CVE-2021-32592: Fortinet FortiClient Path Traversal Flaw

CVE-2021-32592 Overview

CVE-2021-32592 is an unsafe search path vulnerability affecting Fortinet's FortiClientWindows and FortiClientEMS products. This DLL Hijacking vulnerability allows an attacker to execute arbitrary code on affected devices by placing a malicious OpenSSL engine library in the application's search path. The vulnerability stems from improper handling of DLL loading paths, enabling attackers with local access to potentially gain elevated privileges or execute malicious code within the context of the vulnerable application.

Critical Impact

Attackers can achieve code execution with the privileges of the FortiClient application by exploiting the unsafe DLL search path to load a malicious OpenSSL engine library.

Affected Products

• Fortinet FortiClientWindows 7.0.0
• Fortinet FortiClientWindows 6.4.6 and below
• Fortinet FortiClientWindows 6.2.x and 6.0.x
• Fortinet FortiClientEMS 7.0.0
• Fortinet FortiClientEMS 6.4.6 and below
• Fortinet FortiClientEMS 6.2.x and 6.0.x

Discovery Timeline

• December 1, 2021 - CVE-2021-32592 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-32592

Vulnerability Analysis

This vulnerability is classified under CWE-427 (Uncontrolled Search Path Element), which occurs when an application searches for critical resources using an externally-supplied search path that could point to untrusted directories. In the case of FortiClientWindows and FortiClientEMS, the application fails to properly validate or restrict the locations from which DLL files, specifically OpenSSL engine libraries, can be loaded.

DLL Hijacking attacks exploit the Windows DLL search order mechanism. When an application attempts to load a DLL without specifying a fully qualified path, Windows searches through a predefined sequence of directories. An attacker who can place a malicious DLL in a directory that is searched before the legitimate location can intercept the load request and execute arbitrary code.

The local attack vector means an attacker must have some level of access to the target system, either through physical access, remote desktop, or another foothold. While user interaction is required, the attack requires no special privileges to execute, making it a viable persistence or privilege escalation mechanism in enterprise environments where FortiClient is widely deployed.

Root Cause

The root cause of this vulnerability lies in the improper implementation of DLL loading procedures within the FortiClient application. The application does not use absolute paths when loading the OpenSSL engine library, and it does not properly validate or restrict the directories included in the DLL search path. This allows an attacker-controlled directory to be prioritized in the search order, enabling DLL substitution attacks.

Attack Vector

The attack requires local access to the target system where FortiClient is installed. An attacker must place a specially crafted malicious DLL in a directory that the application searches before the legitimate library location. When the FortiClient application is launched or performs an operation that triggers OpenSSL engine loading, the malicious DLL is loaded instead. This results in code execution within the security context of the FortiClient process.

The typical attack scenario involves:

1. An attacker identifies a writable directory in the FortiClient DLL search path
2. The attacker places a malicious OpenSSL engine DLL in this directory
3. When FortiClient loads and attempts to use the OpenSSL engine, it loads the malicious library
4. The attacker's code executes with the privileges of the FortiClient process

Detection Methods for CVE-2021-32592

Indicators of Compromise

• Unexpected DLL files appearing in FortiClient installation directories or application paths
• OpenSSL engine libraries (*.dll) present in non-standard locations on systems with FortiClient installed
• Process creation events showing FortiClient loading DLLs from user-writable directories
• Anomalous file write activity to FortiClient-related paths by unprivileged users

Detection Strategies

• Monitor for DLL loading events from FortiClient processes that reference unusual file paths
• Implement application whitelisting to detect unauthorized DLLs being loaded by FortiClient components
• Deploy file integrity monitoring on FortiClient installation directories to detect unauthorized modifications
• Use endpoint detection and response (EDR) solutions to monitor for DLL sideloading patterns

Monitoring Recommendations

• Enable Windows process auditing to track DLL load events for FortiClient executables
• Configure SIEM alerts for file creation events in FortiClient directories by non-administrator accounts
• Implement behavioral analysis to detect code execution patterns consistent with DLL hijacking
• Review FortiClient logs for unexpected errors or crashes that may indicate exploitation attempts

How to Mitigate CVE-2021-32592

Immediate Actions Required

• Update FortiClientWindows to version 7.0.1 or later, or 6.4.7 or later
• Update FortiClientEMS to version 7.0.1 or later, or 6.4.7 or later
• Audit systems for unauthorized DLL files in FortiClient directories
• Restrict write permissions on directories in the FortiClient search path to administrators only
• Review endpoint security logs for indicators of exploitation

Patch Information

Fortinet has released security updates to address this vulnerability. Organizations should apply patches as documented in the FortiGuard Security Advisory FG-IR-21-088. The advisory provides detailed information on affected versions and the corresponding patched releases.

For FortiClientWindows, upgrade to:

• Version 7.0.1 or later (if running 7.0.0)
• Version 6.4.7 or later (if running 6.4.x)

For FortiClientEMS, upgrade to:

• Version 7.0.1 or later (if running 7.0.0)
• Version 6.4.7 or later (if running 6.4.x)

Workarounds

• Restrict write access to all directories in the DLL search path to prevent unauthorized DLL placement
• Implement application control policies to block execution of unsigned or untrusted DLLs
• Use Windows Defender Application Control or AppLocker to enforce code integrity policies
• Deploy endpoint protection solutions capable of detecting and blocking DLL hijacking attempts
• Monitor and alert on DLL loads from non-standard paths by FortiClient processes

# Restrict permissions on FortiClient directory
icacls "C:\Program Files\Fortinet\FortiClient" /inheritance:r /grant:r Administrators:(OI)(CI)F /grant:r SYSTEM:(OI)(CI)F