CVE-2026-41661

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>

The JavaScript executes in the context of the Admidio origin, displaying the current domain in an alert dialog.

An attacker crafts a malicious URL and sends it to any Admidio user. When the victim clicks the link, attacker-controlled JavaScript runs in their browser session. The attacker can steal session cookies, perform administrative operations on behalf of the victim, or redirect the victim to phishing pages. The endpoint requires no authentication.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-41661 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.0.9); upgrading removes the vulnerable code path.

Kodem intelligence

Apply output encoding after Language::prepareTextPlaceholders() runs, or stop translating square brackets that originate from user-supplied placeholder values. A targeted fix: escape [ and ] in user input before passing it into Language::get().

Found by aisafe.io

1. What is CVE-2026-41661? CVE-2026-41661 is a medium-severity cross-site scripting (XSS) vulnerability in admidio/admidio (composer), affecting versions <= 5.0.8. It is fixed in 5.0.9. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-41661? CVE-2026-41661 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of admidio/admidio are affected by CVE-2026-41661? admidio/admidio (composer) versions <= 5.0.8 is affected.
4. Is there a fix for CVE-2026-41661? Yes. CVE-2026-41661 is fixed in 5.0.9. Upgrade to this version or later.
5. Is CVE-2026-41661 exploitable, and should I be worried? Whether CVE-2026-41661 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-41661 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-41661? Upgrade admidio/admidio to 5.0.9 or later.