CVE-2021-26424 Overview
CVE-2021-26424 is a remote code execution vulnerability in the Microsoft Windows TCP/IP stack. The flaw allows unauthenticated attackers to execute arbitrary code on affected systems over the network with no user interaction. Microsoft disclosed the vulnerability in its August 2021 security update cycle. The issue affects a wide range of Windows client and server operating systems, including Windows 7, Windows 8.1, Windows 10, and Windows Server editions from 2008 through 2019. Successful exploitation grants the attacker full control over confidentiality, integrity, and availability of the target host.
Critical Impact
An unauthenticated remote attacker can execute arbitrary code at the kernel level by sending crafted TCP/IP traffic to a vulnerable Windows host, leading to full system compromise.
Affected Products
- Microsoft Windows 10 (multiple builds: 1607, 1809, 1909, 2004, 20H2, 21H1)
- Microsoft Windows 7 SP1, Windows 8.1, and Windows RT 8.1
- Microsoft Windows Server 2008 SP2, 2008 R2 SP1, 2012, 2012 R2, 2016, and 2019
Discovery Timeline
- 2021-08-12 - CVE-2021-26424 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-26424
Vulnerability Analysis
The vulnerability resides in the Windows TCP/IP networking stack, a kernel-mode component (tcpip.sys) that processes inbound network traffic. Microsoft classifies the flaw as a remote code execution issue triggered by maliciously crafted TCP/IP packets. Because the TCP/IP stack runs in kernel context, successful exploitation grants execution privileges at the highest level of the operating system.
The attack can be launched across the network without authentication and without user interaction. NVD records this issue under [NVD-CWE-noinfo] because Microsoft did not publish a specific weakness classification. Refer to the Microsoft Security Advisory CVE-2021-26424 for vendor details.
Root Cause
The root cause involves improper handling of TCP/IP protocol input within the kernel networking driver. Microsoft has not publicly disclosed the precise memory-corruption primitive. Public analyses associate the August 2021 Windows networking patches with flaws in how the stack parses IPv4 and IPv6 header structures, which can lead to memory corruption when malformed fields are processed.
Attack Vector
An attacker sends specially crafted TCP/IP packets to a vulnerable Windows host that is reachable on the network. The targeted system processes the packets in kernel mode, where the vulnerable parsing logic resides. No credentials, prior access, or user action are required for exploitation. Systems exposing TCP/IP services to untrusted networks face the highest risk, but lateral movement from within a compromised network segment is equally viable.
No verified public proof-of-concept code is available for this CVE. The EPSS percentile of 92.19 indicates a relatively elevated likelihood of exploitation activity compared to most published CVEs.
Detection Methods for CVE-2021-26424
Indicators of Compromise
- Unexpected kernel crashes or bug checks referencing tcpip.sys in MEMORY.DMP or Windows Event Log entries (Event ID 1001, BugCheck).
- Anomalous inbound IPv4 or IPv6 traffic with malformed headers, fragmented packets, or unusual option fields targeting Windows hosts.
- New or unexplained kernel-mode processes, drivers, or scheduled tasks appearing after suspicious inbound network activity.
Detection Strategies
- Deploy network intrusion detection signatures that flag malformed TCP/IP header structures and unexpected fragmentation patterns targeting Windows endpoints.
- Correlate tcpip.sys kernel faults with preceding inbound traffic flows in SIEM data to identify exploitation attempts.
- Hunt for post-exploitation behaviors such as unsigned driver loads, LSASS access, and lateral SMB activity following network anomalies.
Monitoring Recommendations
- Enable Windows Defender Firewall logging and forward connection events to a central logging platform for review.
- Monitor patch compliance for the August 2021 Windows cumulative updates across all affected Windows client and server versions.
- Watch egress traffic from Windows hosts for command-and-control indicators following any suspected exploitation attempts.
How to Mitigate CVE-2021-26424
Immediate Actions Required
- Apply the August 2021 Microsoft security updates to all affected Windows client and server systems without delay.
- Inventory all Windows hosts directly reachable from untrusted networks and prioritize patching of internet-exposed systems.
- Restrict inbound TCP/IP traffic to Windows hosts at perimeter and host-based firewalls where business requirements allow.
Patch Information
Microsoft released patches for all affected Windows versions as part of the August 2021 Patch Tuesday release. Administrators should obtain updates directly from the Microsoft Security Advisory CVE-2021-26424. Windows Update, WSUS, and Microsoft Update Catalog provide the relevant cumulative updates and rollups for each supported Windows build.
Workarounds
- Block untrusted inbound IPv4 and IPv6 traffic at the network perimeter where Windows endpoints do not require external exposure.
- Segment Windows hosts that cannot be patched immediately and restrict access using ACLs or host-based firewall rules.
- Disable unused network protocols and services on Windows hosts to reduce the attack surface of the kernel TCP/IP stack.
# Example: restrict inbound TCP/IP traffic to trusted subnets using Windows Firewall
New-NetFirewallRule -DisplayName "Block-Untrusted-Inbound" -Direction Inbound -Action Block -RemoteAddress Any
New-NetFirewallRule -DisplayName "Allow-Trusted-Subnet" -Direction Inbound -Action Allow -RemoteAddress 10.0.0.0/8
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
