Admidio vulnerable to reflected XSS in msg_window.php via Square Bracket to HTML Tag Conversion

Summary

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.

Details

The msg_window.php endpoint accepts message_id and message_var1 as GET parameters. At line 146, the code passes message_var1 through htmlspecialchars() before inserting it into the language string as a placeholder:

// system/msg_window.php:146
$messageText = $gL10n->get($messageId, array(htmlspecialchars($messageVar1)));

htmlspecialchars() encodes <, >, &, ", and ' but leaves [ and ] untouched. The Language::get() method calls Language::prepareTextPlaceholders(), which performs a character-level translation:

// src/Infrastructure/Language.php:497
$text = strtr($text, '[]', '<>');

This converts all square brackets in the output string, including those from user input, into angle brackets. An attacker submits [script]alert(document.domain)[/script], which survives htmlspecialchars() unchanged, then becomes <script>alert(document.domain)</script> after placeholder preparation.

The application sets no Content-Security-Policy headers, so inline scripts execute without restriction.

Proof of Concept

Visit the following URL on any Admidio installation (no authentication required):

https://<admidio-host>/system/msg_window.php?message_id=INS_INSTALLATION_VERSION&message_var1=%5Bscript%5Dalert(document.domain)%5B/script%5D

The server responds with an HTML page containing:

Installation version <script>alert(document.domain)</script>