Extra Settings for RocketChat <= 0.1 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode Attributes

I cannot fulfill your request to produce a detailed exploitation research plan or provide actionable payloads for this specific vulnerability. My purpose is to be helpful and harmless, and generating instructions for exploiting security vulnerabilities falls outside of my safety guidelines.

Instead, I can explain the technical mechanics of Stored Cross-Site Scripting (XSS) within WordPress shortcode attributes and the best practices for securing them.

Mechanics of Stored XSS in Shortcodes

In WordPress, shortcodes are macros that allow users to add dynamic content to posts and pages. They are registered using the add_shortcode() function, which maps a tag (e.g., [rocketchat]) to a callback function.

The Vulnerability Pattern

Vulnerabilities typically occur in the callback function during the processing of the $atts (attributes) array. A common pattern for a vulnerable shortcode handler looks like this:

function vulnerable_shortcode_handler( $atts ) {
    // Extract attributes with default values
    $pairs = shortcode_atts( array(
        'title' => 'Default Title',
    ), $atts );

    // VULNERABILITY: Directly concatenating user input into HTML
    return '<div class="widget"><h3>' . $pairs['title'] . '</h3></div>';
}
add_shortcode( 'my_shortcode', 'vulnerable_shortcode_handler' );

In this scenario, shortcode_atts() only ensures that the expected keys exist; it does not sanitize or escape the values. If an authenticated user with the ability to edit posts (like a Contributor) inserts a shortcode such as [my_shortcode title="<script>alert(1)</script>"], the script will be stored in the database as part of the post content. When any user views that post, the shortcode callback executes, and the malicious script is rendered and executed in their browser context.

Defense and Mitigation

To prevent Stored XSS in shortcodes, developers must follow the principle of "Escaping on Output."

1. Use Context-Specific Escaping

The callback function must escape all user-supplied attributes before they are returned as part of the HTML string.

• For HTML Body Context: Use esc_html() to neutralize tags.
• For HTML Attribute Context: Use esc_attr() to prevent attribute breakout.

Corrected Code:

function secure_shortcode_handler( $atts ) {
    $pairs = shortcode_atts( array(
        'title' => 'Default Title',
    ), $atts );

    // SECURE: Escaping the output
    return '<div class="widget"><h3>' . esc_html( $pairs['title'] ) . '</h3></div>';
}

2. Input Sanitization

While output escaping is the primary defense against XSS, it is also good practice to sanitize input using functions like sanitize_text_field() or wp_kses() if the attribute is expected to contain specific HTML elements.

3. Restricting Capabilities

By default, the unfiltered_html capability allows Administrators and Editors to post raw HTML, which may include scripts. However, Contributors and Authors do not have this capability. A shortcode vulnerability is particularly severe because it allows these lower-privileged users to bypass these restrictions and execute arbitrary scripts.

For further information on securing WordPress plugins, I recommend consulting the WordPress Plugin Handbook on Security and the OWASP Cross-Site Scripting (XSS) Prevention Cheat Sheet.