LibreNMS alert-rules has a Cross-Site Scripting Vulnerability

Executive Summary

Product: LibreNMS
Vendor: LibreNMS
Vulnerability Type: Cross-Site Scripting (XSS)
CVSS Score: 4.3 (AV:N/AC:L/PR:H/UI:R/S:U/C:L/I:L/A:L)
Affected Version: 25.8.0 (latest at time of discovery)
POC File: Download POC Ticket: ZDI-CAN-28105: LibreNMS Alert Rules Cross-Site Scripting Vulnerability

Vulnerability Details

Description

Trend Micro's Zero Day Initiative has identified a Cross-Site Scripting vulnerability in LibreNMS. The vulnerability exists in the Alert Rules functionality where the alert rule name is not properly sanitized, allowing injection of HTML code.

Technical Details

Version Tested: 25.8.0
Installer File: 25.8.0.tar.gz
Download Link: https://github.com/librenms/librenms/archive/refs/tags/25.8.0.tar.gz
Platform: N/A

Attack Vector

When browsing to Alerts > Alert Rules page, a LibreNMS admin can add and manage alert rules. The alert rule name field is vulnerable to XSS attacks through improper sanitization.

Root Cause Analysis

Vulnerable Request

When creating or updating an alert rule, the following HTTP POST request is sent to /ajax_form.php:

POST /ajax_form.php HTTP/1.1
...

_token=9YjTntCuMIe2ujpumwqJQoENRXUhJzlDt33Xu7kx&device_id=-1&device_name=&rule_id=&type=alert-rules&template_id=&builder_json=%7B%22condition%22%3A%22AND%22%2C%22rules%22%3A%5B%7B%22id%22%3A%22access_points.accesspoint_id%22%2C%22field%22%3A%22access_points.accesspoint_id%22%2C%22type%22%3A%22string%22%2C%22input%22%3A%22text%22%2C%22operator%22%3A%22equal%22%2C%22value%22%3A%2242%22%7D%5D%2C%22valid%22%3Atrue%7D&name=%3Ci%3Efoo%3C%2Fi%3E&builder_rule_0_filter=access_points.accesspoint_id&builder_rule_0_operator=equal&builder_rule_0_value_0=42&severity=warning&count=1&delay=1m&interval=5m&recovery=on&acknowledgement=on&proc=&notes=&adv_query=

Code Flow

1. Request Processing: PHP script includes/html/forms/alert-rules.inc.php processes the request
2. Sanitization Attempt: Calls strip_tags() to sanitize the name parameter
3. Database Operation: Calls dbUpdate() or dbInsert() to save the rule

Bypass Technique

The sanitization can be bypassed using XML character references:

<script>alert(1)</script>

Execution Path

1. Page Load: Victim browses to Alerts > Alert Rules page
2. Script Execution: includes/html/print-alert/rules.php is called
3. Modal Inclusion: Includes includes/html/modal/alert_rule_list.inc.php which returns HTML for modal window
4. Table Rendering: Modal contains HTML table with all rules and inline JavaScript calling bootgrid() function
5. XSS Trigger: The bootgrid() function (http://www.jquery-bootgrid.com/) rewrites table cells, decoding XML character references
6. Code Execution: Browser interprets the decoded payload as HTML tags and executes the injected script

Proof of Concept

Usage

python3 poc.py client ip_addr -U <username> -P <password>

Optional Parameters

• -E [kvp|multipart] - Specify HTTP request parameter encoding

Credit

Discovered by: Simon Humbert of Trend Research, Trend Micro

About Zero Day Initiative (ZDI)

Established by TippingPoint and acquired by Trend Micro, the Zero Day Initiative (ZDI) neither re-sells vulnerability details nor exploit code. Instead, upon notifying the affected product vendor, the ZDI provides its Trend Micro TippingPoint customers with zero day protection through its intrusion prevention technology. Explicit details regarding the specifics of the vulnerability are not exposed to any parties until an official vendor patch is publicly available.

References

• ZDI Website: http://www.zerodayinitiative.com
• Disclosure Policy: http://www.zerodayinitiative.com/advisories/disclosure_policy/