CVE-2026-49800: Windows 10 1809 Privilege Escalation Flaw

CVE-2026-49800 Overview

CVE-2026-49800 is an integer overflow vulnerability in the Windows Web Proxy Auto-Discovery Protocol (WPAD) component. An authorized local attacker can exploit the flaw to elevate privileges on affected systems. Microsoft classifies the issue as high severity, and it affects supported versions of Windows 10, Windows 11, and Windows Server. The underlying weakness maps to [CWE-122] (heap-based buffer overflow) triggered by arithmetic wraparound during memory allocation. Successful exploitation grants attackers full control over confidentiality, integrity, and availability on the target host. No public proof-of-concept, exploit code, or in-the-wild activity has been reported at the time of publication.

Critical Impact
A local, authenticated attacker can leverage an integer wraparound in WPAD to trigger memory corruption and elevate to SYSTEM privileges on affected Windows and Windows Server platforms.

Affected Products

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

Discovery Timeline

2026-07-14 - CVE-2026-49800 published to NVD
2026-07-17 - Last updated in NVD database

Technical Details for CVE-2026-49800

Vulnerability Analysis

The vulnerability resides in the Windows Web Proxy Auto-Discovery Protocol (WPAD) service, which resolves proxy configuration via wpad.dat scripts and PAC file processing. WPAD parses attacker-influenced input while performing arithmetic on size or length fields used for heap allocation. An integer overflow during that calculation produces an undersized buffer, and subsequent writes overflow adjacent heap memory. This corruption pattern aligns with [CWE-122], enabling attackers to overwrite heap metadata or control structures. Because the WPAD service runs in a privileged context, controlled memory corruption translates directly into local privilege escalation.

Root Cause

The root cause is unchecked arithmetic on a size value before a heap allocation in the WPAD parsing path. When the computed size wraps around the integer type boundary, the allocator returns a buffer smaller than the caller expects. The subsequent copy or write operation exceeds the allocated region, corrupting heap state and enabling attacker-controlled memory writes.

Attack Vector

Exploitation requires local access and low-privilege authenticated context on the target system, with no user interaction. The attacker triggers WPAD processing of a crafted input that induces the integer wraparound. Once heap corruption is achieved, standard heap grooming and control-flow hijacking techniques allow elevation to SYSTEM. The scope remains unchanged, meaning the impact is confined to the local security authority of the affected host.

No verified proof-of-concept code is available. Consult the Microsoft Security Update CVE-2026-49800 advisory for authoritative technical details.

Detection Methods for CVE-2026-49800

Indicators of Compromise

Unexpected crashes, restarts, or Windows Error Reporting entries associated with WPAD service processes such as svchost.exe hosting the WinHTTP Web Proxy Auto-Discovery Service.
New or unusual child processes spawned under the WPAD service context, particularly command shells or LOLBins running as SYSTEM.
Anomalous local process access to WPAD-related registry keys under HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings.

Detection Strategies

Monitor for heap corruption crash signatures (c0000374, STATUS_HEAP_CORRUPTION) in WPAD-hosting processes via the Application event log.
Correlate low-privilege user sessions with subsequent SYSTEM-level process creation on the same host to surface privilege escalation chains.
Baseline WPAD service behavior and alert on deviations such as unusual memory allocations, thread creation, or module loads.

Monitoring Recommendations

Enable Sysmon Event ID 1 (process creation) and Event ID 10 (process access) to capture privilege transitions involving WPAD.
Ship Windows Security, System, and Application logs to a centralized analytics platform for correlation across endpoints.
Track patch state across the fleet and alert on hosts running unpatched builds of Windows 10, Windows 11, or Windows Server listed in the advisory.

How to Mitigate CVE-2026-49800

Immediate Actions Required

Apply the Microsoft security updates referenced in the Microsoft Security Update CVE-2026-49800 advisory on all affected Windows and Windows Server systems.
Prioritize patching on multi-user hosts, jump boxes, RDS servers, and VDI environments where low-privileged accounts are common.
Audit local account membership and remove unnecessary interactive logon rights to reduce the pool of potential attackers.

Patch Information

Microsoft has released security updates addressing CVE-2026-49800 for all listed Windows 10, Windows 11, and Windows Server builds. Refer to the Microsoft Security Update CVE-2026-49800 guide for the specific KB article and build numbers corresponding to each affected version.

Workarounds

Disable WPAD where not required by clearing the "Automatically detect settings" option in Internet Options and setting the WpadOverride registry value to block automatic proxy discovery.
Disable the WinHTTP Web Proxy Auto-Discovery Service (WinHttpAutoProxySvc) on hosts that do not rely on PAC-based proxy configuration.
Enforce explicit proxy configuration through Group Policy to eliminate reliance on WPAD resolution on managed endpoints.

bash

# Configuration example: disable the WinHTTP Web Proxy Auto-Discovery Service
sc.exe config WinHttpAutoProxySvc start= disabled
sc.exe stop WinHttpAutoProxySvc

# Block WPAD name resolution via hosts file entry
echo 255.255.255.255 wpad >> %SystemRoot%\System32\drivers\etc\hosts