CVE-2026-62800 Overview
CVE-2026-62800 is a heap-based buffer overflow [CWE-122] in the Windows Server Message Block (SMB) Server. An authenticated attacker on the network can trigger the flaw to execute arbitrary code on the target host. Microsoft assigned the vulnerability a CVSS 3.1 base score of 8.8 and rated it HIGH severity. The issue affects a broad range of client and server SKUs, including Windows 10, Windows 11, and Windows Server releases from 2012 through 2025. The Exploit Prediction Scoring System (EPSS) currently rates the flaw at 0.914%, placing it in the 56.96th percentile of scored CVEs.
Critical Impact
An authorized network attacker can achieve remote code execution against the SMB Server, threatening confidentiality, integrity, and availability of affected Windows hosts.
Affected Products
- Microsoft Windows 10 (versions 1607, 1809, 21H2, 22H2) on x86, x64, and ARM64
- Microsoft Windows 11 (versions 23H2, 24H2, 25H2, 26H1) on x64 and ARM64
- Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, and 2025
Discovery Timeline
- 2026-08-11 - CVE-2026-62800 published to the National Vulnerability Database
- 2026-08-11 - Microsoft published security update guidance for CVE-2026-62800
- 2026-08-13 - Last updated in NVD database
Technical Details for CVE-2026-62800
Vulnerability Analysis
The flaw is a heap-based buffer overflow inside the Windows SMB Server component. SMB is the file-sharing protocol used by Windows clients and servers to expose shares, named pipes, and printer resources on TCP port 445. The vulnerable code path is reachable after successful authentication, meaning an attacker must present valid credentials or reuse an existing session before delivering the malicious request.
Successful exploitation grants arbitrary code execution in the context of the SMB Server process, which typically runs with kernel or SYSTEM-level privileges. Because SMB is enabled by default on Windows Server roles such as file services, domain controllers, and print servers, the attack surface across enterprise fleets is substantial.
Root Cause
Heap-based buffer overflows [CWE-122] occur when code writes data past the end of a heap-allocated buffer. In the SMB Server, this typically stems from a mismatch between the length declared in a client-supplied field of an SMB2 request and the size of the destination buffer allocated on the process heap. Overwriting adjacent heap metadata or object pointers lets an attacker corrupt control data and redirect execution.
Attack Vector
The attack requires network access to TCP port 445 and low-privilege authenticated credentials. An attacker sends a crafted SMB request containing malformed length or offset fields that force the server to copy attacker-controlled bytes beyond the target heap chunk. No user interaction is required, and the scope remains unchanged, but code execution on the SMB Server host is achievable in a single exchange.
Microsoft has not published exploitation details, and no public proof-of-concept has been observed. See the Microsoft Security Update CVE-2026-62800 advisory for vendor guidance.
Detection Methods for CVE-2026-62800
Indicators of Compromise
- Unexpected crashes, restarts, or WER entries for the LanmanServer (srv2.sys, srvnet.sys) components on Windows hosts
- Anomalous child processes such as cmd.exe, powershell.exe, or rundll32.exe spawned by SMB-related system processes
- Inbound SMB (TCP 445) sessions from non-standard internal hosts followed by lateral movement or credential access activity
Detection Strategies
- Hunt for authenticated SMB sessions immediately followed by SYSTEM-level process creation on the target file server
- Correlate Windows Security event IDs 4624 (logon) and 4672 (special privileges) with subsequent SMB write anomalies and service crashes
- Alert on SMB2 requests with malformed length fields or oversized compound requests captured by network sensors
Monitoring Recommendations
- Enable and forward SMB Server auditing (Microsoft-Windows-SMBServer/Audit) and kernel crash telemetry to a central log store
- Baseline which internal hosts legitimately initiate SMB sessions and flag deviations, especially from workstations to domain controllers
- Restrict and monitor east-west SMB traffic using host-based firewall logs and network flow data
How to Mitigate CVE-2026-62800
Immediate Actions Required
- Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-62800 to all affected Windows client and server builds
- Prioritize patching of internet-exposed and high-value SMB endpoints such as file servers, domain controllers, and print servers
- Rotate credentials for any accounts observed authenticating from untrusted hosts prior to patch deployment
Patch Information
Microsoft has released fixes for all listed Windows 10, Windows 11, and Windows Server versions through the standard security update channels. Refer to the Microsoft Security Update CVE-2026-62800 guide for KB article numbers and per-SKU update packages.
Workarounds
- Block inbound TCP 445 at the network perimeter and between segmented internal zones where SMB is not required
- Enforce SMB signing and require modern SMB3 dialects to reduce the population of unauthenticated attackers who could reach the vulnerable code path
- Disable the SMB Server service (LanmanServer) on hosts that do not need to share files or named pipes
# Restrict inbound SMB to trusted management subnets on Windows hosts
New-NetFirewallRule -DisplayName "Block-Inbound-SMB-Untrusted" `
-Direction Inbound -Protocol TCP -LocalPort 445 `
-RemoteAddress Any -Action Block
# Verify installed updates address CVE-2026-62800
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
