CVE-2017-7739 in FortiOS
Summary
by MITRE
A reflected Cross-site Scripting (XSS) vulnerability in web proxy disclaimer response web pages in Fortinet FortiOS 5.6.0, 5.4.0 to 5.4.5, 5.2.0 to 5.2.11 allows an unauthenticated attacker to inject arbitrary web script or HTML in the context of the victim's browser via sending a maliciously crafted URL to the victim.
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Analysis
by VulDB Data Team • 01/23/2021
The vulnerability identified as CVE-2017-7739 represents a critical reflected cross-site scripting flaw within Fortinet FortiOS web proxy disclaimer response pages. This issue affects multiple versions of the FortiOS operating system including 5.6.0, 5.4.0 through 5.4.5, and 5.2.0 through 5.2.11, creating a widespread security concern for organizations utilizing Fortinet's web proxy solutions. The vulnerability specifically manifests in the handling of web proxy disclaimer responses, which are typically displayed to users when accessing certain network resources through the FortiOS web proxy service.
The technical implementation of this flaw occurs when the web proxy service fails to properly sanitize user input parameters within the disclaimer response pages. When a victim clicks on a maliciously crafted URL containing XSS payload, the web proxy server reflects this malicious script back to the victim's browser without adequate input validation or output encoding. This reflection mechanism allows an attacker to inject arbitrary web scripts or HTML code directly into the victim's browser context, effectively bypassing the security boundaries that should separate legitimate web content from malicious payloads.
The operational impact of CVE-2017-7739 extends beyond simple script injection, as it provides attackers with the capability to execute malicious code within the context of the victim's browser session. This vulnerability can be exploited to perform session hijacking, steal cookies, redirect users to malicious websites, or even execute more sophisticated attacks such as credential theft or data exfiltration. The reflected nature of the vulnerability means that attackers do not need to persistently store malicious content on the server, as the attack is carried out through the immediate reflection of the malicious payload in the response. This characteristic makes the vulnerability particularly dangerous in environments where users frequently access web proxy services and where the disclaimer pages are commonly displayed.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to patched versions of FortiOS, implementing proper input validation and output encoding mechanisms, and deploying web application firewalls to filter malicious payloads. The vulnerability aligns with CWE-79 which specifically addresses cross-site scripting flaws, and can be categorized under ATT&CK technique T1212 for exploitation of web application vulnerabilities. Additionally, this issue demonstrates the importance of proper input sanitization in web applications and highlights the critical need for organizations to maintain up-to-date security patches for their network infrastructure components. The reflected XSS nature of this vulnerability also emphasizes the necessity of implementing Content Security Policy headers and other browser-based security mechanisms to provide defense-in-depth against such attacks.