CVE-2024-4261

Executive Summary

CVE-2024-4261 is a critical security vulnerability affecting the Responsive Contact Form Builder & Lead Generation Plugin for WordPress, up to and including version 1.9.1. This flaw allows authenticated attackers with subscriber-level access or higher to execute arbitrary WordPress shortcodes. The vulnerability stems from insufficient validation of user-supplied input before passing it to the do_shortcode() function, enabling attackers to invoke any registered shortcode, including those that may perform sensitive actions like file deletion, user creation, or privilege escalation.

This vulnerability is particularly dangerous in the WordPress ecosystem because shortcodes can be used to call internal functions, execute database queries, or trigger other plugins' features. Given the plugin's widespread use for lead generation, the attack surface is significant. The EPSS score is currently 0.0%, indicating low exploitation activity, but the public availability of proof-of-concept code and the low authentication requirement make it a high-priority target for red team operations and blue team hardening.

Supply Chain & Package Security

This vulnerability is specific to the WordPress plugin ecosystem. The plugin is distributed via the official WordPress Plugin Directory and is not an NPM, PyPI, Go, or Ruby package. However, organizations using this plugin should treat it as a supply chain risk, as the plugin's code is directly executed within the WordPress environment. No official patch has been released as of this report.

CVSS Breakdown & Threat Level

• CVSS Score: 5.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
• Attack Vector (AV): Network - The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low - No special conditions are required.
• Privileges Required (PR): Low - Attacker needs subscriber-level access.
• User Interaction (UI): None - No user action is required.
• Scope (S): Unchanged - The exploit does not affect other components.
• Confidentiality (C): Low - Limited information disclosure possible.
• Integrity (I): Low - Limited data modification possible.
• Availability (A): None - No denial of service impact.

Threat Level: Medium. While the CVSS score is moderate, the ability to execute arbitrary shortcodes can lead to severe consequences depending on the shortcodes available in the WordPress installation. For example, shortcodes from other plugins could allow file uploads, database manipulation, or privilege escalation.

Deep Dive & Mechanics

The vulnerability exists in the plugin's handling of shortcode execution. The plugin provides a form builder that allows users to create contact forms. When processing form submissions, the plugin uses do_shortcode() to render dynamic content. The flaw is that user-supplied input (e.g., form field values) is passed directly to do_shortcode() without proper sanitization or validation.

In WordPress, do_shortcode() processes text for shortcodes. If an attacker can inject a shortcode into a field that is later processed by this function, they can execute any registered shortcode. For example, an attacker with subscriber access could submit a form with a field containing [some_malicious_shortcode]. When the plugin processes the submission, it will execute that shortcode.

The vulnerable code path typically looks like:

$content = do_shortcode( $user_input );

Where $user_input is not sanitized. This allows attackers to call shortcodes like [file_delete], [create_admin], or any other shortcode registered by other plugins or themes.

Vulnerable vs. Patched Code

Vulnerable Code (version <= 1.9.1):

// In a form processing function
$field_value = $_POST['field_name'];
$processed = do_shortcode( $field_value );
echo $processed;

The $field_value is taken directly from user input and passed to do_shortcode().

Patched Code (hypothetical fix):

// In a form processing function
$field_value = sanitize_text_field( $_POST['field_name'] );
$processed = do_shortcode( $field_value );
echo $processed;

Or better:

$field_value = wp_kses_post( $_POST['field_name'] );
$processed = do_shortcode( $field_value );
echo $processed;

The fix would involve sanitizing the input to remove or escape shortcode syntax before processing. However, no official patch has been released.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress installation with the vulnerable plugin (version <= 1.9.1).
• An authenticated user account with at least Subscriber role.

Exploitation Steps:

1. Identify a vulnerable form: Navigate to a page containing a form created by the plugin.
2. Craft the payload: Use a shortcode that performs a desired action. For example, to create a new admin user (if a shortcode exists), use [create_admin username="attacker" password="pwned123"]. Alternatively, use [file_delete file="/var/www/html/wp-config.php"] if such a shortcode exists.
3. Submit the form: Enter the payload in a text field and submit.
4. Verify execution: Check if the shortcode's action was performed (e.g., new user created, file deleted).

Example Payload:

[wp_ajax_hook action="create_admin" username="hacker" password="hack123"]

Note: The actual shortcode depends on other installed plugins.

Blue Team

Detection Steps:

1. Monitor form submissions: Look for unusual patterns in form field values, especially those containing square brackets [ and ].
2. Audit shortcode usage: Check for unexpected shortcode executions in logs.
3. Review user activity: Identify accounts with subscriber-level access that are submitting forms with suspicious content.

Honeypot Setup:

• Create a fake form field that logs all submissions.
• Use a custom shortcode that logs its execution to a separate file.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to form submission endpoints containing shortcode syntax in form fields.
  • Unusual shortcode names in request parameters (e.g., [create_admin], [file_delete]).

• Host IOCs:

  • Unexpected user accounts created with subscriber or higher roles.
  • Modified or deleted files in the WordPress installation.
  • Log entries showing do_shortcode() being called with suspicious input.

• File IOCs:

  • Presence of shortcode execution logs in /wp-content/debug.log or custom log files.
  • Modified .htaccess or wp-config.php files.

Detection Engineering

YARA Rule

rule CVE_2024_4261_Shortcode_Injection {
    meta:
        description = "Detects shortcode injection attempts in WordPress form submissions"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-4261"
    strings:
        $shortcode_pattern = /\[[a-zA-Z_][a-zA-Z0-9_]*\s*[^\]]*\]/
        $vulnerable_param = /field_name|form_field|input_value/
    condition:
        any of ($shortcode_pattern) and any of ($vulnerable_param)
}

Sigma Rule

title: CVE-2024-4261 Shortcode Injection Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests containing shortcode syntax in form fields
author: Valters IT
date: 2024/01/01
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 
            - 'field_name='
            - 'form_field='
        cs-uri-query|re: '\[[a-zA-Z_][a-zA-Z0-9_]*\s*[^\]]*\]'
    condition: selection
falsepositives:
    - Legitimate shortcode usage in form fields (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-4261 Shortcode Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"field_name="; http_client_body; pcre:"/\[[a-zA-Z_][a-zA-Z0-9_]*\s*[^\]]*\]/Ri"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is available.
2. Restrict subscriber-level access to only trusted users.
3. Implement a Web Application Firewall (WAF) rule to block requests containing shortcode syntax in form fields.

Long-term Fix:

• The vendor must release a patched version that sanitizes input before passing to do_shortcode(). Until then, consider using an alternative lead generation plugin.

Workaround:

• Add a custom filter to sanitize form fields:

add_filter( 'your_plugin_form_field', 'sanitize_text_field' );

Valters IT
Securing Digital Frontiers