Colibri Page Builder <= 1.0.345 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode

I cannot fulfill this request. I am prohibited from generating actionable exploitation materials, including detailed plans, payloads, or specific HTTP requests for exploiting vulnerabilities in software like WordPress plugins.

Instead, I can explain the mechanics of shortcode-based Stored Cross-Site Scripting (XSS) in WordPress and how developers can prevent such vulnerabilities.

Understanding Shortcode-Based Stored XSS

In WordPress, shortcodes are small snippets of code (e.g., [my_shortcode attribute="value"]) that allow users to add dynamic content to posts and pages. When a post is rendered, WordPress parses these shortcodes and executes a corresponding PHP callback function.

A Stored XSS vulnerability occurs in this context when:

1. Unsanitized Input: A user with sufficient permissions (like a Contributor or Author) can insert a shortcode with malicious attributes into a post.
2. Insufficient Escaping: The PHP function responsible for rendering the shortcode takes the user-supplied attributes and outputs them back into the HTML of the page without proper escaping.

For example, if a shortcode attribute is used inside an HTML attribute:

// Vulnerable Code Example
function my_shortcode_handler( $atts ) {
    $atts = shortcode_atts( array(
        'color' => 'blue',
    ), $atts );

    // VULNERABLE: The 'color' attribute is not escaped
    return '<div style="color: ' . $atts['color'] . ';">Text</div>';
}
add_shortcode( 'my_shortcode', 'my_shortcode_handler' );

An attacker could use a payload like [my_shortcode color='red; " onmouseover="alert(1)']. If the output is not escaped, the resulting HTML might look like:
<div style="color: red; " onmouseover="alert(1)";">Text</div>
This executes JavaScript in the browser of any user (including administrators) who views the post.

Prevention and Mitigation

To prevent these vulnerabilities, WordPress developers must follow the principle of "Escaping on Output."

1. Use esc_attr() for Attributes: Any user input placed inside an HTML attribute must be passed through esc_attr(). This prevents an attacker from "breaking out" of the attribute.

   // Secure version
   return '<div style="color: ' . esc_attr( $atts['color'] ) . ';">Text</div>';
   

2. Use esc_html() for Text Content: If the input is rendered as text between HTML tags, use esc_html().

3. Use esc_url() for Links: If the shortcode handles URLs, ensure they are validated and escaped using esc_url() to prevent javascript: protocol injections.

4. Restrict Permissions: Ensure that only trusted user roles have the capability to use sensitive shortcodes, although generally, any user with edit_posts capability (Contributor and above) can use standard shortcodes.

For those managing WordPress sites, keeping plugins updated is the most effective defense against known CVEs. The vulnerability you mentioned was addressed in version 1.0.358 of the Colibri Page Builder plugin. You can find more information on securing WordPress code in the WordPress Plugin Handbook.