CVE-2019-25375: OPNsense XSS Vulnerability

CVE-2019-25375 Overview

CVE-2019-25375 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 submitting crafted input to the mailserver parameter within the Monit interface. By sending specially crafted POST requests containing JavaScript payloads, attackers can execute arbitrary code in the browsers of users who interact with the malicious content.

Critical Impact

Successful exploitation enables attackers to steal session cookies, hijack user sessions, redirect users to malicious sites, or perform actions on behalf of authenticated administrators within the OPNsense management interface.

Affected Products

• OPNsense 19.1

Discovery Timeline

• 2026-02-15 - CVE-2019-25375 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2019-25375

Vulnerability Analysis

This reflected XSS vulnerability (CWE-79) exists in the Monit interface of OPNsense 19.1. The vulnerability occurs because user-supplied input to the mailserver parameter is not properly sanitized or encoded before being reflected back to the user's browser. This allows attackers to inject arbitrary JavaScript code that executes within the security context of the OPNsense web interface.

The attack requires user interaction—a victim must click a malicious link or submit a crafted form that sends the payload to the vulnerable endpoint. Since the vulnerability is in the firewall's management interface, successful exploitation could have significant security implications, particularly if targeting administrators with elevated privileges.

Root Cause

The root cause is improper input validation and output encoding in the Monit interface's handling of the mailserver parameter. When processing POST requests, the application fails to sanitize special characters such as angle brackets (<, >), quotes, and other HTML/JavaScript metacharacters. This input is then reflected directly into the HTTP response without proper encoding, allowing injected scripts to execute in the victim's browser context.

Attack Vector

The attack is network-based and requires no authentication. An attacker crafts a malicious URL or form that submits a POST request to the Monit interface with a JavaScript payload embedded in the mailserver parameter. When an authenticated user (particularly an administrator) clicks the link or is tricked into submitting the form, the malicious script executes in their browser session.

The vulnerability is exploited by embedding JavaScript code within the mailserver parameter value. For example, an attacker might include a script tag or event handler that executes when the page renders the unsanitized input. Technical details and proof-of-concept information can be found in the Exploit-DB entry #46351 and the VulnCheck Advisory.

Detection Methods for CVE-2019-25375

Indicators of Compromise

• Unusual POST requests to the Monit interface containing script tags or JavaScript event handlers in the mailserver parameter
• Web server logs showing URL-encoded JavaScript payloads such as %3Cscript%3E or javascript: in request parameters
• Unexpected outbound connections from administrator workstations after accessing the OPNsense interface
• Session anomalies indicating potential cookie theft or session hijacking

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS patterns in POST request parameters
• Monitor web server access logs for suspicious patterns including encoded script tags and event handlers
• Deploy browser-based security tools that detect and block reflected XSS attempts
• Use network intrusion detection systems (IDS) with signatures for common XSS attack patterns

Monitoring Recommendations

• Enable detailed logging on the OPNsense web interface and review logs for unusual parameter values
• Monitor for unexpected administrative actions that could indicate compromised sessions
• Implement Content Security Policy (CSP) headers to mitigate the impact of successful XSS attacks
• Track access patterns to the Monit interface and alert on anomalous activity

How to Mitigate CVE-2019-25375

Immediate Actions Required

• Upgrade OPNsense to a version newer than 19.1 that addresses this vulnerability
• Restrict access to the OPNsense management interface to trusted networks and IP addresses only
• Implement network-level access controls to limit who can reach the administrative interface
• Educate administrators about the risks of clicking untrusted links while authenticated to the firewall

Patch Information

OPNsense users should upgrade to a patched version of the software. Consult the OPNsense official website and OPNsense forum for the latest security updates and upgrade instructions. It is strongly recommended to update to the most recent stable release of OPNsense to ensure all security vulnerabilities are addressed.

Workarounds

• Restrict management interface access to localhost or specific trusted IP addresses using firewall rules
• Use a VPN to access the management interface instead of exposing it directly
• Implement a reverse proxy with XSS filtering capabilities in front of the OPNsense web interface
• Disable the Monit service if it is not required for your deployment

# Example: Restrict access to OPNsense management interface
# Add firewall rule to limit web GUI access to specific management network
# Navigate to Firewall -> Rules -> WAN and create a block rule
# Block all traffic to port 443 except from trusted management IPs

# Alternatively, configure management interface binding
# System -> Settings -> Administration
# Set Listen interfaces to "LAN" only (not WAN)
# Set TCP port to a non-standard port for additional obscurity