CVE-2021-24018: Fortinet FortiOS RCE Vulnerability

CVE-2021-24018 Overview

CVE-2021-24018 is a buffer underwrite vulnerability (CWE-787: Out-of-bounds Write) in the firmware verification routine of Fortinet FortiOS. This security flaw affects FortiOS versions prior to 7.0.1 and may allow an attacker located in the adjacent network to potentially execute arbitrary code via a specifically crafted firmware image.

Critical Impact

An adjacent network attacker can potentially achieve arbitrary code execution on affected FortiOS devices by exploiting the firmware verification routine with a malicious firmware image, potentially leading to complete device compromise.

Affected Products

• Fortinet FortiOS versions prior to 6.0.13
• Fortinet FortiOS versions 6.2.x prior to 6.2.9
• Fortinet FortiOS version 7.0.0

Discovery Timeline

• August 4, 2021 - CVE-2021-24018 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-24018

Vulnerability Analysis

This vulnerability exists within the firmware verification routine of FortiOS, where a buffer underwrite condition can occur during the processing of firmware images. Buffer underwrite vulnerabilities, classified under CWE-787 (Out-of-bounds Write), occur when data is written to memory locations before the beginning of the intended buffer. In this case, the vulnerability manifests when the firmware verification routine improperly handles specially crafted firmware images.

The attack requires the adversary to be on the adjacent network (same physical or logical network segment) to exploit this vulnerability. While no user interaction or prior authentication is required, the adjacent network requirement provides some level of access restriction. Successful exploitation can result in arbitrary code execution with the privileges of the affected process, potentially allowing full compromise of the FortiOS device.

Root Cause

The root cause of this vulnerability is improper boundary checking in the firmware verification routine. When processing firmware images, the verification routine fails to properly validate input boundaries, allowing an attacker to trigger a buffer underwrite condition. This type of memory corruption vulnerability occurs when data is written to memory addresses below the allocated buffer's starting address, corrupting adjacent memory structures.

Attack Vector

The attack vector requires adjacent network access to the target FortiOS device. An attacker must craft a malicious firmware image designed to exploit the buffer underwrite vulnerability during the verification process. The attack scenario involves:

1. The attacker gains access to the same network segment as the target FortiOS device
2. A specially crafted firmware image is presented to the device's firmware verification routine
3. The malformed firmware image triggers the buffer underwrite condition
4. Memory corruption occurs, potentially allowing arbitrary code execution

The vulnerability mechanism exists in the firmware verification routine's boundary handling. When processing firmware image headers or metadata, insufficient validation allows calculated write offsets to become negative or underflow, causing writes to memory locations preceding the intended buffer. For detailed technical information, refer to the FortiGuard Security Advisory FG-IR-21-046.

Detection Methods for CVE-2021-24018

Indicators of Compromise

• Unexpected firmware update attempts or failures on FortiOS devices
• Anomalous network traffic from adjacent network segments targeting firmware management interfaces
• System crashes or unexpected reboots during firmware operations
• Memory corruption evidence in system logs or crash dumps

Detection Strategies

• Monitor FortiOS devices for unauthorized firmware update attempts from non-management systems
• Implement network segmentation to isolate FortiOS management interfaces from general network traffic
• Deploy network intrusion detection systems (IDS) to monitor for suspicious traffic patterns targeting FortiOS devices
• Enable verbose logging on firmware update operations and monitor for anomalies

Monitoring Recommendations

• Configure SIEM rules to alert on firmware update events from unauthorized sources
• Monitor network traffic for unusual patterns targeting FortiOS device management ports
• Implement regular integrity checks on FortiOS device firmware
• Review FortiOS system logs for buffer overflow or memory corruption indicators

How to Mitigate CVE-2021-24018

Immediate Actions Required

• Upgrade FortiOS to version 7.0.1 or later immediately
• For FortiOS 6.2.x branch, upgrade to version 6.2.9 or later
• For FortiOS 6.0.x branch, upgrade to version 6.0.13 or later
• Restrict network access to FortiOS management interfaces to trusted management systems only

Patch Information

Fortinet has released security patches addressing this vulnerability. Affected organizations should upgrade to the following fixed versions:

• FortiOS 7.0.1 or later (for 7.0.x branch)
• FortiOS 6.2.9 or later (for 6.2.x branch)
• FortiOS 6.0.13 or later (for 6.0.x branch)

For complete patch details and download links, refer to the FortiGuard Security Advisory FG-IR-21-046.

Workarounds

• Implement strict network segmentation to isolate FortiOS management interfaces from untrusted network segments
• Restrict physical and logical access to network segments where FortiOS devices reside
• Disable firmware update capabilities from non-management interfaces where possible
• Deploy additional network access controls (802.1X, MAC filtering) on segments with FortiOS devices

# Example: Restrict management access to trusted hosts only
config system admin
    edit "admin"
        set trusthost1 192.168.1.0 255.255.255.0
    next
end

# Enable logging for firmware operations
config log setting
    set fwpolicy-implicit-log enable
end