GHSA-jmqm-f8q4-v7wx LibreNMS: Reflected XSS via Proxmox instance/vmid GET parameters injected into document.title JavaScript assignment

• Packagist librenms/librenms: < 26.5.0 → fixed in 26.5.0

Summary `LegacyController.php:75` writes the page title into a `document.title` JS assignment using string interpolation. `apps/proxmox.inc.php` pushes `$vars['instance']` and `$vars['vmid']` (GET params, only `strip_tags()` applied) directly into `$pagetitle`. A single quote terminates the JS string, executing arbitrary script. Details ```php // LegacyController.php:75 $html .= "\ndocument.title = '$title';\n"; // proxmox.inc.php:38,42 $pagetitle[] = $instance; // GET ?instance= $pagetitle[] = $vars['vmid']; // GET ?vmid= ``` PoC ``` http://target/apps?app=proxmox&instance=%27%3Balert%28document.cookie%29%3B// Confirmed in response: document.title = 'Apps - Proxmox - ';alert(document.cookie);// - LibreNMS'; ``` Fix ```php // LegacyController.php:75 $html .= "\ndocument.title = " . json_encode($title) . ";\n"; ``` Also wrap `$instance` and `$vars['vmid']` in `htmlspecialchars()` in proxmox.inc.php. Prerequisite Any authenticated session. Victim must follow a crafted link.