Microsoft Windows NTLM: Heap-based buffer overflow allows local privilege escalation
Summary
A heap-based buffer overflow in the Windows NTLM authentication component allows privilege escalation. The CVE record lists Windows 10 Version 1607 and Windows Server 2016 among the affected products. An attacker with a valid local user account and low privileges can exploit it to gain higher privileges on the system. It is not remotely exploitable, so it is mainly a risk on shared or multi-user systems after an initial compromise.
Affected
- Windows 10 Version 1607 (builds below 10.0.14393.9612)
- Windows Server 2016 and Server Core installation, as listed in the CVE record (builds below 10.0.14393.9612; the research notes ask for confirmation on MSRC)
Fixed in
- Windows 10 Version 1607: build 10.0.14393.9612 or later, from the October 2026 security update
- Knowledge base (KB) number for Windows 10 1607: Unknown.
Workaround
Unknown.
Exploitation
- CISA SSVC data: exploitation none, automatable no, technical impact total.
- EPSS score 0.00353 (very low).
- No public proof of concept or CISA KEV listing was found in limited searches. This is not confirmation that no exploitation exists.
Our recommendation
- Inventory all Windows 10 1607 and Windows Server 2016 systems.
- Install the October 2026 security update to reach build 10.0.14393.9612 or later, or replace devices that can no longer be patched.
- Check Microsoft's lifecycle pages for the support status of Windows 10 1607.
- Restrict local logon rights and remove unnecessary local administrator accounts.
- Give priority to multi-user systems, terminal servers and any unisolated 1607 machine.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Elevation of privilege – Windows NTLM
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation unlikely
- Product family
- Windows, Windows ESU (Extended Security Updates)
- Updates (KB)
- KB5127648, KB5126651, KB5126655, KB5126657, KB5126646, KB5126653
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Response Center: CVE-2026-83709 msrc.microsoft.com
- OpenCVE: CVE-2026-83709 (NVD data, CVSS, SSVC) app.opencve.io
- VulDB: vulnerability 418899 vuldb.com
- Vulners: CVE-2026-83709 (version ranges) vulners.com
- Vulners: CVELIST CVE-2026-83709 (EPSS) vulners.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 2 Nov 2026, 03:15.
Original description (NVD)
Heap-based buffer overflow in Windows NTLM allows an authorized attacker to elevate privileges locally.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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