CVE-2021-43072: Fortinet FortiAnalyzer Buffer Overflow

CVE-2021-43072 Overview

CVE-2021-43072 is a classic buffer overflow vulnerability (CWE-120) affecting multiple Fortinet products including FortiAnalyzer, FortiManager, FortiOS, and FortiProxy. The vulnerability stems from improper size validation when copying input data to a buffer, allowing an authenticated attacker with high privileges to execute unauthorized code or commands on affected systems.

The vulnerability is exploitable through crafted CLI operations specifically targeting the execute restore image and execute certificate remote commands when using the TFTP protocol. This buffer overflow condition occurs because the affected products fail to properly validate the size of input data before copying it into a fixed-size buffer.

Critical Impact

Authenticated attackers with administrative privileges can achieve arbitrary code execution on Fortinet network infrastructure devices, potentially compromising network security monitoring and management capabilities.

Affected Products

• Fortinet FortiAnalyzer versions 7.0.2 and below, 6.4.7 and below, 6.2.9 and below, 6.0.11 and below, 5.6.11 and below
• Fortinet FortiManager versions 7.0.2 and below, 6.4.7 and below, 6.2.9 and below, 6.0.11 and below, 5.6.11 and below
• Fortinet FortiOS versions 7.0.0 through 7.0.4, 6.4.0 through 6.4.8, 6.2.0 through 6.2.10, and 6.0.x
• Fortinet FortiProxy versions 7.0.0 through 7.0.3, 2.0.0 through 2.0.8, 1.2.x, 1.1.x, and 1.0.x

Discovery Timeline

• July 18, 2023 - CVE-2021-43072 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-43072

Vulnerability Analysis

This vulnerability represents a classic buffer overflow condition categorized under CWE-120 (Buffer Copy without Checking Size of Input). The flaw exists within the CLI command processing functionality of multiple Fortinet products. When an authenticated administrator executes specific restore or certificate operations via the command line interface using TFTP as the transfer protocol, the system fails to properly validate the size of incoming data before copying it into memory buffers.

Buffer overflow vulnerabilities of this nature can lead to memory corruption, potentially allowing an attacker to overwrite critical memory regions including return addresses and function pointers. In successful exploitation scenarios, this can result in arbitrary code execution with the same privileges as the affected process, typically running with elevated system-level permissions on these network security appliances.

Root Cause

The root cause is the absence of proper bounds checking when processing input data during execute restore image and execute certificate remote CLI operations. The affected code paths copy data received via the TFTP protocol into fixed-size buffers without verifying that the input size does not exceed the buffer capacity. This classic buffer overflow pattern allows data to overflow beyond the intended buffer boundaries, corrupting adjacent memory regions.

Attack Vector

The attack requires local access and high privileges, meaning an attacker must have authenticated administrative access to the CLI of the affected Fortinet device. The exploitation path involves:

1. Establishing an authenticated administrative session on the target device
2. Setting up a malicious TFTP server with specially crafted payload data
3. Executing either execute restore image or execute certificate remote commands pointing to the attacker-controlled TFTP server
4. The oversized payload from the TFTP server triggers the buffer overflow during data processing

The local attack vector and high privilege requirement limit the exposure, though compromised administrator credentials or insider threats remain viable attack scenarios. The vulnerability affects critical network infrastructure devices that often have broad visibility into network traffic and management capabilities.

Detection Methods for CVE-2021-43072

Indicators of Compromise

• Unexpected CLI activity involving execute restore image or execute certificate remote commands from unusual sources or at unusual times
• TFTP connections from Fortinet devices to untrusted or unknown external servers
• System crashes, unexpected reboots, or abnormal behavior on FortiAnalyzer, FortiManager, FortiOS, or FortiProxy devices following restore or certificate operations
• Unusual process activity or memory consumption anomalies on affected devices

Detection Strategies

• Monitor CLI command logs for execute restore image and execute certificate remote operations, particularly those using TFTP protocol
• Implement network monitoring to detect TFTP traffic originating from Fortinet management devices to non-standard or unauthorized destinations
• Deploy behavioral analysis to detect anomalous administrative actions or process behavior on affected Fortinet products
• Review authentication logs for administrative access patterns that deviate from baseline activity

Monitoring Recommendations

• Enable comprehensive CLI command logging on all affected Fortinet products and forward logs to a centralized SIEM
• Configure network segmentation and firewall rules to restrict TFTP traffic from management devices to authorized internal servers only
• Implement alerting for any TFTP protocol usage from FortiAnalyzer, FortiManager, or FortiOS devices to external destinations
• Conduct regular reviews of administrative account activity and access patterns

How to Mitigate CVE-2021-43072

Immediate Actions Required

• Upgrade all affected Fortinet products to patched versions as specified in the FortiGuard PSIRT Advisory
• Restrict administrative CLI access to trusted networks and implement multi-factor authentication for administrative accounts
• Audit and review all administrative accounts to ensure no unauthorized access exists
• Disable or restrict TFTP-based restore and certificate operations until patches are applied

Patch Information

Fortinet has released security updates addressing this vulnerability. Organizations should consult the FortiGuard Security Advisory FG-IR-21-206 for specific patched version information and upgrade guidance. The advisory provides detailed information on fixed versions for each affected product line including FortiAnalyzer, FortiManager, FortiOS, and FortiProxy.

Workarounds

• Restrict administrative CLI access to only trusted internal networks using firewall rules and access control lists
• Implement strict access controls limiting which administrators can execute restore and certificate operations
• Block or monitor TFTP protocol traffic from affected devices at the network perimeter
• Consider using alternative protocols for image restore and certificate operations where supported until patches can be applied

# Network segmentation configuration example
# Restrict TFTP traffic from FortiAnalyzer/FortiManager management interfaces
# Apply appropriate firewall rules to limit outbound TFTP (UDP port 69) traffic
# to authorized internal servers only

# Example: Verify current firmware version on FortiOS
get system status

# Example: Review CLI command history for suspicious activity
execute log filter category event
execute log filter field subtype system
execute log display