CVE-2026-11766: Ultimate Member WordPress XSS Vulnerability

CVE-2026-11766 Overview

CVE-2026-11766 is a stored Cross-Site Scripting (XSS) vulnerability in the Ultimate Member WordPress plugin before version 2.12.0. The plugin fails to sanitize and escape values submitted through custom textarea profile fields before rendering them on user profile pages. Authenticated users with Subscriber-level access or higher can inject JavaScript that executes in the browser of any visitor who views the profile, including administrators. Successful exploitation can lead to session theft, account takeover, and privileged actions performed on behalf of an administrator. The vulnerability carries a CVSS 3.1 base score of 8.0 and impacts confidentiality, integrity, and availability.

Critical Impact
A low-privileged Subscriber account can achieve administrator account takeover through stored JavaScript executed on profile views.

Affected Products

Ultimate Member WordPress plugin versions prior to 2.12.0
WordPress installations exposing user profile pages via the plugin
Sites permitting Subscriber-level registration with custom textarea profile fields

Discovery Timeline

2026-07-06 - CVE-2026-11766 published to NVD
2026-07-06 - Last updated in NVD database

Technical Details for CVE-2026-11766

Vulnerability Analysis

The vulnerability resides in how the Ultimate Member plugin handles custom textarea fields within user profiles. When a user submits content through a custom textarea field, the plugin stores the raw input and later outputs it on the rendered profile without applying proper HTML escaping or sanitization. Any HTML or JavaScript included in the field executes in the browser of every user who views the profile.

Because the payload persists in the WordPress database, this is a stored (persistent) XSS rather than a reflected one. The malicious script fires automatically each time an administrator or moderator navigates to the attacker's profile, without any additional interaction beyond viewing the page.

The attacker requires only Subscriber-level authentication, a role that many WordPress sites grant by default to self-registered users. This lowers the barrier significantly compared to vulnerabilities that require elevated privileges. See the WPScan Vulnerability Report for additional details.

Root Cause

The root cause is missing output escaping in the code path that renders custom textarea profile fields. The plugin does not apply WordPress functions such as esc_html(), esc_attr(), or wp_kses() before echoing user-supplied data. This maps to [CWE-79] Improper Neutralization of Input During Web Page Generation.

Attack Vector

An attacker registers or uses an existing Subscriber account, then edits their profile and enters a JavaScript payload into a vulnerable custom textarea field. When an administrator views the attacker's profile in the front-end or back-end, the script executes within the administrator's authenticated session. Typical outcomes include exfiltration of authentication cookies, creation of new administrator accounts through the WordPress REST API, or injection of persistent backdoors via theme or plugin editors.

The vulnerability manifests when unsanitized textarea content is written to the profile HTML. No verified proof-of-concept code has been published; refer to the WPScan Vulnerability Report for technical validation details.

Detection Methods for CVE-2026-11766

Indicators of Compromise

Unexpected <script> tags, event handlers, or encoded JavaScript within wp_usermeta rows tied to Ultimate Member custom fields
New WordPress administrator accounts created shortly after an administrator viewed a Subscriber profile
Outbound HTTP requests from browser sessions to unfamiliar domains during profile page views
Modified theme or plugin PHP files with timestamps aligned to administrator activity on user profiles

Detection Strategies

Audit the wp_usermeta table for values containing <script, onerror=, onload=, javascript:, or base64-encoded JavaScript patterns in Ultimate Member field keys
Inspect web server access logs for POST requests to /wp-admin/profile.php or Ultimate Member profile update endpoints followed by administrator profile views
Deploy web application firewall rules that flag script content submitted to user profile update requests

Monitoring Recommendations

Alert on creation of new administrator or editor accounts outside standard provisioning workflows
Monitor WordPress audit logs for plugin or theme file modifications following any Subscriber account activity
Track anomalous browser-originated API calls to /wp-json/wp/v2/users from administrator sessions

How to Mitigate CVE-2026-11766

Immediate Actions Required

Upgrade the Ultimate Member plugin to version 2.12.0 or later on all affected WordPress installations
Review all Subscriber and higher-level accounts for suspicious profile content and remove injected payloads
Rotate administrator passwords and invalidate active sessions after patching
Audit user roles and remove unnecessary Subscriber self-registration if not required for business operations

Patch Information

The vendor addressed the issue in Ultimate Member 2.12.0 by applying proper output escaping to custom textarea profile fields. Administrators should update through the WordPress plugin dashboard or by replacing the plugin directory with the fixed release. Verify the installed version under Plugins > Installed Plugins after updating.

Workarounds

Disable custom textarea profile fields in the Ultimate Member form builder until the patch can be applied
Restrict new user registration or set the default role to a non-privileged custom role that cannot edit profile fields
Deploy a WAF rule blocking HTML tags such as <script> in profile update POST parameters

bash

# Update Ultimate Member via WP-CLI
wp plugin update ultimate-member --version=2.12.0
wp plugin list --name=ultimate-member --fields=name,version,status