CVE-2023-40721: Fortinet FortiOS RCE Vulnerability

CVE-2023-40721 Overview

CVE-2023-40721 is a use of externally-controlled format string vulnerability (CWE-134) affecting multiple Fortinet products. This vulnerability allows a privileged attacker to execute arbitrary code or commands via specially crafted requests. Format string vulnerabilities occur when user-controlled input is improperly used as a format string argument in functions like printf(), potentially allowing attackers to read from or write to memory locations.

Critical Impact

A privileged attacker can leverage this format string vulnerability to execute arbitrary code or commands on affected Fortinet devices, potentially leading to complete system compromise.

Affected Products

• Fortinet FortiOS
• Fortinet FortiSwitchManager
• Fortinet FortiProxy
• Fortinet FortiPAM

Discovery Timeline

• 2025-02-11 - CVE-2023-40721 published to NVD
• 2026-01-14 - Last updated in NVD database

Technical Details for CVE-2023-40721

Vulnerability Analysis

This vulnerability is classified as a use of externally-controlled format string issue (CWE-134). Format string vulnerabilities arise when an application passes user-supplied input directly to a formatting function without proper sanitization. In the context of Fortinet products, a privileged attacker can craft malicious requests containing format specifiers that manipulate the behavior of string formatting functions.

The attack requires local access and high privileges, which limits the attack surface but increases the potential impact once exploited. Successful exploitation could result in unauthorized code execution with the privileges of the vulnerable process, potentially compromising the confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2023-40721 lies in improper handling of user-supplied input in string formatting operations. When externally-controlled data is passed directly to format string functions without validation, attackers can inject format specifiers such as %s, %x, %n, and others to read stack memory, leak sensitive information, or write to arbitrary memory locations.

Attack Vector

The vulnerability requires local access to the affected system with high privileges. An authenticated attacker with administrative access can craft specially formatted requests that exploit the format string handling weakness. The attack does not require user interaction and can be executed directly against the vulnerable component.

The attacker can leverage format specifiers to:

• Read sensitive data from the stack or memory
• Crash the application through invalid memory reads
• Execute arbitrary code by overwriting function pointers or return addresses using the %n format specifier

Due to the local attack vector and high privilege requirement, this vulnerability would typically be leveraged as part of a post-compromise attack chain where an attacker has already gained administrative access and seeks to escalate privileges or maintain persistence.

Detection Methods for CVE-2023-40721

Indicators of Compromise

• Unusual administrative activity or requests containing format string specifiers (%s, %x, %n, %p)
• Unexpected process crashes or memory corruption errors in Fortinet product logs
• Anomalous command execution patterns from privileged accounts
• Evidence of memory disclosure or stack content in logs or network traffic

Detection Strategies

• Monitor Fortinet device logs for unusual patterns in administrative requests or commands
• Implement file integrity monitoring on critical Fortinet configuration files and binaries
• Deploy network monitoring to detect anomalous traffic patterns to management interfaces
• Review authentication logs for suspicious privileged account activity

Monitoring Recommendations

• Enable verbose logging on all affected Fortinet products to capture detailed request information
• Implement Security Information and Event Management (SIEM) rules to alert on format string patterns in logs
• Monitor for unexpected service restarts or crashes on affected devices
• Regularly audit administrative account usage and access patterns

How to Mitigate CVE-2023-40721

Immediate Actions Required

• Review the FortiGuard Security Advisory for specific patch information and affected versions
• Apply available security updates to all affected Fortinet products immediately
• Restrict administrative access to trusted networks and personnel only
• Implement additional authentication controls for privileged accounts

Patch Information

Fortinet has released security updates to address this vulnerability. Organizations should consult the FortiGuard Security Advisory FG-IR-23-261 for detailed information on affected versions and available patches. The advisory provides specific guidance on upgrading to fixed versions for FortiOS, FortiSwitchManager, FortiProxy, and FortiPAM.

Workarounds

• Limit administrative access to the management interface from trusted networks only
• Implement network segmentation to isolate management interfaces from general network traffic
• Enable multi-factor authentication for all administrative accounts
• Monitor and audit all privileged user activity on affected devices
• Consider implementing a jump server architecture for administrative access to limit direct exposure

# Example: Restrict management interface access to trusted networks
config system interface
    edit "mgmt"
        set allowaccess ping https ssh
        set trusted-hosts 10.0.0.0/24 192.168.1.0/24
    next
end