started recording who sent them.

## References

- https://wpsec.com/vuln/WPSEC-2026-2043/
- https://plugins.svn.wordpress.org/activitypub/tags/9.4.0/
- https://wordpress.org/plugins/activitypub/

Source: WPSec, https://wpsec.com/vuln/WPSEC-2026-2043/

License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Free to use, share and adapt, also commercially, if you credit WPSec and link to this advisory.