CVE-2026-62799 Overview
CVE-2026-62799 is a heap-based buffer overflow [CWE-122] in the Windows Server Message Block (SMB) Client component of Microsoft Windows 11 26h1. An authorized local attacker can trigger the overflow to elevate privileges on the host. The vulnerability affects both arm64 and x64 builds of Windows 11 26h1. Microsoft published the advisory on 2026-08-11, and the entry was last updated on 2026-08-13.
Critical Impact
Successful exploitation grants elevated privileges on the local system, resulting in high impact to confidentiality, integrity, and availability of the affected host.
Affected Products
- Microsoft Windows 11 26h1 (x64)
- Microsoft Windows 11 26h1 (arm64)
- Windows SMB Client component
Discovery Timeline
- 2026-08-11 - CVE-2026-62799 published to NVD
- 2026-08-11 - Microsoft releases security update via MSRC advisory
- 2026-08-13 - Last updated in NVD database
Technical Details for CVE-2026-62799
Vulnerability Analysis
The flaw resides in the Windows SMB Client, which handles outbound SMB protocol traffic on Windows endpoints. A heap-based buffer overflow occurs when the client processes crafted response data. The overflow corrupts adjacent heap structures used by the SMB Client during message parsing.
An attacker with local, authenticated access can trigger the vulnerable code path from a low-privileged user context. Because the SMB Client executes with elevated system privileges when handling certain operations, memory corruption in this component can be leveraged to gain higher privileges. The CWE-122 classification confirms heap metadata or adjacent heap objects as the corruption target.
Root Cause
The root cause is missing or incorrect bounds validation on data copied into a heap-allocated buffer inside the SMB Client. When input length exceeds the destination allocation, the client writes beyond the buffer boundary. This overwrites adjacent heap chunks, function pointers, or object metadata used by SMB Client internals.
Attack Vector
Exploitation requires local access and low-privilege authentication on the target Windows 11 26h1 host. No user interaction is required. An attacker typically induces the SMB Client to communicate with an attacker-controlled endpoint or crafted local resource that returns malformed SMB traffic. Successful heap layout manipulation followed by controlled overflow yields code execution in the security context of the SMB Client, resulting in local privilege escalation. See the Microsoft Vulnerability Update CVE-2026-62799 advisory for vendor-supplied details.
Detection Methods for CVE-2026-62799
Indicators of Compromise
- Unexpected SMB Client crashes or Windows Error Reporting entries referencing mrxsmb.sys, mrxsmb20.sys, or related SMB client modules.
- Local processes running as low-privileged users spawning SMB connections to loopback or attacker-controlled endpoints.
- New child processes launched from svchost.exe hosting the SMB client service with atypical command lines.
Detection Strategies
- Monitor for anomalous SMB client-initiated connections originating from non-administrative user sessions.
- Alert on kernel or user-mode crash dumps whose faulting module is an SMB client component on Windows 11 26h1 hosts.
- Correlate local logon events with subsequent process token elevation to identify potential post-exploitation activity.
Monitoring Recommendations
- Ingest Windows Security, System, and Sysmon logs into a centralized SIEM for correlation across endpoints.
- Enable Windows Defender Exploit Guard and Attack Surface Reduction telemetry to flag suspicious child-process creation from service hosts.
- Track patch compliance for Windows 11 26h1 fleets and flag any hosts missing the August 2026 security update.
How to Mitigate CVE-2026-62799
Immediate Actions Required
- Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-62799 to all Windows 11 26h1 endpoints.
- Restrict local logon rights on sensitive systems to reduce the pool of users able to trigger the vulnerable code path.
- Audit endpoints for unauthorized local accounts and reset credentials on systems suspected of compromise.
Patch Information
Microsoft issued a security update for Windows 11 26h1 (x64 and arm64) addressing the heap-based buffer overflow in the SMB Client. Administrators should deploy the update through Windows Update, Windows Server Update Services (WSUS), or Microsoft Intune. Verify installation by confirming the updated SMB client binary versions after reboot. Consult the vendor advisory for KB identifiers and file version details.
Workarounds
- Block outbound SMB traffic (TCP 445) at the host firewall where SMB client functionality is not required for business operations.
- Disable the LanmanWorkstation service on systems that do not need to access remote SMB shares, after validating operational impact.
- Enforce SMB signing and require SMBv3 encryption via Group Policy to reduce exposure to malicious SMB responses.
# Configuration example: block outbound SMB and disable Workstation service via PowerShell
New-NetFirewallRule -DisplayName "Block Outbound SMB 445" -Direction Outbound -Protocol TCP -RemotePort 445 -Action Block
Set-Service -Name LanmanWorkstation -StartupType Disabled
Stop-Service -Name LanmanWorkstation -Force
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
