Microsoft Security Response CenterCVE-2026-40378

Windows Local Security Authority Subsystem Service (LSASS) Denial of Service Vulnerability

Memory allocation with excessive size value in Windows Local Security Authority Subsystem Service (LSASS) allows an unauthorized attacker to deny service over a network.

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7.5CVSS out of 10High severity

Scope

What the vendor says is affected

Windows 10 Version 1809 for 32-bit Systems
Windows 10 Version 1809 for x64-based Systems
Windows Server 2019
Windows Server 2019 (Server Core installation)
Windows Server 2022
Windows 10 Version 21H2 for 32-bit Systems
Windows 10 Version 21H2 for ARM64-based Systems
Windows 10 Version 21H2 for x64-based Systems
Windows 10 Version 22H2 for x64-based Systems
Windows 10 Version 22H2 for ARM64-based Systems
Windows 10 Version 22H2 for 32-bit Systems
Windows Server 2025 (Server Core installation)
Windows 11 Version 25H2 for ARM64-based Systems
Windows 11 Version 25H2 for x64-based Systems
Windows 11 Version 23H2 for ARM64-based Systems
Windows 11 Version 23H2 for x64-based Systems
Windows 11 Version 24H2 for ARM64-based Systems
Windows 11 Version 24H2 for x64-based Systems
Windows Server 2025
Windows 11 version 26H1 for x64-based Systems
Windows 11 Version 26H1 for ARM64-based Systems
Windows 10 Version 1607 for 32-bit Systems
Windows 10 Version 1607 for x64-based Systems
Windows Server 2016

Versions the vendor lists as fixed

10.0.17763.9020
10.0.20348.5386
10.0.19044.7548
10.0.19045.7548
10.0.26100.33158
10.0.26200.8875
10.0.22631.7376
10.0.26100.8875
10.0.28000.2525
10.0.28000.2269
10.0.14393.9339
6.2.9200.26226
6.3.9600.23291

Next step

What the vendor recommends

Install KB5099538. Install KB5099540. Install KB5099539. Install KB5099536. Install KB5101650. Install KB5099414. Install KB5101649. Install KB5095051. Install KB5099535. Install KB5099445. Install KB5099444.

Review the complete instructions on the vendor's site

Timeline

When this advisory changed

Jul 14, 2026 at 7:00 AM UTC
Published by Microsoft Security Response Center
The publication date reported by the vendor.
Sep 2, 2026 at 11:20 PM UTC
Added to SecurityAlert
We collected the advisory from the official source.
Sep 3, 2026 at 5:25 AM UTC
Confirmed at the source
Our collector saw this advisory during a later source check.

Vulnerabilities

CVEs named in this advisory

CVE-2026-40378View CVSS, KEV, EPSS, and threat context

Source

Start with the vendor's bulletin

SecurityAlert organizes the fields the vendor published. The original bulletin remains the source of record for affected versions, fixes, and later changes.

Vendor: Microsoft Security Response Center
Advisory ID: CVE-2026-40378
Last source check: Sep 13, 2026 at 2:40 AM UTC

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What the vendor says is affected

Versions the vendor lists as fixed

What the vendor recommends

When this advisory changed

CVEs named in this advisory

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