CVE-2026-21250: Windows 11 24H2 Privilege Escalation Flaw

CVE-2026-21250 Overview

CVE-2026-21250 is an untrusted pointer dereference vulnerability in Windows HTTP.sys that allows an authorized attacker to elevate privileges locally. The vulnerability exists in the HTTP protocol stack driver (HTTP.sys), a kernel-mode component responsible for handling HTTP requests and responses in Windows operating systems. When exploited, this flaw enables attackers with local access to escalate their privileges to SYSTEM level, potentially gaining complete control over affected systems.

Critical Impact

Local privilege escalation via untrusted pointer dereference in HTTP.sys kernel driver could allow attackers to gain SYSTEM privileges and execute arbitrary code with elevated permissions.

Affected Products

• Microsoft Windows 11 24H2 (x64 and ARM64)
• Microsoft Windows 11 25H2 (x64 and ARM64)
• Microsoft Windows Server 2022 23H2
• Microsoft Windows Server 2025

Discovery Timeline

• 2026-02-10 - CVE-2026-21250 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-21250

Vulnerability Analysis

This vulnerability is classified under CWE-822 (Untrusted Pointer Dereference), which occurs when a program dereferences a pointer that it expects to be valid but is actually attacker-controlled or otherwise untrusted. In the context of HTTP.sys, this kernel-mode driver processes HTTP requests and manages request queues, connection pooling, and caching for Windows web servers and services.

The untrusted pointer dereference allows an authenticated local attacker to manipulate memory in a way that bypasses normal security boundaries. Because HTTP.sys operates in kernel mode, successful exploitation grants the attacker the highest level of system privileges, enabling them to install programs, modify data, create new accounts with full user rights, or disable security features.

The attack requires local access and low-privileged authentication, meaning an attacker would need initial access to the system through another vector before exploiting this vulnerability. However, once local access is achieved, the exploitation complexity is low with no user interaction required.

Root Cause

The root cause stems from improper validation of pointer values before dereferencing them within the HTTP.sys driver. When processing certain requests or operations, the driver accepts pointer values from sources that may be controllable by local users without sufficient verification. This allows an attacker to supply a malicious pointer value that, when dereferenced, can lead to arbitrary memory access or code execution in kernel mode.

Attack Vector

The vulnerability requires local access to the target system. An attacker must first gain a foothold on the machine through valid credentials or another exploitation technique. Once authenticated, the attacker can interact with the HTTP.sys driver through system calls or IOCTLs (I/O Control codes) that trigger the vulnerable code path. By carefully crafting input that includes a malicious pointer value, the attacker can cause the driver to dereference this pointer, leading to privilege escalation.

The local attack vector combined with low attack complexity and no user interaction requirement makes this vulnerability particularly concerning for environments where multiple users share system access or where initial compromise through other means is possible.

Detection Methods for CVE-2026-21250

Indicators of Compromise

• Unexpected system crashes or BSOD errors related to HTTP.sys that may indicate exploitation attempts
• Unusual privilege escalation events where low-privileged accounts suddenly gain SYSTEM-level access
• Anomalous process creation from kernel mode or HTTP.sys context
• Suspicious IOCTL calls to \Device\Http\* device objects from user-mode processes

Detection Strategies

• Monitor Windows Security Event Logs for Event ID 4672 (Special privileges assigned to new logon) associated with unexpected accounts
• Deploy endpoint detection rules that flag unusual interactions with HTTP.sys driver interfaces
• Implement kernel-mode monitoring to detect suspicious pointer operations and memory access patterns
• Use SentinelOne's behavioral AI to identify privilege escalation attempts that deviate from normal system behavior

Monitoring Recommendations

• Enable verbose logging for HTTP.sys through Event Tracing for Windows (ETW) providers
• Configure SIEM rules to correlate privilege escalation events with HTTP.sys activity
• Implement process integrity monitoring to detect unexpected elevation of process tokens
• Deploy memory protection solutions that can detect kernel-mode exploitation attempts

How to Mitigate CVE-2026-21250

Immediate Actions Required

• Apply Microsoft security updates immediately through Windows Update or WSUS
• Audit local user accounts and remove unnecessary access privileges
• Review and restrict local logon rights using Group Policy
• Enable Credential Guard and other virtualization-based security features where supported

Patch Information

Microsoft has released security updates addressing this vulnerability. Administrators should consult the Microsoft Security Response Center advisory for detailed patch information and specific KB articles for each affected Windows version. The patches should be applied as soon as possible, particularly on systems that are accessible to multiple users or have been previously compromised.

Workarounds

• Limit local access to affected systems to only essential personnel until patches are applied
• Implement application whitelisting to prevent unauthorized executables from running
• Use the principle of least privilege to minimize the impact of successful exploitation
• Consider network segmentation to isolate critical servers running affected Windows versions

# Check HTTP.sys driver version and status
sc query http
driverquery /v | findstr /i "http.sys"

# Review local user privileges
whoami /priv
net localgroup administrators

# Enable advanced auditing for privilege use
auditpol /set /subcategory:"Sensitive Privilege Use" /success:enable /failure:enable