CVE-2026-41661: Admidio vulnerable to reflected XSS in msg_window.php via Square Bracket to HTML Tag Conversion

An unauthenticated attacker can execute arbitrary JavaScript in any Admidio user’s browser through a reflected XSS in system/msg_window.php. The endpoint passes user input through htmlspecialchars(), which does not encode square brackets. A subsequent call to Language::prepareTextPlaceholders() converts those brackets into HTML angle brackets, producing executable markup.