CVE-2021-42757: Fortinet FortiADC RCE Vulnerability

CVE-2021-42757 Overview

A buffer overflow vulnerability (CWE-121) exists in the TFTP client library of FortiOS before version 6.4.7 and FortiOS 7.0.0 through 7.0.2. This vulnerability may allow an authenticated local attacker to achieve arbitrary code execution via specially crafted command line arguments. The flaw affects multiple Fortinet products that share the vulnerable TFTP client library component.

Critical Impact

An authenticated local attacker can exploit this buffer overflow to execute arbitrary code on affected Fortinet devices, potentially leading to complete system compromise.

Affected Products

• Fortinet FortiOS (versions before 6.4.7 and 7.0.0 through 7.0.2)
• Fortinet FortiOS-6K7K
• Fortinet FortiProxy (including versions 7.0.0 and 7.0.1)
• Fortinet FortiADC
• Fortinet FortiAnalyzer
• Fortinet FortiManager
• Fortinet FortiMail
• Fortinet FortiNDR
• Fortinet FortiPortal
• Fortinet FortiVoice
• Fortinet FortiWeb (including versions 6.4.0 and 6.4.1)
• Fortinet FortiRecorder
• Fortinet FortiSwitch

Discovery Timeline

• 2021-12-08 - CVE-2021-42757 published to NVD
• 2025-10-16 - Last updated in NVD database

Technical Details for CVE-2021-42757

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-121) combined with out-of-bounds write (CWE-787) in the TFTP client library used across multiple Fortinet products. The flaw occurs when processing command line arguments passed to TFTP client functionality, where insufficient bounds checking allows an attacker to overflow a fixed-size buffer.

The vulnerability requires local access and elevated privileges to exploit, which somewhat limits the attack surface. However, successful exploitation grants the attacker the ability to execute arbitrary code with the privileges of the TFTP client process, potentially leading to full device compromise.

The impact includes complete confidentiality, integrity, and availability compromise of the affected system. Given that Fortinet devices typically operate as network security appliances at critical infrastructure points, successful exploitation could have cascading effects on the security posture of the entire network.

Root Cause

The root cause of CVE-2021-42757 is improper bounds checking in the TFTP client library when handling command line arguments. The vulnerable code fails to validate the length of user-supplied input before copying it into a fixed-size stack buffer, resulting in a classic stack-based buffer overflow condition.

This type of vulnerability falls under CWE-120 (Buffer Copy without Checking Size of Input) and CWE-787 (Out-of-bounds Write), both of which indicate fundamental issues with memory safety in the affected code path.

Attack Vector

The attack vector for this vulnerability is local, requiring an authenticated attacker with high privileges to execute the exploit. The attacker must craft specially malformed command line arguments that overflow the vulnerable buffer when processed by the TFTP client library.

The exploitation mechanism involves providing oversized or malformed input through command line parameters that exceed the expected buffer size. When the vulnerable function copies this data without proper length validation, the attacker can overwrite adjacent memory locations on the stack, potentially including the return address. By carefully controlling the overflow data, an attacker can redirect program execution to arbitrary code of their choosing.

Since the vulnerability requires local authenticated access with elevated privileges, the most likely attack scenarios involve insider threats, compromised administrator accounts, or privilege escalation chains where this vulnerability serves as a final step to achieve code execution.

Detection Methods for CVE-2021-42757

Indicators of Compromise

• Unusual TFTP client process crashes or restarts on Fortinet devices
• Unexpected command line arguments passed to TFTP-related processes
• Anomalous process spawning from TFTP client components
• Signs of memory corruption in system logs related to TFTP operations

Detection Strategies

• Monitor Fortinet device logs for TFTP client abnormalities or crash reports
• Implement behavioral analysis to detect unusual command execution patterns on network appliances
• Use endpoint detection and response (EDR) solutions capable of monitoring embedded device behavior
• Review administrative access logs for suspicious authenticated sessions preceding TFTP operations

Monitoring Recommendations

• Enable verbose logging on all affected Fortinet products to capture TFTP client activity
• Configure SIEM rules to alert on repeated TFTP process failures or unexpected terminations
• Monitor for privilege escalation attempts that may precede exploitation of this vulnerability
• Establish baseline behavior for TFTP client usage and alert on deviations

How to Mitigate CVE-2021-42757

Immediate Actions Required

• Upgrade FortiOS to version 6.4.7 or later for the 6.x branch, or 7.0.3 or later for the 7.x branch
• Review and update all affected Fortinet products to their respective patched versions
• Audit administrative access to identify and restrict unnecessary privileged accounts
• Implement network segmentation to limit lateral movement if a device is compromised

Patch Information

Fortinet has released security patches addressing this vulnerability. Administrators should consult the FortiGuard Security Advisory FG-IR-21-173 for specific version requirements for each affected product. The advisory provides detailed guidance on which versions contain the fix for each product in the affected portfolio.

For FortiOS specifically:

• Version 6.4.7 and later addresses this vulnerability in the 6.4.x branch
• Version 7.0.3 and later addresses this vulnerability in the 7.0.x branch

Workarounds

• Restrict local administrative access to only essential personnel
• Implement strict access controls and multi-factor authentication for device management
• Monitor and audit all administrative sessions on affected Fortinet devices
• Consider disabling TFTP functionality if not required for operations (where applicable)

# Example: Restrict administrative access (FortiOS CLI)
# Limit admin access to specific trusted hosts
config system admin
    edit "admin"
        set trusthost1 192.168.1.0 255.255.255.0
    next
end