Can network attackers remotely execute code on unpatched Windows 11 via critical Microsoft QUIC flaw?

The short version

• Microsoft has identified a critical vulnerability in its QUIC implementation.
• Attackers can execute code over a network without prior authentication.
• Several recent Windows 11 versions are affected by this use-after-free issue.

The details

Microsoft has published an advisory for CVE-2026-100219, describing a use-after-free vulnerability within the Microsoft QUIC component. According to the National Vulnerability Database, this flaw allows an unauthorised attacker to execute code over a network. The issue stems from improper memory management, specifically classified under CWE-416. This type of defect occurs when a program continues to use a pointer after the memory it references has been freed, potentially leading to unpredictable behaviour or code execution.

The vulnerability affects multiple editions of Microsoft Windows 11. Specific builds remain at risk if they have not been updated to the latest cumulative patches. For Windows 11 version 23H2, systems running builds before 10.0.22631.7652 are exposed. Similarly, version 24H2 requires updates to build 10.0.26100.9711 or later. Version 25H2 must be updated to build 10.0.26200.9711, while version 26H1 needs build 10.0.28000.3231. Version 26H2 is affected until build 10.0.26300.9711 is installed.

The context

The severity of this flaw is underscored by its CVSS score of 9.8, which the NVD rates as critical. This high score reflects the potential for an attacker to exploit the vulnerability over a network. The QUIC protocol is widely used for secure communications, making this component a significant target. Security teams must prioritise patching these specific builds to mitigate the risk of remote code execution.

Next steps

Organisations should immediately audit their Windows 11 environments to identify affected builds. Inventory systems running version 23H2, 24H2, 25H2, 26H1, and 26H2 to check their current build numbers. Compare these against the safe versions listed in the advisory. Deploy the necessary cumulative updates to bring all systems above the minimum safe build levels. Verify installation success through system logs or update history reports.

What defenders should look for

Security operations teams should monitor for unusual network connections initiated by the QUIC process. Look for unexpected outbound traffic or connections to unknown external IP addresses. Check for signs of memory corruption or crashes in system logs related to network services. Investigate any new processes spawned by network-facing services. Isolate affected endpoints from the network to prevent lateral movement. Validate file integrity on critical system directories to detect any malicious payloads. Review firewall logs for blocked or allowed suspicious QUIC traffic.

What to do and how to stay safe: Microsoft

• Verify current Windows 11 build numbers against the safe versions provided by Microsoft.
• Install the latest cumulative updates for all affected Windows 11 versions immediately.
• Monitor network traffic for anomalous QUIC protocol activity or unexpected connections.
• Review system logs for signs of memory corruption or unauthorised code execution.

Step-by-step guide: How Virtual Patching Works: The Mechanics Behind Inline Protection

General security guidance from the Honeypot Daily newsroom. It is not confirmed advice from the organisations named in this story.

Frequently asked questions

What is the CVSS score for CVE-2026-100219?

The National Vulnerability Database rates CVE-2026-100219 as critical with a CVSS score of 9.8.

Which Windows 11 versions are affected by this vulnerability?

Windows 11 versions 23H2, 24H2, 25H2, 26H1, and 26H2 are affected until specific build updates are applied.

What type of vulnerability is CVE-2026-100219?

It is a use-after-free vulnerability in the Microsoft QUIC component that allows remote code execution.