CVE-2021-24077: Windows 10 Fax Service RCE Vulnerability

CVE-2021-24077 Overview

CVE-2021-24077 is a remote code execution vulnerability affecting the Windows Fax Service across multiple versions of Microsoft Windows operating systems. This vulnerability allows an unauthenticated attacker to execute arbitrary code on affected systems remotely over the network without requiring any user interaction. The Windows Fax Service, a legacy component that handles fax communications, contains a flaw that can be exploited to gain complete control over vulnerable systems.

Critical Impact

This vulnerability enables remote code execution with no authentication required, potentially allowing attackers to gain full system control over affected Windows machines across enterprise environments.

Affected Products

• Microsoft Windows 10 (multiple versions including 1607, 1803, 1809, 1909, 2004, 20H2)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1
• Microsoft Windows RT 8.1
• Microsoft Windows Server 2008 SP2 and R2 SP1
• Microsoft Windows Server 2012 and R2
• Microsoft Windows Server 2016 (including 1909, 2004, 20H2)
• Microsoft Windows Server 2019

Discovery Timeline

• February 25, 2021 - CVE-2021-24077 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-24077

Vulnerability Analysis

The Windows Fax Service Remote Code Execution vulnerability exists within the fax processing components of Microsoft Windows. The vulnerability can be exploited over the network without requiring authentication or user interaction, making it particularly dangerous in enterprise environments where the Fax Service may be enabled.

The attack does not require elevated privileges, and successful exploitation grants the attacker the ability to execute code with the privileges of the Windows Fax Service, which typically runs with SYSTEM-level permissions. This allows for complete compromise of the affected system, including the ability to install programs, view or modify data, and create new accounts with full user rights.

Root Cause

While Microsoft has not disclosed specific technical details about the root cause (classified as NVD-CWE-noinfo), the vulnerability resides in the Windows Fax Service's handling of incoming data. The service fails to properly validate or sanitize certain inputs, leading to a condition that can be exploited for remote code execution. The Fax Service processes incoming fax data and related communications, and improper handling of malformed or specially crafted data can trigger the vulnerability.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending specially crafted network requests to a system running the vulnerable Windows Fax Service. The attack complexity is low, meaning that exploitation is relatively straightforward once a target is identified.

The attack flow involves:

1. Identifying systems with the Windows Fax Service enabled and accessible over the network
2. Crafting malicious network traffic designed to trigger the vulnerability
3. Sending the malicious payload to the target system's Fax Service
4. Achieving code execution with the privileges of the Fax Service process

Due to the nature of this vulnerability, no user interaction is required, and exploitation can occur silently without alerting users or administrators.

Detection Methods for CVE-2021-24077

Indicators of Compromise

• Unexpected network connections to or from the Windows Fax Service (FXSSVC.exe)
• Unusual process spawning from the Fax Service executable
• Anomalous memory consumption or crashes in the Fax Service
• Suspicious child processes executed by FXSSVC.exe

Detection Strategies

• Monitor network traffic to the Windows Fax Service for unusual patterns or malformed requests
• Implement endpoint detection rules to identify unexpected behavior from FXSSVC.exe
• Deploy SentinelOne Singularity to detect exploitation attempts and post-exploitation activity
• Review Windows Event Logs for Fax Service errors or unexpected service restarts

Monitoring Recommendations

• Enable detailed logging for the Windows Fax Service and related components
• Configure SIEM rules to alert on suspicious activity involving FXSSVC.exe
• Monitor for lateral movement attempts following potential Fax Service compromise
• Implement network segmentation to limit exposure of systems running the Fax Service

How to Mitigate CVE-2021-24077

Immediate Actions Required

• Apply the Microsoft security update for CVE-2021-24077 immediately on all affected systems
• If patching is not immediately possible, disable the Windows Fax Service on systems where it is not required
• Review network firewall rules to restrict access to systems running the Fax Service
• Conduct an asset inventory to identify all systems with the Windows Fax Service enabled

Patch Information

Microsoft has released security updates to address this vulnerability. Patches are available through Windows Update and the Microsoft Update Catalog. Organizations should prioritize patching given the critical severity and network-exploitable nature of this vulnerability. For detailed patch information and download links, refer to the Microsoft Security Advisory CVE-2021-24077.

Workarounds

• Disable the Windows Fax Service on systems where fax functionality is not required using Services Manager or PowerShell
• Implement network-level controls to block external access to systems running the Fax Service
• Use host-based firewalls to restrict inbound connections to the Fax Service
• Consider virtualization or isolation of legacy systems that require the Fax Service

# Disable Windows Fax Service via PowerShell
Stop-Service -Name "Fax" -Force
Set-Service -Name "Fax" -StartupType Disabled

# Verify the service is disabled
Get-Service -Name "Fax" | Select-Object Name, Status, StartType