WPBakery Page Builder Addons by Livemesh <= 3.9.4 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode Attributes

I cannot fulfill this request. I am prohibited from generating functional exploit plans, actionable attack payloads, or specific step-by-step instructions for exploiting vulnerabilities in software. I can, however, provide information on the defensive side of this vulnerability class and how to implement proper security controls in WordPress plugins.

Understanding Stored XSS via Shortcode Attributes

The vulnerability described (Stored Cross-Site Scripting via Shortcode Attributes) occurs when a WordPress plugin allows users to provide input through shortcodes that is then stored and later rendered on a page without proper sanitization or escaping.

In the case of CVE-2026-2030, the vulnerability arises from an "attribute breakout." This happens when user-supplied attributes are embedded into an HTML tag's attribute (like data-settings) in a way that allows an attacker to terminate the attribute and inject new ones, such as event handlers (e.g., onmouseover).

The Vulnerable Pattern

Developers often use wp_json_encode() to pass complex configuration data from PHP to JavaScript via HTML data-* attributes. A common mistake is to place the resulting JSON string inside single quotes without further escaping:

// VULNERABLE CODE EXAMPLE
$atts = shortcode_atts( array( 'speed' => '500' ), $user_provided_atts );
$settings = wp_json_encode( $atts );

// If speed contains a single quote, it can break out of the data-settings attribute
echo '<div class="carousel" data-settings=\'' . $settings . '\'></div>';

If an attacker sets speed to something like 500' onmouseover='alert(1), the resulting HTML might look like this:

<div class="carousel" data-settings='{"speed":"500' onmouseover='alert(1)"}'></div>

The single quote after 500 closes the data-settings attribute, and the browser interprets onmouseover='alert(1)' as a new attribute on the div element.

Defensive Best Practices

To prevent this type of vulnerability, developers must adhere to the principle of context-aware escaping.

1. Always use esc_attr(): Any value placed inside an HTML attribute must be passed through esc_attr(). This function ensures that quotes, ampersands, and angle brackets are correctly encoded, preventing attribute breakout.

   // SECURE CODE EXAMPLE
   echo '<div class="carousel" data-settings="' . esc_attr( wp_json_encode( $atts ) ) . '"></div>';
   

2. Use Double Quotes for Attributes: While HTML allows single quotes for attributes, using double quotes and ensuring the content is escaped via esc_attr() is the standard practice in WordPress development.

3. Sanitize Input Early: Although escaping on output is the primary defense against XSS, sanitizing input when the shortcode is processed adds an extra layer of security. Use functions like sanitize_text_field() or absint() for numeric values.

4. Enforce Least Privilege: Vulnerabilities triggered by Contributor-level users highlight the importance of security even for authenticated roles. Always assume that any user-controlled input, regardless of the user's role, could be malicious.

For more information on securing WordPress plugins, I recommend consulting the WordPress Plugin Handbook's Security section and the OWASP Top Ten project.