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Privacy Statement Third-Party Cookies Accept Reject Manage cookies MSRC  Customer Guidance  Security Update Guide  Vulnerabilities  CVE-2026-40401 Windows TCP/IP Denial of Service Vulnerability New On this page  CVE-2026-40401  Subscribe RSS PowerShell  API  CSAF Security Vulnerability Released: May 12, 2026 Assigning CNA Microsoft CVE.org link CVE-2026-40401  Impact Denial of Service Max Severity Important Weakness CWE-476: NULL Pointer Dereference CVSS Source Microsoft Vector String CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:U/RL:O/RC:C Metrics CVSS:3.1 7.1 / 6.2  Base score metrics: 7.1 / Temporal score metrics: 6.2  Expand all  Collapse all Metric Value   Base score metrics(8) Attack Vector Local Attack Complexity Low Privileges Required None User Interaction None Scope Changed Confidentiality None Integrity None Availability High   Temporal score metrics(3) Exploit Code Maturity Unproven Remediation Level Official Fix Report Confidence Confirmed Please see Common Vulnerability Scoring System for more information on the definition of these metrics. Executive Summary Null pointer dereference in Windows TCP/IP allows an unauthorized attacker to deny service locally. Exploitability The following table provides an exploitability assessment for this vulnerability at the time of original publication. Publicly disclosed No Exploited No Exploitability assessment Exploitation Unlikely FAQ According to the CVSS metric, a successful exploitation could lead to a scope change (S:C). What does this mean for this vulnerability? In this case, a successful attack could be performed from a low privilege Hyper-V guest. The attacker could traverse the guest's security boundary to cause denial of service on the Hyper-V host environment. Acknowledgements Windows Attack Research & Protection (WARP) with Microsoft Microsoft recognizes the efforts of those in the security community who help us protect customers through coordinated vulnerability disclosure. See Acknowledgements for more information. Security Updates To determine the support lifecycle for your software, see the Microsoft Support Lifecycle. Release date Descending  Edit columns  Download  Filters  Product Family  Max Severity  Impact  Platform   Clear Release date  Product Platform Impact Max Severity Article Download Build Number Assigning CNA Customer Action Required Title: Release date, Content: May 12, 2026 Windows Server 2012 R2 (Server Core installation) - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2012 R2 (Server Core installation), Content:, 1 link 5087471  Title: Download Security Update for Windows Server 2012 R2 (Server Core installation), Content:, 1 link Monthly Rollup  Title: Build numbers, Content: 6.3.9600.23181 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2012 R2 - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2012 R2, Content:, 1 link 5087471  Title: Download Security Update for Windows Server 2012 R2, Content:, 1 link Monthly Rollup  Title: Build numbers, Content: 6.3.9600.23181 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2012 (Server Core installation) - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2012 (Server Core installation), Content:, 1 link 5087470  Title: Download Security Update for Windows Server 2012 (Server Core installation), Content:, 1 link Monthly Rollup  Title: Build numbers, Content: 6.2.9200.26079 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2012 - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2012, Content:, 1 link 5087470  Title: Download Security Update for Windows Server 2012, Content:, 1 link Monthly Rollup  Title: Build numbers, Content: 6.2.9200.26079 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2016 (Server Core installation) - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2016 (Server Core installation), Content:, 1 link 5087537  Title: Download Security Update for Windows Server 2016 (Server Core installation), Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.14393.9140 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2016 - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2016, Content:, 1 link 5087537  Title: Download Security Update for Windows Server 2016, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.14393.9140 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 10 Version 1607 for x64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 10 Version 1607 for x64-based Systems, Content:, 1 link 5087537  Title: Download Security Update for Windows 10 Version 1607 for x64-based Systems, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.14393.9140 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 Version 26H1 for ARM64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 Version 26H1 for ARM64-based Systems, Content:, 1 link 5089548  Title: Download Security Update for Windows 11 Version 26H1 for ARM64-based Systems, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.28000.2113 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 version 26H1 for x64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 version 26H1 for x64-based Systems, Content:, 1 link 5089548  Title: Download Security Update for Windows 11 version 26H1 for x64-based Systems, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.28000.2113 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2025 - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2025, Content:, 2 links 5087539  5087423  Title: Download Security Update for Windows Server 2025, Content:, 2 links Security Update  Security Hotpatch Update  Title: Build numbers, Content: 10.0.26100.32860 10.0.26100.32772 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 Version 24H2 for x64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 Version 24H2 for x64-based Systems, Content:, 2 links 5089549  5089466  Title: Download Security Update for Windows 11 Version 24H2 for x64-based Systems, Content:, 2 links Security Update  Security Hotpatch Update  Title: Build numbers, Content: 10.0.26100.8457 10.0.26100.8390 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 Version 24H2 for ARM64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 Version 24H2 for ARM64-based Systems, Content:, 2 links 5089549  5089466  Title: Download Security Update for Windows 11 Version 24H2 for ARM64-based Systems, Content:, 2 links Security Update  Security Hotpatch Update  Title: Build numbers, Content: 10.0.26100.8457 10.0.26100.8390 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows Server 2022, 23H2 Edition (Server Core installation) - Denial of Service Important Title: Knowledge Base Articles for Windows Server 2022, 23H2 Edition (Server Core installation), Content:, 1 link 5087541  Title: Download Security Update for Windows Server 2022, 23H2 Edition (Server Core installation), Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.25398.2330 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 Version 23H2 for x64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 Version 23H2 for x64-based Systems, Content:, 1 link 5087420  Title: Download Security Update for Windows 11 Version 23H2 for x64-based Systems, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.22631.7079 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 2026 Windows 11 Version 23H2 for ARM64-based Systems - Denial of Service Important Title: Knowledge Base Articles for Windows 11 Version 23H2 for ARM64-based Systems, Content:, 1 link 5087420  Title: Download Security Update for Windows 11 Version 23H2 for ARM64-based Systems, Content:, 1 link Security Update  Title: Build numbers, Content: 10.0.22631.7079 Title: Assigning CNA, Content: Microsoft Title: Customer action required, Content: Required Title: Release date, Content: May 12, 20