CVE-2019-25371: OPNsense 19.1 XSS Vulnerability

CVE-2019-25371 Overview

CVE-2019-25371 is a reflected cross-site scripting (XSS) vulnerability affecting OPNsense 19.1, an open-source firewall and routing platform. The vulnerability allows unauthenticated attackers to inject malicious scripts by exploiting insufficient input validation in the host parameter of the diag_ping.php endpoint. When a victim accesses a crafted URL or submits a malicious form, arbitrary JavaScript executes in their browser context, potentially leading to session hijacking, credential theft, or further attacks against the firewall management interface.

Critical Impact

Unauthenticated attackers can execute arbitrary JavaScript in users' browsers by exploiting insufficient input validation in the diagnostic ping functionality, potentially compromising firewall administrator sessions.

Affected Products

• OPNsense 19.1

Discovery Timeline

• 2026-02-15 - CVE-2019-25371 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2019-25371

Vulnerability Analysis

This reflected XSS vulnerability exists in the diagnostic ping functionality of OPNsense 19.1. The diag_ping.php endpoint accepts user-supplied input through the host parameter without proper sanitization or encoding before reflecting it back in the HTTP response. This allows attackers to craft malicious requests containing JavaScript payloads that execute in the context of an authenticated user's browser session.

The attack requires user interaction, as the victim must be tricked into clicking a malicious link or submitting a crafted form. However, given that OPNsense is a firewall management interface typically accessed by administrators with elevated privileges, successful exploitation could lead to significant security implications including session hijacking, administrative credential theft, or unauthorized configuration changes to the firewall.

Root Cause

The root cause is improper input validation and output encoding in the diag_ping.php script. The host parameter is processed and reflected in the HTTP response without adequate sanitization, violating the security principle of treating all user input as untrusted. The application fails to implement proper context-aware output encoding when rendering user-supplied data in HTML contexts.

Attack Vector

The attack is network-based and requires no authentication. An attacker crafts a malicious POST request to the diag_ping.php endpoint with a script payload embedded in the host parameter. The attacker then distributes this payload via phishing emails, malicious websites, or other social engineering techniques to trick OPNsense administrators into executing the request. When the vulnerable page processes the request and reflects the unsanitized input, the malicious JavaScript executes in the victim's browser with the same privileges as their authenticated session.

The vulnerability can be exploited by submitting crafted POST requests containing JavaScript payloads in the host parameter. When the server processes these requests and reflects the unsanitized input back to the user's browser, the injected script executes in the context of the authenticated session. Detailed technical information is available in the Exploit-DB #46351 entry and the VulnCheck Advisory for OPNsense.

Detection Methods for CVE-2019-25371

Indicators of Compromise

• Suspicious POST requests to diag_ping.php containing script tags or JavaScript event handlers in the host parameter
• Web server logs showing encoded characters like %3Cscript%3E or %22onmouseover%3D in requests to diagnostic endpoints
• Unusual administrative session activity following user interaction with external links

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing common XSS payloads targeting the host parameter
• Deploy browser-based security controls such as Content Security Policy (CSP) headers to mitigate script injection attacks
• Enable detailed logging for the OPNsense web interface and monitor for anomalous POST requests to diag_ping.php

Monitoring Recommendations

• Review web server access logs for requests to diag_ping.php with unusual or encoded payload characters
• Monitor authentication events for suspicious session activity following potential XSS exploitation
• Implement real-time alerting for requests containing known XSS patterns targeting OPNsense administrative endpoints

How to Mitigate CVE-2019-25371

Immediate Actions Required

• Upgrade OPNsense to a version newer than 19.1 that addresses this vulnerability
• Restrict access to the OPNsense web management interface to trusted networks only
• Implement network segmentation to limit exposure of the firewall management interface
• Educate administrators about phishing attacks and the risks of clicking untrusted links while authenticated

Patch Information

Users should upgrade to a patched version of OPNsense. For the latest security updates and version information, refer to the OPNsense Official Website. Additional discussion about this vulnerability can be found in the OPNsense Forum Topic.

Workarounds

• Restrict access to the OPNsense web interface to specific trusted IP addresses using firewall rules
• Implement a reverse proxy with WAF capabilities in front of the OPNsense web interface to filter malicious requests
• Disable the diagnostic ping functionality if not required until an upgrade can be performed
• Use separate browser sessions or profiles for OPNsense administration to minimize session hijacking risks

# Example: Restrict OPNsense web interface access to management network only
# Add firewall rule to limit access to the web interface
# Navigate to Firewall > Rules > WAN and create a blocking rule for port 443/80
# Allow only from trusted management subnet (e.g., 10.0.0.0/24)