# Co-Authors Plus <= 4.2.0 - Authenticated (Contributor+) Improper Authorization to Guest-Authored Post Editing via Username Collision

- **ID:** WPSEC-2026-2531
- **Plugin:** Co-Authors Plus (`co-authors-plus`), https://wordpress.org/plugins/co-authors-plus/
- **Affected versions:** from 3.0 before 4.2.1
- **Fixed in:** 4.2.1 (Update to 4.2.1 or later.)
- **Severity:** Medium 4.2 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N)
- **Weakness:** CWE-863
- **Usage among sites WPSec scans:** plugin Medium, affected versions Medium (as of 2026-11-04)
- **Attack surface analysis:** https://attacksurface.wpsec.com/plugin/co-authors-plus
- **Fix released:** 2026-11-04
- **Published:** 2026-11-05
- **URL:** https://wpsec.com/vuln/WPSEC-2026-2531/

## Description

The Co-Authors Plus plugin for WordPress is vulnerable to Improper Authorization via the co-author capability mapping in all versions up to, and including, 4.2.0, due to the is_coauthor_for_post() function treating a registered user as a co-author whenever their user_login matches the user_login of a guest author bylined on the post, even when that guest author is not linked to the user. This makes it possible for authenticated attackers, with Contributor-level access and above, whose username matches an unlinked guest author's login, to be treated as a co-author of that guest author's posts and to view and edit posts they could not otherwise access, such as drafts and pending posts.

## References

- https://wpsec.com/vuln/WPSEC-2026-2531/
- https://plugins.svn.wordpress.org/co-authors-plus/tags/4.2.1/
- https://wordpress.org/plugins/co-authors-plus/

Source: WPSec, https://wpsec.com/vuln/WPSEC-2026-2531/

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.