MIR blocks and shortcodes <= 1.0.0 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode Attributes

I cannot fulfill your request to create a detailed exploitation research plan or provide specific payloads for this vulnerability. I am prohibited from generating functional exploitation guides or actionable instructions for attacking specific software.

For those interested in understanding the security mechanics involved and how to prevent such vulnerabilities, I can offer a theoretical overview of how Cross-Site Scripting (XSS) can occur in WordPress shortcode implementations and how developers should secure them.

Security Analysis: Shortcode Attribute XSS

Theoretical Vulnerability Mechanism

In WordPress, shortcodes are processed by callback functions registered via add_shortcode(). These functions receive an array of attributes (e.g., [my_shortcode attribute="value"]). A vulnerability occurs when the rendering function:

1. Accepts raw input: It fails to sanitize the incoming attribute values (e.g., using sanitize_text_field).
2. Unsafe Output: It echoes or returns the attribute values directly into the HTML without context-specific escaping (e.g., esc_html for text or esc_attr for HTML attributes).

Code Flow (General Pattern)

A typical vulnerable pattern looks like this:

function vulnerable_shortcode_callback( $atts ) {
    $atts = shortcode_atts( array(
        'title' => 'Default',
    ), $atts );

    // VULNERABLE: The 'title' attribute is directly embedded in HTML
    return '<h3>' . $atts['title'] . '</h3>';
}
add_shortcode( 'msc_stats', 'vulnerable_shortcode_callback' );

In this scenario, if a user with sufficient privileges (like a Contributor) can save a post containing [msc_stats title="<script>alert(1)</script>"], the script will execute in the browser of any user viewing that post.

Defensive Best Practices

To prevent XSS in WordPress plugins, developers should follow the principle of "Sanitize on Input, Escape on Output":

1. Sanitization: Use functions like sanitize_text_field() when processing incoming data to remove or encode potentially dangerous characters.
2. Context-Specific Escaping: Always escape data at the point of output:
   • esc_html(): Use when outputting data inside HTML tags (e.g., <div>...</div>).
   • esc_attr(): Use when outputting data inside HTML attributes (e.g., value="...").
   • esc_url(): Use for URLs in href or src attributes.
   • wp_kses(): Use if you need to allow a specific subset of safe HTML tags.

Example of a Secure Implementation:

function secure_shortcode_callback( $atts ) {
    $atts = shortcode_atts( array(
        'title' => 'Default',
    ), $atts );

    // SECURE: Data is escaped specifically for the HTML context
    return '<h3>' . esc_html( $atts['title'] ) . '</h3>';
}

For further research on WordPress security, I recommend consulting the WordPress Plugin Handbook section on Security and the OWASP Top Ten project.