CVE-2025-9891: 
WordPress vulnerability analysis and mitigation

CVE-2025-9891 is a Cross-Site Request Forgery (CSRF) vulnerability in the User Sync – Remote User Sync plugin for WordPress, affecting all versions up to and including 1.0.2. The flaw allows unauthenticated attackers to deactivate the plugin on a target site by tricking an authenticated administrator into clicking a malicious link. It was published on September 17, 2025, with a CVSS v3.1 base score of 4.3 (Medium) (Wordfence, Red Hat CVE).

The root cause is missing or incorrect nonce validation in the mo_user_sync_form_handler() function (CWE-352: Cross-Site Request Forgery). Because WordPress nonces are not properly verified before processing form submissions, an attacker can craft a forged HTTP request that, when triggered by an authenticated administrator, causes the plugin to execute privileged actions — specifically, deactivating the User Sync plugin. The vulnerable code path is visible in the plugin source at version 1.0.1 (WordPress Trac). Exploitation requires social engineering to induce administrator interaction, such as visiting a malicious page or clicking a crafted link.

Successful exploitation allows an unauthenticated attacker to deactivate the User Sync – Remote User Sync plugin on an affected WordPress site, disrupting user synchronization functionality. The integrity impact is low and there is no direct confidentiality or availability impact per the CVSS scoring. However, disabling the plugin could indirectly affect site operations that depend on remote user synchronization, potentially causing authentication or access control disruptions for synchronized users (Wordfence, ENISA EUVD).

No public proof-of-concept exploit code or in-the-wild exploitation has been reported for CVE-2025-9891. The EPSS score is approximately 0.012% (0.000120), indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires tricking a site administrator into performing an action, which limits the attack surface (Wordfence, Red Hat CVE).

1. Reconnaissance: Identify WordPress sites running the User Sync – Remote User Sync plugin at version 1.0.2 or earlier, using tools like WPScan or by inspecting publicly accessible plugin metadata.
2. Craft malicious request: Create an HTML page or link containing a forged form submission targeting the vulnerable mo_user_sync_form_handler() function endpoint, with parameters that trigger plugin deactivation.
3. Social engineering: Deliver the malicious link or page to a site administrator via phishing email, forum post, or other communication channel, inducing them to click the link while authenticated to their WordPress dashboard.
4. CSRF execution: When the administrator's browser loads the attacker-controlled page, it automatically submits the forged request to the target WordPress site. Because nonce validation is absent or incorrect, the server processes the request and deactivates the User Sync plugin (Wordfence, WordPress Trac).

• Logs: WordPress admin action logs showing unexpected plugin deactivation events for the user-sync plugin, particularly if not initiated by a known administrator session.
• Logs: Web server access logs showing POST requests to the WordPress admin endpoint (e.g., wp-admin/admin-post.php or similar) from unexpected referrer origins or with missing/invalid nonce values.
• File System: Absence of active User Sync plugin functionality or plugin listed as inactive in wp-content/plugins/ without a corresponding administrator action record.

Users should update the User Sync – Remote User Sync plugin to a version beyond 1.0.2 that includes proper nonce validation in the mo_user_sync_form_handler() function. The fix was introduced in changeset 3360328 (WordPress Trac Changeset). As a general workaround, administrators should exercise caution when clicking links from untrusted sources while logged into their WordPress dashboard, and consider using a Web Application Firewall (WAF) with CSRF protection rules. Wordfence users with the firewall enabled receive additional protection against CSRF-based attacks (Wordfence).

Wordfence disclosed and reported this vulnerability as part of their weekly WordPress vulnerability report for the period of September 15–21, 2025, assigning it a medium severity rating (Wordfence Weekly Report). No significant broader media coverage or notable researcher commentary beyond the standard disclosure has been identified.