CVE-2021-26092: Fortinet FortiProxy XSS Vulnerability

CVE-2021-26092 Overview

CVE-2021-26092 is a reflected Cross-Site Scripting (XSS) vulnerability affecting the SSL VPN web portal of Fortinet FortiOS and FortiProxy products. The vulnerability stems from a failure to properly sanitize user-supplied input, allowing remote unauthenticated attackers to inject malicious scripts by sending crafted requests to the error page with malicious GET parameters.

Critical Impact

Unauthenticated attackers can execute arbitrary JavaScript in the context of a victim's browser session, potentially leading to session hijacking, credential theft, or phishing attacks targeting SSL VPN users.

Affected Products

• Fortinet FortiOS versions 5.2.10 through 5.2.15, 5.4.0 through 5.4.13, 5.6.0 through 5.6.14, 6.0.0 through 6.0.12, 6.2.0 through 6.2.7, 6.4.0 through 6.4.4
• Fortinet FortiProxy versions 1.2.0 through 1.2.9
• Fortinet FortiProxy versions 2.0.0 through 2.0.1

Discovery Timeline

• 2022-02-24 - CVE-2021-26092 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-26092

Vulnerability Analysis

This vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists within the SSL VPN web portal's error page handling mechanism, where user-controlled input from GET parameters is reflected back to the browser without proper sanitization or encoding.

When a user accesses the SSL VPN portal's error page, the application processes GET parameters and incorporates them into the rendered HTML response. Due to insufficient input validation and output encoding, an attacker can craft a malicious URL containing JavaScript code that executes in the victim's browser when clicked.

The attack requires user interaction, as the victim must click on a malicious link or be redirected to the crafted URL. However, no authentication is required to exploit this vulnerability, making it accessible to any remote attacker who can deliver the malicious link to potential victims.

Root Cause

The root cause of CVE-2021-26092 is improper input sanitization in the SSL VPN web portal's error handling functionality. The application fails to adequately validate and encode user-supplied GET parameters before including them in the HTML response. This allows special characters and JavaScript code to be interpreted as executable content rather than being rendered as plain text.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no authentication. An attacker exploits this flaw through the following approach:

1. The attacker identifies a vulnerable FortiOS or FortiProxy SSL VPN portal
2. A malicious URL is crafted containing JavaScript payload within GET parameters targeting the error page
3. The attacker distributes this URL to potential victims via phishing emails, social media, or other delivery mechanisms
4. When a victim clicks the link while having an active session, the malicious script executes in their browser context
5. The script can then steal session cookies, capture credentials, redirect users to phishing pages, or perform actions on behalf of the authenticated user

Since this vulnerability targets the SSL VPN web portal—a critical remote access component—successful exploitation could lead to compromise of VPN credentials or session tokens, enabling further network intrusion.

Detection Methods for CVE-2021-26092

Indicators of Compromise

• Unusual URL patterns in web server logs containing encoded JavaScript or HTML tags in GET parameters directed at the SSL VPN error page
• Browser-based alerts or anomalous behavior reported by users accessing the SSL VPN portal
• Network traffic containing reflected XSS payloads in HTTP responses from the FortiOS or FortiProxy SSL VPN endpoints
• User reports of unexpected redirects or credential prompts when accessing the VPN portal

Detection Strategies

• Deploy Web Application Firewall (WAF) rules to detect and block common XSS payload patterns in incoming requests
• Monitor SSL VPN access logs for requests containing suspicious characters such as <script>, javascript:, onerror=, or encoded variants in URL parameters
• Implement Content Security Policy (CSP) headers to reduce the impact of successful XSS attacks
• Use endpoint detection solutions to identify suspicious browser behavior indicative of XSS exploitation

Monitoring Recommendations

• Regularly audit SSL VPN web portal access logs for anomalous request patterns
• Configure alerting for high-frequency error page access with unusual parameter lengths or character sets
• Monitor for outbound connections from client browsers to unknown domains following VPN portal access
• Track changes in user session behavior that could indicate session hijacking

How to Mitigate CVE-2021-26092

Immediate Actions Required

• Update FortiOS to version 6.4.5 or later, 6.2.8 or later, 6.0.13 or later, 5.6.15 or later, or 5.4.14 or later
• Update FortiProxy to version 2.0.2 or later, or 1.2.10 or later
• Review SSL VPN access logs for evidence of exploitation attempts
• Implement Web Application Firewall rules to block XSS payloads as an interim measure
• Educate users about the risks of clicking suspicious links related to VPN access

Patch Information

Fortinet has released security updates addressing this vulnerability. Administrators should consult the FortiGuard Security Advisory FG-IR-20-199 for specific patch versions and detailed remediation guidance. The advisory provides version-specific upgrade paths for all affected FortiOS and FortiProxy releases.

Workarounds

• Deploy a reverse proxy or WAF in front of the SSL VPN portal configured to sanitize incoming requests
• Implement strict Content Security Policy headers to prevent inline script execution where possible
• Restrict SSL VPN portal access to trusted IP ranges while awaiting patching
• Consider temporarily disabling customized error pages if they can be modified to remove vulnerable parameter reflection
• Use network segmentation to limit the impact of potential credential compromise from XSS attacks