LibreNMS is a network monitoring system. In versions up to and including 26.4.0, the Proxmox application view is vulnerable to reflected cross-site scripting through the user-supplied instance and vmid GET parameters, which are reflected into the page title without adequate encoding. The parameters are placed into the page title with only strip_tags applied, and the title is then written into an inline document.title assignment through string interpolation, so a single quote terminates the JavaScript string and the remaining input runs as script. An attacker who lures an authenticated user into following a crafted link can execute script in that user's session, enabling actions such as theft of session data. This issue is fixed in version 26.5.0.

Affected: Packagist: librenms/librenms

### 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 $html .= "<script>\ndocument.title = '$title';\n</script>";

// 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 $html .= "<script>\ndocument.title = " . json_encode($title) . ";\n</script>"; ``` Also wrap `$instance` and `$vars['vmid']` in `htmlspecialchars()` in proxmox.inc.php.

### Prerequisite Any authenticated session. Victim must follow a crafted link.