CVE-2026-6203: WordPress User Registration Open Redirect

CVE-2026-6203 Overview

The User Registration & Membership plugin for WordPress contains an Open Redirect vulnerability in versions up to and including 5.1.4. This security flaw stems from insufficient validation of user-supplied URLs passed via the redirect_to_on_logout GET parameter before redirecting users. The vulnerability allows attackers to craft malicious links that redirect users to external, potentially malicious URLs after logout, creating opportunities for phishing attacks and credential theft.

Critical Impact

Attackers can craft specially formed links that redirect authenticated users to malicious external sites after logout, potentially enabling phishing attacks, credential harvesting, or malware distribution through social engineering.

Affected Products

• User Registration & Membership Plugin for WordPress versions up to and including 5.1.4

Discovery Timeline

• 2026-04-13 - CVE CVE-2026-6203 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-6203

Vulnerability Analysis

This Open Redirect vulnerability (CWE-601) exists due to improper URL validation in the logout redirect functionality. The core issue lies in how the plugin handles the redirect_to_on_logout GET parameter, which accepts user-controlled input and passes it directly to WordPress's wp_redirect() function rather than the more secure wp_safe_redirect() function.

While the plugin does apply esc_url_raw() sanitization to handle malformed URLs, this function only ensures the URL is properly formatted—it does not restrict redirect destinations to the local domain. This distinction is critical: wp_safe_redirect() explicitly validates that the target URL belongs to an allowed host list, while wp_redirect() permits redirects to any valid URL.

The vulnerability enables attackers to construct legitimate-looking links that initially point to the trusted WordPress site but ultimately redirect users to attacker-controlled domains after the logout process completes.

Root Cause

The root cause is the use of wp_redirect() instead of wp_safe_redirect() for handling the redirect_to_on_logout parameter in the plugin's template functions. The vulnerable code path is located in functions-ur-template.php at line 39. The esc_url_raw() function applied to the input sanitizes malformed URLs but does not enforce domain restrictions, allowing arbitrary external redirects.

Attack Vector

This vulnerability is exploitable over the network and requires user interaction. An attacker crafts a malicious URL containing the redirect_to_on_logout parameter pointing to an attacker-controlled domain. When a logged-in user clicks this link and logs out, they are automatically redirected to the malicious site.

The attack flow typically involves:

1. Attacker creates a link to the legitimate WordPress site with a malicious redirect_to_on_logout value
2. Victim receives the link through email, social media, or other communication channels
3. Victim clicks the link and performs a logout action
4. The plugin redirects the victim to the attacker's site without validation
5. The attacker's site may present a fake login page to harvest credentials

For technical details on the vulnerable code path, see the WordPress Plugin Repository and the Wordfence Vulnerability Analysis.

Detection Methods for CVE-2026-6203

Indicators of Compromise

• HTTP requests containing the redirect_to_on_logout parameter with external domain URLs
• Referrer logs showing users arriving at external domains from WordPress logout pages
• User reports of unexpected redirects after logging out
• Phishing campaigns targeting users of affected WordPress sites

Detection Strategies

• Monitor web server access logs for logout requests containing the redirect_to_on_logout parameter with non-local domain values
• Implement URL parameter analysis rules in web application firewalls to detect external redirect attempts
• Review referrer headers in security logs to identify users being redirected to suspicious external domains
• Deploy browser-based security tools that warn users about redirects to untrusted domains

Monitoring Recommendations

• Configure alerting for GET requests to logout endpoints containing URLs with external domains in the redirect_to_on_logout parameter
• Implement SentinelOne Singularity to monitor for suspicious redirect patterns and phishing indicators
• Enable detailed HTTP request logging to capture full query string parameters for security analysis
• Establish baseline user behavior patterns to identify anomalous redirect activity

How to Mitigate CVE-2026-6203

Immediate Actions Required

• Update the User Registration & Membership plugin to a version newer than 5.1.4 that addresses this vulnerability
• Review web server logs for evidence of exploitation attempts targeting the redirect_to_on_logout parameter
• Implement web application firewall rules to block or sanitize external redirect attempts in logout parameters
• Notify users about potential phishing risks and advise them to verify URLs before entering credentials

Patch Information

The vulnerability is addressed by replacing wp_redirect() with wp_safe_redirect() in the logout redirect functionality. This ensures that redirect destinations are validated against the allowed hosts list maintained by WordPress. Site administrators should update to the latest version of the User Registration & Membership plugin available through the WordPress plugin repository.

For detailed patch information, refer to the updated plugin code.

Workarounds

• Implement a web application firewall rule to strip or validate the redirect_to_on_logout parameter from incoming requests
• Use server-side URL validation to restrict redirect destinations to the local domain before processing logout requests
• Consider temporarily disabling logout redirect functionality until the plugin is updated
• Educate users to manually navigate to the site rather than clicking logout links from external sources

# Apache .htaccess configuration to block external redirects
# Add to WordPress .htaccess file
RewriteEngine On
RewriteCond %{QUERY_STRING} redirect_to_on_logout=https?://(?!your-domain\.com) [NC]
RewriteRule ^(.*)$ /$1? [R=302,L]