Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2019-25376 is a reflected cross-site scripting (XSS) vulnerability found in OPNsense version 19.1 and earlier.'}, {'type': 'paragraph', 'content': 'This vulnerability occurs because the application does not properly neutralize input when generating web pages, specifically in the handling of the "ignoreLogACL" parameter.'}, {'type': 'paragraph', 'content': 'Unauthenticated attackers can exploit this by sending specially crafted POST requests to the proxy endpoint, injecting malicious JavaScript code via the "ignoreLogACL" parameter.'}, {'type': 'paragraph', 'content': 'When users access the affected endpoint, the injected scripts execute in their browsers, potentially leading to unauthorized actions or data exposure.'}] [1]

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Impact Analysis

This vulnerability allows attackers to execute arbitrary scripts in the browsers of users who visit the affected endpoint.

Such script execution can lead to theft of sensitive information, session hijacking, or performing actions on behalf of the user without their consent.

Since the attack requires user interaction (the user must access the affected endpoint), the impact depends on users visiting the vulnerable interface.

The CVSS v4.0 base score of 5.1 indicates a medium severity level, with no privileges required for the attacker and low attack complexity.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by monitoring for suspicious POST requests sent to the proxy endpoint containing the parameter "ignoreLogACL" with potentially malicious JavaScript payloads.'}, {'type': 'paragraph', 'content': 'A practical approach is to capture and inspect HTTP traffic targeting the OPNsense proxy endpoint to identify any crafted requests attempting to inject scripts via the ignoreLogACL parameter.'}, {'type': 'paragraph', 'content': 'For example, using command-line tools like curl or network monitoring tools such as tcpdump or Wireshark can help detect such attempts.'}, {'type': 'list_item', 'content': 'Use curl to test the vulnerability by sending a crafted POST request: curl -X POST -d "ignoreLogACL=<script>alert(1)</script>" http://<opnsense-ip>/proxy_endpoint'}, {'type': 'list_item', 'content': "Use tcpdump to capture HTTP POST requests to the proxy endpoint: tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'"}] [1]

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Mitigation Strategies

The immediate mitigation step is to upgrade OPNsense to version 19.1.1 or later, where this reflected XSS vulnerability has been fixed.

Until the upgrade can be applied, consider restricting access to the proxy endpoint to trusted users only and implement web application firewall (WAF) rules to block requests containing suspicious payloads in the ignoreLogACL parameter.

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