OS Command Injection via ‘lang’ Parameter in Fortinet VPN SSL Interface by | Sep 2, 2025 | Articles, External Pentesting Write ups

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Introduction
While automated tools are useful for maintaining baseline security, they often miss sophisticated, multi-layered vulnerabilities that require a manual, strategic approach to uncover. At Black Hat Ethical Hacking (BHEH), our Red Team employs advanced manual testing, real-world attack simulations, and in-depth system analysis to uncover vulnerabilities that automated methods often overlook. This process highlights the importance of human expertise and creativity in identifying and exploiting complex weaknesses that could compromise even well-protected systems.
 

Executive Summary
During an external pentesting, a critical OS Command Injection vulnerability was identified by our team in a Fortinet SSL VPN web interface, specifically through manipulation of the lang (language) parameter. This flaw allows an unauthenticated remote attacker to execute arbitrary system-level commands on the VPN appliance due to improper input sanitization. In testing, a simple time-delay command injected into the lang parameter caused the VPN server to pause its response (demonstrating code execution) and then return a normal page, instead of an error. Both manual exploitation (using tools like OWASP ZAP) and automated scanning confirmed the vulnerability’s impact and reliability, underlining its critical severity.
Security Lesson Learned: This vulnerability highlights that even seemingly harmless parameters like lang can become critical attack vectors when not properly sanitized. Developers must apply strict input validation and whitelisting on...