CVE-2019-25377: OPNsense 19.1 XSS Vulnerability

CVE-2019-25377 Overview

CVE-2019-25377 is a reflected cross-site scripting (XSS) vulnerability in OPNsense 19.1 that affects the system_advanced_sysctl.php endpoint. This vulnerability allows attackers to inject malicious JavaScript code via the value parameter, which is then executed in the context of authenticated user sessions. Attackers can craft specially constructed POST requests containing script payloads to exploit this flaw, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of legitimate users.

Critical Impact

Attackers can execute arbitrary JavaScript in authenticated OPNsense administrator sessions, potentially leading to complete firewall compromise through session hijacking or credential theft.

Affected Products

• OPNsense 19.1

Discovery Timeline

• 2026-02-15 - CVE-2019-25377 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2019-25377

Vulnerability Analysis

This reflected XSS vulnerability exists due to insufficient input sanitization in the system_advanced_sysctl.php endpoint within OPNsense 19.1. The endpoint accepts user-supplied input through the value parameter without properly encoding or validating it before reflecting it back in the HTTP response. This allows attackers to inject arbitrary JavaScript code that executes within the security context of the victim's browser session.

The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), which encompasses cross-site scripting flaws. When exploited against an authenticated administrator, the attacker gains the ability to perform any action the administrator could perform, including modifying firewall rules, creating new administrative accounts, or exfiltrating sensitive configuration data.

Root Cause

The root cause of this vulnerability is improper input validation and output encoding in the system_advanced_sysctl.php script. The value parameter is accepted from POST requests and reflected in the page response without adequate sanitization. Specifically, the application fails to properly escape or encode special HTML characters such as <, >, ", and ' before including user input in the rendered page content. This allows script tags and JavaScript event handlers to be injected and executed by the browser.

Attack Vector

The attack requires an authenticated user to be tricked into submitting a malicious POST request to the vulnerable endpoint. This is typically achieved through social engineering tactics such as:

1. Crafting a malicious HTML page that automatically submits a form to the target OPNsense instance when visited
2. Embedding the malicious form in a phishing email or compromised website
3. Using CSRF techniques in conjunction with the XSS payload to maximize impact

When the victim visits the attacker-controlled page while authenticated to OPNsense, the malicious JavaScript payload executes in the context of their session. The attack requires network access to the OPNsense management interface and depends on user interaction to trigger the payload.

The vulnerability is exploited via specially crafted POST requests to the system_advanced_sysctl.php endpoint. The value parameter accepts script payloads that are reflected without sanitization. Detailed technical information and proof-of-concept code is available in the Exploit-DB #46351 advisory.

Detection Methods for CVE-2019-25377

Indicators of Compromise

• Unusual POST requests to system_advanced_sysctl.php containing <script> tags or JavaScript event handlers in the value parameter
• HTTP access logs showing encoded script payloads (%3Cscript%3E, %22onclick%3D) targeting the sysctl endpoint
• Browser console errors or unexpected JavaScript execution on OPNsense administrative pages
• Administrative actions logged without corresponding legitimate user activity

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing common XSS payload patterns targeting the value parameter
• Monitor HTTP POST requests to system_advanced_sysctl.php for suspicious characters or encoded script content
• Deploy intrusion detection signatures to identify XSS attack patterns in network traffic to OPNsense management interfaces
• Review OPNsense access logs for unusual patterns or requests from unexpected source IP addresses

Monitoring Recommendations

• Enable detailed logging for all administrative interface access on OPNsense appliances
• Configure SIEM alerts for requests containing script injection patterns targeting firewall management interfaces
• Monitor for unauthorized configuration changes that may indicate successful exploitation
• Implement network segmentation monitoring to detect unexpected access to management interfaces

How to Mitigate CVE-2019-25377

Immediate Actions Required

• Upgrade OPNsense to a version newer than 19.1 that includes the security fix
• Restrict access to the OPNsense management interface to trusted networks only using firewall rules
• Implement network segmentation to limit exposure of administrative interfaces
• Enable Content Security Policy (CSP) headers if supported by the OPNsense version

Patch Information

OPNsense users should upgrade to a version newer than 19.1 to address this vulnerability. The latest stable release can be obtained from the Official OPNsense Website. Additional discussion regarding this vulnerability can be found in the OPNsense Forum Discussion. The Vulncheck OPNsense XSS Advisory provides additional technical details about the vulnerability.

Workarounds

• Restrict management interface access to localhost or specific trusted IP addresses using firewall rules
• Implement a reverse proxy with WAF capabilities in front of the OPNsense management interface to filter malicious requests
• Disable remote administrative access when not actively needed for management tasks
• Train administrators to be cautious of suspicious links and avoid clicking untrusted URLs while authenticated to OPNsense

# Configuration example - Restrict management interface access
# Add these rules to your OPNsense firewall to limit administrative access

# Allow management access only from trusted management network
# Navigate to: Firewall > Rules > LAN (or appropriate interface)
# Create rule: Pass | IPv4 | TCP | Source: Management_Network | Destination: This Firewall | Port: 443
# Create rule: Block | IPv4 | TCP | Source: Any | Destination: This Firewall | Port: 443