CVE-2021-41024: Fortinet FortiProxy Path Traversal Flaw

CVE-2021-41024 Overview

CVE-2021-41024 is a relative path traversal vulnerability (CWE-22/CWE-23) affecting Fortinet FortiOS versions 7.0.0 and 7.0.1 as well as FortiProxy version 7.0.0. This vulnerability allows an unauthenticated, unauthorized attacker to inject path traversal character sequences to disclose sensitive information from the server via the GET request of the login page.

Critical Impact

Unauthenticated attackers can exploit this vulnerability remotely to access sensitive server information without any user interaction, potentially exposing configuration files, credentials, or other critical data.

Affected Products

• Fortinet FortiOS version 7.0.0
• Fortinet FortiOS version 7.0.1
• Fortinet FortiProxy version 7.0.0

Discovery Timeline

• 2021-12-08 - CVE-2021-41024 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41024

Vulnerability Analysis

This path traversal vulnerability exists in the login page handling mechanism of FortiOS and FortiProxy. The affected products fail to properly sanitize user-supplied input in GET requests directed at the login page, allowing attackers to include path traversal sequences (such as ../) to navigate outside intended directories and access sensitive files on the server.

The vulnerability can be exploited remotely over the network without requiring any authentication or user interaction. While the impact is limited to confidentiality—allowing information disclosure only—the severity is significant given that Fortinet devices often serve as critical network security infrastructure components. Exposed information could include system configuration files, internal network details, or other sensitive data that attackers could leverage for further attacks.

Root Cause

The root cause of CVE-2021-41024 is improper input validation in the web interface's login page handler. The application fails to adequately sanitize or validate user-supplied path information in GET request parameters, allowing relative path traversal sequences to be processed. This lack of proper input filtering enables attackers to break out of the intended web root directory and traverse the file system.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no authentication or privileges. An attacker can craft a malicious GET request to the login page endpoint that includes path traversal character sequences (e.g., ../../../etc/passwd). When the vulnerable FortiOS or FortiProxy device processes this request, it follows the traversal path and returns the contents of the requested file if accessible.

The attack requires no special conditions—the attacker simply needs network access to the device's web management interface. This makes Internet-facing FortiOS and FortiProxy devices particularly vulnerable to exploitation.

Detection Methods for CVE-2021-41024

Indicators of Compromise

• Unusual GET requests to the login page containing path traversal sequences such as ../, ..%2f, or ..%252f
• Web server access logs showing repeated attempts to access files outside the web root directory
• Anomalous traffic patterns targeting the FortiOS/FortiProxy management interface from external sources
• Evidence of sensitive file content in HTTP responses that should not be accessible via the web interface

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block path traversal patterns in incoming requests
• Configure intrusion detection systems (IDS) to alert on GET requests containing directory traversal sequences targeting Fortinet devices
• Enable comprehensive logging on FortiOS and FortiProxy devices and monitor for suspicious access patterns
• Deploy network traffic analysis to identify anomalous requests to the management interface

Monitoring Recommendations

• Monitor access logs for the FortiOS/FortiProxy login page for unusual request patterns or path traversal attempts
• Set up alerts for high volumes of requests from single IP addresses targeting the management interface
• Implement real-time log analysis to detect patterns consistent with path traversal exploitation attempts
• Review audit logs regularly for evidence of unauthorized information access

How to Mitigate CVE-2021-41024

Immediate Actions Required

• Upgrade FortiOS to a patched version beyond 7.0.1 as specified in the vendor advisory
• Upgrade FortiProxy to a patched version beyond 7.0.0 as specified in the vendor advisory
• Restrict access to the management interface to trusted networks and IP addresses only
• Ensure management interfaces are not exposed to the public Internet

Patch Information

Fortinet has released security updates to address this vulnerability. Administrators should consult the FortiGuard Security Advisory FG-IR-21-181 for specific patch versions and upgrade instructions. It is critical to apply these patches as soon as possible, particularly for devices with Internet-facing management interfaces.

Workarounds

• If immediate patching is not possible, restrict management interface access to trusted internal networks using firewall rules
• Implement a web application firewall (WAF) in front of the management interface to filter path traversal attempts
• Disable web-based management access from untrusted networks until patches can be applied
• Consider using VPN-only access for device management to limit exposure

# Example: Restrict management interface access via FortiOS CLI
config system interface
    edit "mgmt"
        set allowaccess ping https ssh
        set trust-ip 10.0.0.0/8 192.168.0.0/16
    next
end