CVE-2026-3551: WordPress Custom New User Plugin XSS Flaw

CVE-2026-3551 Overview

The Custom New User Notification plugin for WordPress contains a Stored Cross-Site Scripting (XSS) vulnerability affecting all versions up to and including 1.2.0. This security flaw exists in the plugin's admin settings page, where multiple input fields fail to properly sanitize user input and escape output. The vulnerability stems from the plugin's use of register_setting() without sanitize callbacks, combined with direct echoing of values retrieved via get_option() into HTML input value attributes without proper escaping using esc_attr().

Critical Impact

Authenticated attackers with Administrator-level access can inject malicious JavaScript into the plugin settings page. In WordPress multisite environments, this allows subsite administrators to potentially target and compromise super administrators through persistent script execution.

Affected Products

• Custom New User Notification plugin for WordPress versions ≤ 1.2.0
• WordPress multisite installations using the affected plugin
• Single-site WordPress installations with multiple administrator accounts

Discovery Timeline

• April 16, 2026 - CVE-2026-3551 published to NVD
• April 16, 2026 - Last updated in NVD database

Technical Details for CVE-2026-3551

Vulnerability Analysis

This Stored XSS vulnerability exploits a fundamental weakness in how the Custom New User Notification plugin handles user-supplied data in its administrative interface. The vulnerability affects six distinct settings fields: 'User Mail Subject', 'User From Name', 'User From Email', 'Admin Mail Subject', 'Admin From Name', and 'Admin From Email'.

When an administrator saves malicious JavaScript code within any of these settings fields, the plugin stores the unvalidated input directly in the WordPress options database. Subsequently, when any user with access to the plugin's settings page loads that page, the stored payload is retrieved and rendered without proper output escaping, causing the malicious script to execute in the victim's browser context.

The attack is particularly concerning in WordPress multisite environments where a compromised or malicious subsite administrator could leverage this vulnerability to execute JavaScript in the context of a super administrator's session, potentially leading to full network compromise.

Root Cause

The root cause is two-fold: First, the plugin registers its settings using register_setting() without implementing sanitization callbacks to filter dangerous input. Second, when displaying stored values in the admin interface, the plugin uses get_option() to retrieve values and echoes them directly into HTML input value attributes without wrapping them in esc_attr() or similar escaping functions. This combination of missing input sanitization and missing output escaping creates the conditions necessary for Stored XSS exploitation.

Attack Vector

The attack requires network access and authenticated access with Administrator-level privileges. An attacker with these permissions navigates to the Custom New User Notification settings page and injects JavaScript payloads into one or more of the vulnerable settings fields. Common payloads include session cookie theft, keylogging, or DOM manipulation scripts. Once saved, the payload persists in the database and executes automatically when any administrator visits the settings page.

The vulnerability mechanism involves direct output of unsanitized database values into HTML attributes. When the settings page renders, values from the database are placed directly into input field value attributes without proper escaping. For example, if an attacker stores "><script>alert(document.cookie)</script> as a field value, the resulting HTML breaks out of the value attribute and executes the script. Technical details can be found in the WordPress Plugin Code references and the Wordfence Vulnerability Advisory.

Detection Methods for CVE-2026-3551

Indicators of Compromise

• Unusual JavaScript code or HTML entities present in the Custom New User Notification plugin settings stored in the wp_options database table
• Unexpected script tags or event handlers in the plugin's configuration values (User Mail Subject, User From Name, User From Email, Admin Mail Subject, Admin From Name, Admin From Email)
• Browser console errors or unexpected script execution when loading the plugin settings page
• Evidence of unauthorized settings changes in WordPress audit logs

Detection Strategies

• Implement Content Security Policy (CSP) headers to detect and block unauthorized inline script execution
• Deploy web application firewalls (WAF) with rules to detect XSS payloads in POST requests to WordPress admin pages
• Use WordPress security plugins capable of scanning option values for malicious content
• Monitor for HTML-encoded special characters or script patterns in database queries to the wp_options table

Monitoring Recommendations

• Enable detailed access logging for WordPress administrative pages, particularly /wp-admin/options-general.php and plugin settings pages
• Configure alerting for modifications to Custom New User Notification plugin settings
• Review administrator account activity in WordPress multisite environments for unusual cross-site access patterns
• Implement real-time monitoring for JavaScript execution anomalies on administrative interfaces

How to Mitigate CVE-2026-3551

Immediate Actions Required

• Audit current plugin settings for any suspicious or unexpected content containing HTML or JavaScript
• Temporarily disable the Custom New User Notification plugin if a patched version is not available
• Review WordPress administrator accounts and limit access to only essential personnel
• In multisite environments, audit subsite administrator privileges and restrict plugin management capabilities

Patch Information

At the time of publication, users should check the WordPress plugin repository for an updated version that addresses this vulnerability. The fix should implement proper sanitization callbacks in register_setting() and use esc_attr() when outputting stored values in HTML attributes. Monitor the WordPress Plugin Repository for security updates.

Workarounds

• Remove the Custom New User Notification plugin until a patched version is available if the functionality is not critical
• Implement a custom plugin or functions.php filter to sanitize the affected option values before they are saved
• Use a WordPress security plugin that provides virtual patching capabilities for known XSS vulnerabilities
• Restrict access to the WordPress admin area using IP allowlisting or additional authentication layers

# Verify plugin version and check for suspicious option values
wp plugin list --name=custom-new-user-notification --fields=name,version,status

# Check for potentially malicious content in plugin options
wp option get custom_new_user_notification_settings --format=json | grep -E '<script|javascript:|on\w+='

# Disable the plugin temporarily if needed
wp plugin deactivate custom-new-user-notification