CVE-2026-40414: Windows 10 1607 DOS Vulnerability

CVE-2026-40414 Overview

CVE-2026-40414 is a null pointer dereference vulnerability in the Microsoft Windows TCP/IP networking stack. An unauthenticated attacker positioned on an adjacent network can send crafted traffic that triggers the dereference and crashes the affected host. The flaw is tracked under CWE-476 and affects a broad range of Windows client and server releases, including Windows 10, Windows 11 (23H2 through 26H1), and Windows Server 2012 through Windows Server 2025. Exploitation does not require user interaction or prior credentials, and successful attacks result in a high-impact denial of service against the targeted system.

Critical Impact

An adjacent, unauthenticated attacker can crash Windows hosts by triggering a null pointer dereference in the TCP/IP stack, producing a high-impact denial of service.

Affected Products

• Microsoft Windows 10 (1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (23H2, 24H2, 25H2, 26H1)
• Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2026-05-12 - CVE-2026-40414 published to NVD
• 2026-05-15 - Last updated in NVD database

Technical Details for CVE-2026-40414

Vulnerability Analysis

The vulnerability resides in the Windows TCP/IP driver, which processes inbound network packets in kernel mode. When the stack handles a specific malformed or unexpected protocol input, a code path fails to validate that a pointer has been initialized before dereferencing it. The resulting kernel-mode null pointer dereference triggers a bug check and forces the host to reboot. Because the affected component is the core TCP/IP stack, every Windows role that exposes a network interface on a local link segment is reachable. The scope is marked as changed because a crash in the kernel networking driver impacts the entire operating system, not just the network component.

Root Cause

The root cause is missing pointer validation in a packet-handling routine within the TCP/IP stack, classified as [CWE-476]. A function consumes a structure field or callback pointer that was never populated under the attacker-induced state, then dereferences it, causing an access violation in kernel space.

Attack Vector

The attack vector is adjacent network, meaning the attacker must be on the same broadcast or logical network segment, such as a shared Wi-Fi network, VLAN, or IPv6 link-local domain. No authentication and no user interaction are required. The attacker sends one or more crafted packets to the target, which the TCP/IP stack parses before dropping into the vulnerable code path. The vulnerability does not yield code execution or data disclosure, but it can be used to repeatedly take hosts offline. Refer to the Microsoft Security Update CVE-2026-40414 advisory for protocol-level technical detail.

Detection Methods for CVE-2026-40414

Indicators of Compromise

• Unexpected BugCheck events in the Windows System event log, often referencing tcpip.sys or related networking modules, followed by an unplanned reboot.
• Repeated kernel crash dumps generated in %SystemRoot%\Minidump that point at the TCP/IP driver call stack.
• Bursts of unusual link-local or adjacent-segment traffic preceding host unavailability across multiple endpoints on the same VLAN.

Detection Strategies

• Correlate Windows Event IDs 1001 (Windows Error Reporting) and 41 (Kernel-Power unexpected shutdown) with network capture data from the same time window.
• Hunt for clusters of Windows hosts on a shared subnet rebooting within a short interval, which suggests an adjacent-network DoS rather than a single faulty machine.
• Inspect IDS/IPS and NDR telemetry for malformed IPv4, IPv6, or related TCP/IP option fields targeting unpatched Windows builds.

Monitoring Recommendations

• Forward kernel bug-check and Kernel-Power events to a centralized log platform and alert on repeat crashes referencing networking drivers.
• Monitor switch and wireless infrastructure for abnormal volumes of link-local multicast or broadcast traffic from a single source.
• Track patch deployment status for the Microsoft May 2026 security update across all Windows client and server SKUs listed in the advisory.

How to Mitigate CVE-2026-40414

Immediate Actions Required

• Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-40414 advisory to all affected Windows 10, Windows 11, and Windows Server builds.
• Prioritize patching hosts on shared network segments such as guest Wi-Fi, lab VLANs, and DMZ subnets where adjacent attackers are most likely to reach them.
• Validate that domain controllers, file servers, and other availability-sensitive Windows Server systems receive the update during the next maintenance window.

Patch Information

Microsoft has released cumulative security updates that remediate CVE-2026-40414 for all listed Windows versions. The fixes are delivered through Windows Update, WSUS, and the Microsoft Update Catalog. Administrators should consult the Microsoft Security Update CVE-2026-40414 page for the specific KB numbers that apply to each operating system build.

Workarounds

• Segment networks so that untrusted devices cannot share a Layer 2 broadcast domain or IPv6 link-local segment with critical Windows servers.
• Disable unused network protocols and tunneling features on Windows hosts when business requirements permit, reducing the parser surface in tcpip.sys.
• Enforce wireless client isolation and private VLANs on guest and BYOD networks to limit adjacent-network reachability until patches are deployed.

# Example: query a Windows host for the installed update and confirm patch presence
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10

# Example: enable client isolation on a managed Wi-Fi SSID (vendor syntax varies)
# set wlan ssid <name> client-isolation enable