CVE-2018-9186 in FortiAuthenticator
Summary
by MITRE
A cross-site scripting (XSS) vulnerability in Fortinet FortiAuthenticator below 5.3.0 versions "CSRF validation failure" page allows attacker to execute unauthorized script code via inject malicious scripts in HTTP referer header.
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Analysis
by VulDB Data Team • 02/10/2020
The vulnerability identified as CVE-2018-9186 represents a critical cross-site scripting flaw within Fortinet FortiAuthenticator authentication platform affecting versions prior to 5.3.0. This security weakness stems from inadequate input validation mechanisms that fail to properly sanitize user-supplied data within the CSRF validation failure page. The vulnerability specifically manifests when the application processes HTTP referer headers without adequate sanitization, creating an avenue for malicious actors to inject and execute unauthorized script code within the context of authenticated user sessions. The flaw resides in the application's failure to implement proper output encoding and input validation controls, particularly when handling HTTP headers that are typically considered trusted components of web traffic. This vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is classified as a critical weakness in web applications due to its potential for user session hijacking and data exfiltration.
The technical exploitation of this vulnerability occurs through the manipulation of HTTP referer headers during CSRF validation failure scenarios. When a user encounters a CSRF validation error, the FortiAuthenticator system displays a failure page that improperly processes the referer header value. Attackers can craft malicious referer values containing script payloads that get executed when the page renders, particularly affecting users with administrative privileges or those who interact with the error page. The vulnerability demonstrates a classic case of insufficient input validation where the application assumes referer headers are safe and does not perform proper sanitization or encoding before displaying them. This weakness allows for both reflected and stored XSS scenarios depending on how the application processes and stores the referer information, with the potential for persistent script execution across multiple user sessions.
The operational impact of CVE-2018-9186 extends beyond simple script execution, as it provides attackers with the capability to hijack user sessions, steal authentication tokens, and potentially escalate privileges within the FortiAuthenticator environment. An attacker could leverage this vulnerability to execute malicious scripts that redirect users to phishing sites, capture session cookies, or perform unauthorized administrative actions on behalf of legitimate users. The attack surface is particularly concerning given that FortiAuthenticator serves as an authentication platform where successful exploitation could compromise the entire authentication infrastructure. This vulnerability directly maps to ATT&CK technique T1531 for Credential Access and T1059 for Command and Scripting Interpreter, as it enables attackers to execute malicious code within the context of authenticated sessions and manipulate the authentication flow. The impact is amplified in environments where FortiAuthenticator is used as a central authentication point for multiple services and applications, potentially allowing lateral movement and privilege escalation throughout the network infrastructure.
Mitigation strategies for CVE-2018-9186 require immediate implementation of input validation and output encoding controls within the FortiAuthenticator application. Organizations should upgrade to FortiAuthenticator version 5.3.0 or later, which includes proper sanitization of HTTP referer headers and enhanced CSRF protection mechanisms. Additional defensive measures include implementing proper header validation, applying Content Security Policy headers to limit script execution, and configuring web application firewalls to monitor and block suspicious referer header values. Security teams should also conduct comprehensive code reviews to ensure all HTTP header processing follows secure coding practices and implement regular vulnerability scanning to identify similar weaknesses in other components of the authentication infrastructure. The remediation process must include thorough testing to verify that referer header sanitization does not break legitimate functionality while effectively preventing XSS exploitation. Organizations should also consider implementing additional monitoring and alerting mechanisms to detect potential exploitation attempts and maintain detailed logs of authentication error conditions for forensic analysis.