CVE-2021-43247: Windows 10 TCP/IP Privilege Escalation

CVE-2021-43247 Overview

CVE-2021-43247 is an elevation of privilege vulnerability in the Windows TCP/IP Driver that affects multiple versions of Microsoft Windows operating systems. This vulnerability allows a local attacker with low privileges to escalate their privileges on the affected system, potentially gaining full control over the compromised machine.

The vulnerability resides in the tcpip.sys driver, a critical kernel-mode component responsible for handling TCP/IP network communications in Windows. Due to improper handling of memory operations, an attacker can exploit this flaw to execute code with elevated privileges.

Critical Impact

Successful exploitation enables local privilege escalation, allowing attackers to gain SYSTEM-level access and achieve complete compromise of the affected Windows system.

Affected Products

• Microsoft Windows 10 (versions 1809, 1909, 2004, 20H2, 21H1, 21H2) - x86, x64, and ARM64 architectures
• Microsoft Windows 11 (x64 and ARM64)
• Microsoft Windows Server 2016 (version 2004)
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022 and 20H2

Discovery Timeline

• December 15, 2021 - CVE-2021-43247 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-43247

Vulnerability Analysis

This vulnerability is classified as an out-of-bounds write (CWE-787) in the Windows TCP/IP Driver. The flaw occurs when the driver improperly validates the bounds of memory write operations, allowing an attacker to write data outside the intended buffer boundaries in kernel memory.

The attack requires local access to the target system and low-level privileges to execute. No user interaction is required for exploitation, making it particularly dangerous in environments where attackers may have limited initial access. The vulnerability affects the confidentiality, integrity, and availability of the system, as successful exploitation grants the attacker elevated privileges that can be used to access sensitive data, modify system configurations, and disrupt normal operations.

According to the Zero Day Initiative Advisory ZDI-21-1554, the vulnerability exists within the handling of TCP/IP operations in the kernel driver, making it a significant risk for enterprise environments where lateral movement from compromised user accounts is a concern.

Root Cause

The root cause of CVE-2021-43247 is improper bounds checking in the Windows TCP/IP Driver (tcpip.sys). When processing certain network-related operations, the driver fails to properly validate the size or boundaries of data being written to memory buffers. This oversight allows an attacker to craft malicious input that triggers an out-of-bounds write condition, corrupting adjacent kernel memory structures.

Out-of-bounds write vulnerabilities in kernel-mode drivers are particularly severe because they can be leveraged to overwrite critical kernel data structures, enabling privilege escalation from a standard user account to SYSTEM-level access.

Attack Vector

The attack vector for this vulnerability is local, meaning an attacker must have authenticated access to the target system to exploit it. The typical attack scenario involves:

1. An attacker gains initial access to a Windows system with low-privilege credentials
2. The attacker executes a specially crafted application or uses system calls to trigger the vulnerable code path in the TCP/IP driver
3. The out-of-bounds write corrupts kernel memory, allowing the attacker to manipulate execution flow or overwrite security tokens
4. The attacker elevates their privileges to SYSTEM, gaining complete control over the machine

This type of vulnerability is commonly exploited as part of multi-stage attacks where initial access is obtained through phishing or other means, and privilege escalation is needed to achieve persistence or access sensitive resources.

Detection Methods for CVE-2021-43247

Indicators of Compromise

• Unexpected crashes or blue screen errors (BSOD) involving tcpip.sys driver
• Unusual privilege escalation events in Windows Security logs (Event ID 4672, 4688)
• Suspicious processes running with SYSTEM privileges that originated from low-privilege user sessions
• Memory anomalies or kernel pool corruption detected by endpoint protection solutions

Detection Strategies

• Monitor for abnormal system call patterns targeting the TCP/IP driver stack
• Implement behavioral analysis to detect processes attempting privilege escalation techniques
• Deploy kernel-level monitoring to identify suspicious memory write operations in the network driver space
• Use SentinelOne's Behavioral AI to detect post-exploitation activities associated with privilege escalation

Monitoring Recommendations

• Enable Windows Event Log auditing for process creation and privilege use events
• Configure EDR solutions to alert on suspicious kernel driver interactions
• Implement network segmentation to limit lateral movement if a system is compromised
• Regularly review and correlate security events for indicators of privilege escalation attempts

How to Mitigate CVE-2021-43247

Immediate Actions Required

• Apply the Microsoft security update from the December 2021 Patch Tuesday release immediately
• Prioritize patching for internet-facing systems and high-value targets
• Restrict local login access to only authorized users and administrators
• Enable attack surface reduction rules in Microsoft Defender for Endpoint

Patch Information

Microsoft has released security updates to address this vulnerability as part of the December 2021 security update cycle. Administrators should obtain the appropriate patch from the Microsoft Security Advisory for CVE-2021-43247.

The patches address the out-of-bounds write condition by implementing proper bounds validation in the affected TCP/IP driver code paths. Organizations should test patches in a staging environment before deploying to production systems, and ensure all affected Windows versions are updated.

Workarounds

• Implement the principle of least privilege to limit the number of users with local access to systems
• Use application whitelisting to prevent unauthorized executables from running
• Deploy network segmentation to contain potential compromises
• Monitor for exploitation attempts while patches are being deployed

# Verify patch installation status using PowerShell
Get-HotFix | Where-Object {$_.HotFixID -match "KB500"} | Select-Object HotFixID, InstalledOn

# Check Windows TCP/IP driver version
Get-WmiObject Win32_PnPSignedDriver | Where-Object {$_.DeviceName -like "*TCP/IP*"} | Select-Object DeviceName, DriverVersion, DriverDate