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

bash

# 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