CVE-2026-56649: Windows 10 1607 Race Condition Vulnerability

CVE-2026-56649 Overview

CVE-2026-56649 is a race condition vulnerability in the Windows Network File System (NFS) service. The flaw stems from concurrent execution using a shared resource with improper synchronization [CWE-362]. An unauthenticated remote attacker can exploit this weakness to execute arbitrary code over a network against a vulnerable Windows host running the NFS role.

Microsoft published the advisory on July 14, 2026, covering client and server editions of Windows from Windows 10 1607 through Windows 11 26H1 and Windows Server 2012 through Windows Server 2025.

Critical Impact
Successful exploitation grants remote code execution on the NFS server process, potentially leading to full system compromise across Windows client and server versions where the NFS role is enabled.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-56649 published to NVD and Microsoft advisory released
2026-07-17 - Last updated in NVD database

Technical Details for CVE-2026-56649

Vulnerability Analysis

The vulnerability resides in the Windows NFS server component, which handles remote file operations from NFS clients over the network. Two or more concurrent request handlers operate on shared state without adequate locking or serialization. An attacker who can trigger the racing code paths in the correct order and timing can corrupt internal data structures.

Once the shared resource is left in an inconsistent state, subsequent processing can cause the NFS service to dereference attacker-influenced memory. This behavior is used to achieve arbitrary code execution in the context of the NFS service, which typically runs with elevated privileges on Windows Server hosts.

Exploitation requires winning a timing window, which raises attack complexity. However, no authentication or user interaction is required, and the vulnerability is reachable over the network wherever NFS ports (TCP/UDP 111 and 2049) are exposed.

Root Cause

The root cause is a Time-of-Check Time-of-Use (TOCTOU) style flaw in the NFS request-processing path, classified under [CWE-362]. Multi-threaded handlers access the same object or buffer without acquiring a mutex or performing atomic validation, allowing a concurrent request to mutate state between validation and use.

Attack Vector

An attacker sends crafted NFS RPC requests over the network to a Windows host with the NFS server role enabled. By issuing overlapping requests, the attacker triggers the race, corrupts shared state, and pivots to code execution. No credentials are required, and the attack traverses standard NFS network protocols. Refer to the Microsoft CVE-2026-56649 Advisory for vendor technical details.

Detection Methods for CVE-2026-56649

Indicators of Compromise

Unexpected crashes or restarts of the NfsService or nfssvc.exe process recorded in the Windows Application or System event logs.
New or unusual child processes spawned by the NFS service, particularly cmd.exe, powershell.exe, or rundll32.exe.
Outbound network connections initiated by the NFS service to previously unseen external hosts.

Detection Strategies

Monitor NFS RPC traffic on TCP/UDP ports 111 (portmap) and 2049 for high volumes of concurrent or malformed requests from a single source.
Alert on Windows Error Reporting entries citing access violations within NFS server binaries.
Correlate NFS service anomalies with process-creation telemetry to catch post-exploitation activity.

Monitoring Recommendations

Enable verbose logging for the NFS server role and forward logs to a centralized SIEM for correlation.
Baseline normal NFS client sources and flag connections from unexpected subnets or the internet.
Track service restart counts for NfsService as an early indicator of exploitation attempts.

How to Mitigate CVE-2026-56649

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-56649 Advisory to all affected Windows 10, Windows 11, and Windows Server hosts.
Inventory systems with the NFS server role enabled and prioritize patching internet-exposed or DMZ-facing hosts first.
Restrict NFS ports (111 and 2049) at the perimeter and internal firewalls so only authorized clients can reach the service.

Patch Information

Microsoft has released cumulative security updates addressing CVE-2026-56649 across all supported Windows client and server versions. Administrators should install the July 2026 Patch Tuesday updates or later via Windows Update, WSUS, or Microsoft Update Catalog. Details are available in the Microsoft CVE-2026-56649 Advisory.

Workarounds

Disable the Windows NFS server role on systems that do not require it using Disable-WindowsOptionalFeature -Online -FeatureName ServerForNFS-Infrastructure.
Block inbound traffic to TCP/UDP ports 111 and 2049 from untrusted networks using Windows Firewall or upstream network access controls.
Segment NFS servers into isolated VLANs with explicit allow-lists for approved client IP ranges.

bash

# Disable Windows NFS Server role (PowerShell, run as Administrator)
Disable-WindowsOptionalFeature -Online -FeatureName ServerForNFS-Infrastructure

# Block NFS ports at Windows Firewall until patch is applied
New-NetFirewallRule -DisplayName "Block NFS TCP 2049" -Direction Inbound -Protocol TCP -LocalPort 2049 -Action Block
New-NetFirewallRule -DisplayName "Block NFS UDP 2049" -Direction Inbound -Protocol UDP -LocalPort 2049 -Action Block
New-NetFirewallRule -DisplayName "Block Portmap TCP 111" -Direction Inbound -Protocol TCP -LocalPort 111 -Action Block
New-NetFirewallRule -DisplayName "Block Portmap UDP 111" -Direction Inbound -Protocol UDP -LocalPort 111 -Action Block