Click to Chat <= 4.39 - Authenticated (Contributor+) Stored Cross-Site Scripting via 'num' Shortcode Parameter

The Click to Chat – WA Widget plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the [chat] shortcode 'num' parameter in all versions up to, and including, 4.38. This is due to insufficient escaping when embedding user-supplied shortcode attribute values inside JavaScript string literals that are then placed in HTML event-handler attributes. The CCW_Shortcode::shortcode() function applies esc_attr() to the 'num' parameter (line 157), which converts single quotes to the HTML entity &#039;. This entity-encoded value is then interpolated directly into a JavaScript window.open() call string delimited by single quotes (line 194/221), and that complete string is placed verbatim into an HTML onclick attribute in the style template files (e.g., sc-style-1.php line 6). Because browsers HTML-decode event attribute values before executing the embedded JavaScript, the &#039; entities are decoded back to literal single quotes at runtime, allowing the injected payload to break out of the JavaScript string context and execute arbitrary code. This makes it possible for authenticated attackers with Contributor-level access and above to inject arbitrary web scripts into pages that will execute whenever a user clicks the WhatsApp chat button rendered by the [chat] shortcode.