CVE-2026-40406: Windows 10 1607 Use-After-Free Vulnerability

CVE-2026-40406 Overview

CVE-2026-40406 is a use-after-free vulnerability [CWE-416] in the Microsoft Windows TCP/IP networking stack. An unauthenticated remote attacker can exploit the flaw over the network to disclose sensitive information from affected systems. The vulnerability requires no user interaction and no prior privileges, making it reachable from any host that can send traffic to a vulnerable Windows endpoint.

The issue affects a broad range of Windows client and server releases, including Windows 10, Windows 11, and Windows Server editions from 2012 through 2025. Microsoft published the advisory on May 12, 2026.

Critical Impact

Unauthenticated network attackers can read memory contents from the Windows TCP/IP stack, potentially exposing kernel pointers, session data, or other sensitive information useful for chaining further attacks.

Affected Products

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

Discovery Timeline

• 2026-05-12 - Microsoft publishes advisory for CVE-2026-40406
• 2026-05-12 - CVE-2026-40406 published to NVD
• 2026-05-15 - Last updated in NVD database

Technical Details for CVE-2026-40406

Vulnerability Analysis

The vulnerability is a use-after-free condition within the Windows TCP/IP driver stack. Use-after-free flaws occur when code continues to reference a memory object after it has been freed, allowing an attacker to influence the contents read from that freed allocation. In this case, the impact is restricted to confidentiality, meaning an attacker can read residual data rather than corrupt execution flow or crash the host.

Because the TCP/IP stack processes packets in kernel mode, leaked memory contents may include kernel addresses, connection metadata, or fragments of buffers from other sessions. This information can be used to defeat Kernel Address Space Layout Randomization (KASLR) or to assist in chaining with separate code execution vulnerabilities.

The EPSS score is 0.068%, indicating low observed exploitation probability at publication. No public proof-of-concept or in-the-wild exploitation has been reported.

Root Cause

The root cause is improper lifecycle management of a memory object inside the Windows TCP/IP stack. A network packet handler dereferences a pointer to a structure that has already been freed by another code path, returning the stale memory contents to the network layer where they can be observed by the remote attacker.

Attack Vector

Exploitation requires only network reachability to the target system. An attacker sends specially crafted TCP/IP traffic that triggers the race or sequence of operations leading to the dangling reference. The response or subsequent network behavior includes residual memory content, which the attacker captures and analyzes to extract sensitive data. No authentication or user interaction is required.

No verified exploit code is publicly available. See the Microsoft Security Response Center advisory for vendor technical details.

Detection Methods for CVE-2026-40406

Indicators of Compromise

• Unusual sequences of malformed or fragmented TCP/IP packets directed at Windows hosts from a single external source
• Anomalous IPv6 extension header traffic or unexpected protocol field values targeting Windows endpoints
• Spikes in low-level network errors or stack-related Event Log entries on affected Windows systems

Detection Strategies

• Deploy network intrusion detection signatures that flag malformed TCP/IP header combinations targeting Windows hosts
• Correlate kernel-mode crash dumps or memory diagnostic events with inbound network activity from untrusted sources
• Monitor for repeated probing patterns against TCP/IP services from the same source IP across short time windows

Monitoring Recommendations

• Centralize Windows network and kernel event telemetry for correlation across endpoints
• Track patch deployment status for all affected Windows 10, 11, and Server builds to identify unpatched assets
• Alert on outbound data volumes from edge-facing Windows servers that deviate from established baselines

How to Mitigate CVE-2026-40406

Immediate Actions Required

• Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-40406 to all affected Windows clients and servers
• Prioritize patching of internet-facing Windows Server systems and any hosts exposing TCP/IP services across untrusted network segments
• Inventory all Windows 10, Windows 11, and Windows Server endpoints to confirm patch coverage across the affected build matrix

Patch Information

Microsoft has released security updates addressing CVE-2026-40406 through the standard Windows Update channels. Refer to the Microsoft CVE-2026-40406 Update advisory for the specific KB articles corresponding to each affected Windows version and architecture (x86, x64, ARM64).

Workarounds

• Restrict inbound TCP/IP traffic to Windows hosts using host-based and perimeter firewalls where the patch cannot be applied immediately
• Segment legacy Windows Server 2012 and 2016 systems into isolated network zones with limited external exposure
• Disable unused network protocols and bindings on affected hosts to reduce the reachable attack surface

# Verify installed updates on a Windows host
wmic qfe list brief /format:table

# Restrict inbound traffic to trusted management subnets via Windows Firewall
New-NetFirewallRule -DisplayName "Restrict TCPIP CVE-2026-40406" -Direction Inbound -Action Block -RemoteAddress Any -Profile Public