CVE-2025-62676: Fortinet FortiClient Privilege Escalation

CVE-2025-62676 Overview

An Improper Link Resolution Before File Access vulnerability (CWE-59), also known as a Link Following vulnerability, has been identified in Fortinet FortiClientWindows. This security flaw allows a local attacker with low privileges to perform arbitrary file write operations with elevated permissions by exploiting crafted named pipe messages. The vulnerability affects multiple versions of FortiClientWindows across the 7.x release branch.

Critical Impact

Local privilege escalation enabling arbitrary file write with elevated permissions, potentially allowing attackers to overwrite critical system files or plant malicious payloads in privileged locations.

Affected Products

• FortiClientWindows 7.4.0 through 7.4.4
• FortiClientWindows 7.2.0 through 7.2.12
• FortiClientWindows 7.0 all versions

Discovery Timeline

• 2026-02-10 - CVE CVE-2025-62676 published to NVD
• 2026-02-12 - Last updated in NVD database

Technical Details for CVE-2025-62676

Vulnerability Analysis

This vulnerability stems from improper handling of symbolic links (symlinks) during file operations performed by the FortiClientWindows service. When the privileged FortiClient service processes named pipe messages, it fails to properly validate and resolve file paths before performing write operations. An attacker can exploit this by creating symbolic links that redirect file write operations to arbitrary locations on the filesystem.

The attack leverages the elevated privileges of the FortiClient service component. Since the service runs with SYSTEM-level permissions, any file write operations it performs will inherit these elevated privileges. By manipulating the link resolution process through specially crafted named pipe messages, a low-privileged local user can cause the service to write attacker-controlled content to protected system locations.

Root Cause

The root cause is a classic Time-of-Check Time-of-Use (TOCTOU) race condition combined with improper symlink handling. The FortiClientWindows service fails to implement proper safeguards when resolving file paths received through named pipe communications. Specifically, the service does not verify whether the target path contains symbolic links or junction points that could redirect write operations to unintended destinations. This allows an attacker to create a symlink between the time the path is validated and when the actual file write occurs.

Attack Vector

The attack requires local access to the system where FortiClientWindows is installed. An attacker with low-privilege access can exploit this vulnerability by:

1. Identifying the named pipe endpoint used by FortiClientWindows for inter-process communication
2. Creating a symbolic link in a user-writable directory that points to a privileged location (e.g., C:\Windows\System32)
3. Sending crafted named pipe messages that reference the symlink path
4. The FortiClient service, running with elevated privileges, follows the symlink and writes data to the target location

This attack pattern can be used to overwrite DLLs, configuration files, or other critical system components, potentially leading to full system compromise through DLL hijacking or service manipulation.

Detection Methods for CVE-2025-62676

Indicators of Compromise

• Unusual symbolic links or junction points in FortiClient working directories
• Unexpected file modifications in privileged system directories originating from FortiClient processes
• Named pipe communication anomalies with the FortiClient service
• New or modified files in C:\Windows\System32 or other protected paths with FortiClient as the source process

Detection Strategies

• Monitor file system operations performed by FortiClient service processes for writes to protected directories
• Implement endpoint detection rules to identify symlink creation followed by named pipe activity targeting FortiClient
• Enable Windows Security Event logging for symbolic link creation (Event ID 4663 with object type FILE)
• Deploy behavioral detection for TOCTOU exploitation patterns involving privileged services

Monitoring Recommendations

• Configure SentinelOne to monitor FortiClient service processes for suspicious file write operations to system directories
• Enable enhanced file integrity monitoring for critical Windows system files
• Audit named pipe connections to FortiClient endpoints from non-standard processes
• Implement real-time alerting for privilege escalation indicators involving endpoint protection software

How to Mitigate CVE-2025-62676

Immediate Actions Required

• Update FortiClientWindows to a patched version as specified in the Fortinet security advisory
• Review systems for signs of exploitation including unauthorized files in privileged directories
• Restrict local user permissions where possible to limit attack surface
• Enable enhanced logging for FortiClient service activities

Patch Information

Fortinet has released security patches addressing this vulnerability. Organizations should immediately consult the Fortinet Security Advisory FG-IR-25-661 for specific patch versions and upgrade instructions. The advisory provides detailed guidance on updating FortiClientWindows to remediate this link following vulnerability.

Workarounds

• Apply the principle of least privilege to limit local user accounts that could potentially exploit this vulnerability
• Monitor and alert on symbolic link creation in user-writable directories that interface with FortiClient
• Consider temporary network isolation of affected systems until patches can be applied
• Implement application whitelisting to prevent execution of unauthorized code even if files are overwritten

Administrators should prioritize patching as the primary remediation approach, as workarounds may not fully mitigate the risk of exploitation.