CVE-2021-36173: Fortinet FortiOS Buffer Overflow Flaw

CVE-2021-36173 Overview

A heap-based buffer overflow vulnerability exists in the firmware signature verification function of Fortinet FortiOS. This critical flaw allows an attacker to execute arbitrary code by crafting specially designed firmware installation images. The vulnerability affects multiple FortiOS versions across various FortiGate hardware appliances, potentially compromising the security of enterprise network infrastructure.

Critical Impact

Successful exploitation of this vulnerability could allow attackers to execute arbitrary code on FortiGate devices, potentially leading to complete device compromise, network infiltration, and unauthorized access to protected network segments.

Affected Products

• Fortinet FortiOS versions 7.0.0 through 7.0.1
• Fortinet FortiOS versions 6.4.0 through 6.4.6
• Fortinet FortiOS versions 6.2.0 through 6.2.9
• Fortinet FortiOS versions 6.0.0 through 6.0.13
• Fortinet FortiGate Hardware Appliances (40F, 60F, 200F, 400E, 600E, 1100E, 1800F, 2200E, 2600F, 3300E, 3500F, 3600E, 7121F)

Discovery Timeline

• 2021-12-08 - CVE-2021-36173 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-36173

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), specifically manifesting as a heap-based buffer overflow in the firmware signature verification routine of FortiOS. The flaw occurs during the processing of firmware installation images, where improper boundary checks allow an attacker to write data beyond the allocated heap buffer.

The firmware signature verification function is a critical security component designed to ensure only legitimate, Fortinet-signed firmware can be installed on FortiGate devices. When this process fails to properly validate input sizes or boundaries, specially crafted installation images can trigger a buffer overflow condition. This memory corruption can be leveraged to overwrite adjacent heap memory structures, potentially enabling code execution with elevated privileges.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the firmware signature verification function. When processing firmware images, the affected code fails to properly check the size of input data before copying it to a heap-allocated buffer. This allows an attacker to supply a maliciously crafted firmware image that exceeds expected boundaries, resulting in heap memory corruption.

The lack of proper bounds checking during cryptographic verification operations creates a window where controlled heap overflow can modify function pointers or other critical data structures in adjacent memory regions.

Attack Vector

The attack requires network access and some user interaction to be exploited. An attacker could potentially deliver a malicious firmware image through various vectors:

The exploitation involves crafting a specially designed firmware installation image that, when processed by the signature verification function, triggers the heap overflow. The attacker must convince an administrator to initiate a firmware update using the malicious image, or gain sufficient access to initiate the firmware update process themselves.

The heap overflow allows the attacker to corrupt heap metadata or adjacent data structures, potentially redirecting execution flow to attacker-controlled code. Given the nature of FortiGate devices as security appliances, successful exploitation could provide attackers with a foothold in privileged network positions.

Detection Methods for CVE-2021-36173

Indicators of Compromise

• Unexpected firmware update attempts or firmware version changes on FortiGate devices
• Anomalous memory allocation patterns or crash logs in FortiOS system diagnostics
• Presence of unrecognized or unsigned firmware images in administrative interfaces
• Unusual outbound network connections originating from FortiGate management interfaces

Detection Strategies

• Monitor FortiGate system logs for failed or suspicious firmware update attempts
• Implement file integrity monitoring for firmware image storage locations
• Configure alerts for any administrative actions related to firmware management
• Deploy network monitoring to detect exfiltration of firmware images or related reconnaissance

Monitoring Recommendations

• Enable verbose logging on all FortiGate devices to capture firmware-related events
• Implement centralized log collection for all FortiGate administrative activities
• Establish baseline firmware versions and alert on any unauthorized changes
• Conduct regular security audits of FortiGate device configurations and firmware integrity

How to Mitigate CVE-2021-36173

Immediate Actions Required

• Identify all FortiGate devices running vulnerable FortiOS versions in your environment
• Restrict firmware update capabilities to trusted administrators only
• Verify firmware image integrity using Fortinet's official checksums before any updates
• Isolate management interfaces from untrusted networks to reduce attack surface

Patch Information

Fortinet has released security updates to address this vulnerability. Administrators should upgrade FortiOS to the latest available patched versions as specified in the FortiGuard Security Advisory FG-IR-21-115. It is critical to only download firmware images from official Fortinet sources and verify their cryptographic signatures before installation.

Recommended upgrade paths:

• FortiOS 7.0.x: Upgrade to 7.0.2 or later
• FortiOS 6.4.x: Upgrade to 6.4.7 or later
• FortiOS 6.2.x: Upgrade to 6.2.10 or later
• FortiOS 6.0.x: Upgrade to 6.0.14 or later

Workarounds

• Restrict administrative access to FortiGate devices to trusted networks and personnel only
• Implement multi-factor authentication for all administrative access to FortiGate devices
• Disable firmware update capabilities via management interfaces until patches can be applied
• Monitor for and block any attempts to upload firmware via untrusted channels

# Example: Restrict administrative access to specific trusted hosts
config system admin
    edit "admin"
        set trusthost1 10.0.0.0 255.255.255.0
        set trusthost2 192.168.1.0 255.255.255.0
    next
end