CVE-2026-62739: Windows 10 1809 Privilege Escalation Flaw

CVE-2026-62739 Overview

CVE-2026-62739 is a heap-based buffer overflow [CWE-122] in the Windows HTTP.sys kernel-mode driver. An authorized local attacker can trigger the overflow to elevate privileges on affected Windows client and server systems. The vulnerability affects a broad range of supported Windows versions, including Windows 10, Windows 11, and Windows Server 2019 through 2025. Microsoft assigned a CVSS 3.1 base score of 7.8 with the vector AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H, reflecting local attack vector, low complexity, and high impact to confidentiality, integrity, and availability.

Critical Impact
Successful exploitation grants an authorized user elevated privileges on the target system through corruption of kernel heap memory in HTTP.sys.

Affected Products

Microsoft Windows 10 (1809, 21H2, 22H2) across x86, x64, and ARM64
Microsoft Windows 11 (23H2, 24H2, 25H2, 26H1) across x64 and ARM64
Microsoft Windows Server 2019, 2022, and 2025

Discovery Timeline

2026-08-11 - CVE-2026-62739 published to NVD
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-62739

Vulnerability Analysis

The flaw resides in HTTP.sys, the kernel-mode HTTP protocol stack used by Internet Information Services (IIS), WinRM, WSDAPI, and other Windows components that accept HTTP traffic. HTTP.sys allocates buffers on the kernel heap when parsing and processing HTTP requests and responses. Insufficient bounds validation during this processing allows attacker-controlled data to write beyond the allocated buffer boundary.

Because HTTP.sys executes in kernel context, heap corruption inside the driver enables an authorized local user to overwrite adjacent kernel structures. Manipulating these structures yields SYSTEM-level execution, bypassing standard user account restrictions. The Exploit Prediction Scoring System places the near-term exploitation probability at a low value, but the impact of successful exploitation is complete host compromise.

Root Cause

The underlying weakness is classified as [CWE-122] Heap-based Buffer Overflow. A code path in HTTP.sys writes data into a heap allocation without correctly enforcing the destination buffer size against the length of incoming data. When the attacker provides input that exceeds the expected boundary, the write crosses into adjacent heap chunks and metadata.

Attack Vector

Exploitation requires local access and low-privilege authentication on the target host. The attacker interacts with an HTTP.sys-backed endpoint reachable from the local session, submits crafted HTTP request data that triggers the vulnerable code path, and manipulates the resulting heap corruption to hijack execution flow in the kernel. No user interaction is required. Refer to the Microsoft Security Update Guide for vendor-provided technical detail.

Detection Methods for CVE-2026-62739

Indicators of Compromise

Unexpected crashes, bugchecks, or memory dumps referencing HTTP.sys on hosts running IIS, WinRM, or other HTTP-consuming services.
Sudden token elevation or process integrity changes for processes spawned from low-privilege user sessions.
Anomalous local HTTP requests to HTTP.sys endpoints originating from non-service user accounts.

Detection Strategies

Monitor Windows Event Log for kernel bugcheck events (Event ID 1001) that reference HTTP.sys or http! symbols in the stack.
Correlate local authentication events with subsequent SYSTEM-context process creations to identify privilege elevation chains.
Deploy EDR behavioral rules that flag user-mode processes spawning SYSTEM-privileged children shortly after interacting with local HTTP endpoints.

Monitoring Recommendations

Enable and forward Windows kernel crash telemetry and Sysmon Event ID 1 (process creation) with parent-child integrity level tracking.
Track patch compliance status for the August 2026 Microsoft security update across all Windows 10, 11, and Server SKUs listed by the vendor.
Baseline the set of accounts that legitimately interact with local HTTP.sys services and alert on deviations.

How to Mitigate CVE-2026-62739

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-62739 on all affected Windows 10, Windows 11, and Windows Server systems.
Prioritize patching on multi-user hosts, jump servers, and terminal servers where low-privilege users share the system.
Audit local account inventories and remove unnecessary interactive logon rights to reduce the pool of potential attackers.

Patch Information

Microsoft has published fixed builds via the Microsoft Security Update Guide. Administrators should deploy the update through Windows Update, Windows Server Update Services (WSUS), Microsoft Intune, or their preferred patch management tooling. Verify build numbers after installation to confirm the vulnerable HTTP.sys binary has been replaced.

Workarounds

No official workaround has been published by Microsoft; applying the security update is the supported remediation path.
Where immediate patching is not possible, restrict local logon rights and disable non-essential services that load HTTP.sys such as unused IIS sites or WinRM listeners.
Enforce least-privilege policies and application allowlisting to limit the ability of low-privilege users to run arbitrary binaries that could trigger the vulnerable code path.

bash

# Example: query HTTP.sys URL reservations and disable unneeded listeners
netsh http show urlacl
netsh http delete urlacl url=http://+:80/UnneededApp/
Stop-Service -Name W3SVC -Force
Set-Service -Name W3SVC -StartupType Disabled