CVE-2024-10261

Executive Summary

CVE-2024-10261 is a critical arbitrary shortcode execution vulnerability affecting the WordPress plugin "Paid Membership Subscriptions – Effortless Memberships, Recurring Payments & Content Restriction" by Cozmoslabs, in all versions up to and including 2.13.0. This vulnerability allows an unauthenticated attacker to execute arbitrary WordPress shortcodes by exploiting a missing validation check before the do_shortcode() function is called. Successful exploitation can lead to unauthorized content injection, data exfiltration, or privilege escalation depending on the shortcodes available in the WordPress environment. The vulnerability has a CVSS score of 7.3 (High) and currently has no known public exploit or patch.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin distributed via the official WordPress plugin repository. The plugin is not an NPM, PyPI, Go, or Ruby package, but it is a critical component of the WordPress supply chain. Users are advised to monitor the plugin's update channel and consider temporary disabling or replacing the plugin until a patch is released.

CVSS Breakdown & Threat Level

• CVSS Score: 7.3 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: Low

The vulnerability is easily exploitable remotely without authentication, making it a high-priority threat for WordPress sites using the affected plugin.

Deep Dive & Mechanics

The vulnerability lies in the plugin's handling of user-supplied input that is passed to the do_shortcode() function without proper validation or sanitization. WordPress shortcodes are powerful macros that can execute arbitrary PHP code, query the database, or render dynamic content. By injecting a malicious shortcode, an attacker can:

• Execute arbitrary SQL queries via shortcodes like [query].
• Read or write files if a file-manipulation shortcode exists.
• Escalate privileges by executing admin-level shortcodes.
• Deface the site or inject malicious JavaScript.

The specific vulnerable code path is triggered when an unauthenticated user submits a request that includes a parameter (likely a POST or GET parameter) that is directly passed to do_shortcode(). The plugin fails to check if the user has the necessary permissions or if the input is a valid shortcode.

Vulnerable vs. Patched Code

Vulnerable Code (example from version 2.13.0):

// In some handler function
$shortcode = $_POST['shortcode_input'];
echo do_shortcode( $shortcode );

Patched Code (hypothetical fix):

// Validate and sanitize input
$shortcode = sanitize_text_field( $_POST['shortcode_input'] );
if ( current_user_can( 'manage_options' ) ) {
    echo do_shortcode( $shortcode );
} else {
    echo 'Unauthorized';
}

The patched version would include proper capability checks and input sanitization before executing any shortcode.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A WordPress site with the vulnerable plugin installed and activated (version <= 2.13.0).
• No authentication required.

Exploitation Steps:

1. Identify the vulnerable endpoint. This is typically an AJAX action or form handler that accepts user input and passes it to do_shortcode().
2. Craft a POST request with a malicious shortcode payload. For example:

   POST /wp-admin/admin-ajax.php HTTP/1.1
   Content-Type: application/x-www-form-urlencoded
   
   action=vulnerable_action&shortcode_input=[wp_php_eval code='echo "PWNED";']
   

3. If the shortcode is executed, the server will respond with the output of the shortcode.

Example using cURL:

curl -X POST http://target.com/wp-admin/admin-ajax.php \
  -d "action=vulnerable_action&shortcode_input=[wp_php_eval code='system(\"id\");']"

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual POST requests to admin-ajax.php or other plugin-specific endpoints containing shortcode-like patterns (e.g., [ and ]).
• Look for spikes in do_shortcode() calls in PHP error logs.
• Use WAF rules to block requests containing do_shortcode or suspicious shortcode patterns.

Defense:

• Immediately disable the plugin if no patch is available.
• Implement a Web Application Firewall (WAF) rule to block requests containing [ and ] in parameters that are not expected to contain shortcodes.
• Restrict access to the vulnerable endpoint via .htaccess or server-level rules.

Indicators of Compromise (IOCs)

• Network IOCs:

  • POST requests to /wp-admin/admin-ajax.php with parameters containing [ and ] (e.g., shortcode_input=[...]).
  • Unusual HTTP response bodies containing output from executed shortcodes (e.g., system commands, database dumps).

• File IOCs:

  • Unexpected files in the WordPress uploads directory (e.g., shell.php).
  • Modified .htaccess files with malicious redirects.

• Log IOCs:

  • PHP error logs showing do_shortcode() calls with unexpected arguments.
  • Web server access logs showing repeated POST requests to the vulnerable endpoint.

Detection Engineering

YARA Rule

rule CVE_2024_10261_Shortcode_Injection {
    meta:
        description = "Detects attempts to exploit CVE-2024-10261 via shortcode injection in WordPress"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-10261"
    strings:
        $shortcode_pattern = /\[[a-zA-Z_]+[^\]]*\]/
        $vulnerable_param = "shortcode_input"
        $do_shortcode = "do_shortcode"
    condition:
        any of ($shortcode_pattern) and $vulnerable_param and $do_shortcode
}

Sigma Rule

title: CVE-2024-10261 WordPress Shortcode Injection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects POST requests to WordPress admin-ajax.php with shortcode injection attempts
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'admin-ajax.php'
        cs-uri-query|contains: 'shortcode_input'
        cs-uri-query|re: '\[.*\]'
    condition: selection
falsepositives:
    - Legitimate use of shortcodes in admin panels (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-10261 WordPress Shortcode Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"admin-ajax.php"; http_uri; content:"shortcode_input"; http_client_body; pcre:"/\[[a-zA-Z_]+[^\]]*\]/"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if it is not critical to business operations.
2. Apply a virtual patch using a WAF to block requests containing shortcode_input parameter with shortcode syntax.
3. Monitor logs for exploitation attempts.

Long-term Fix:

• The vendor (Cozmoslabs) must release a patched version that includes proper input validation and capability checks before calling do_shortcode().
• Until then, consider replacing the plugin with an alternative that provides similar functionality.

Workaround:

• Add the following rule to .htaccess to block direct access to the vulnerable endpoint:

  <IfModule mod_rewrite.c>
      RewriteEngine On
      RewriteCond %{REQUEST_URI} ^/wp-admin/admin-ajax\.php
      RewriteCond %{QUERY_STRING} shortcode_input
      RewriteRule .* - [F,L]
  </IfModule>
  

Valters IT
Securing Digital Frontiers