Microsoft QUIC: Security feature bypass due to insecure default initialization
Summary
Microsoft QUIC, the transport protocol used by HTTP/3 and Windows services such as IIS and SMB over QUIC, initializes an internal variable with an insecure default value (CWE-453). A remote, unauthenticated attacker can bypass a security feature over the network without user interaction. Attack complexity is high, the scope changes, and the impact is mainly on availability with a limited integrity impact.
Affected
- Windows 11 Version 23H2 (x64 and ARM64): 10.0.22631.0 to below 10.0.22631.7652
- Windows 11 Version 24H2: 10.0.26100.0 to below 10.0.26100.9711
- Windows 11 Version 25H2: 10.0.26200.0 to below 10.0.26200.9711
- Windows 11 Version 26H1: 10.0.28000.0 to below 10.0.28000.3231
- Windows 11 Version 26H2: 10.0.26300.0 to below 10.0.26300.9711
- Windows Server 2022: 10.0.20348.0 to below 10.0.20348.5761
- Windows Server 2025 (including Server Core): 10.0.26100.0 to below 10.0.26100.33650
Fixed in
- Windows 11 23H2: build 10.0.22631.7652 (KB5126653, per third-party mirrors; one mirror shows a typo, verify on MSRC)
- Windows 11 24H2: build 10.0.26100.9711
- Windows 11 25H2: build 10.0.26200.9711
- Windows 11 26H1: build 10.0.28000.3231
- Windows 11 26H2: build 10.0.26300.9711
- Windows Server 2022: build 10.0.20348.5761
- Windows Server 2025: build 10.0.26100.33650
- Build numbers are provisional until confirmed in the MSRC Security Update Guide.
Workaround
Unknown.
Exploitation
- No exploitation in the wild known.
- No public proof of concept found.
- CISA SSVC: exploitation none, not automatable, partial technical impact.
- EPSS about 0.5 percent.
Our recommendation
- Install the October 2026 cumulative update on affected Windows 11 and Windows Server systems, with priority on servers and hosts exposed to the internet.
- Confirm the fixed build numbers on the MSRC Security Update Guide before rollout.
- Check whether HTTP/3, IIS, or SMB over QUIC is in use; if not needed, restrict inbound UDP 443 at the perimeter.
- Plan the upgrade of Windows 11 23H2 to a supported release, as servicing for Enterprise and Education editions is expected to end in November 2026.
Information from Microsoft (Patch Tuesday October 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide (CVE-2026-103577) msrc.microsoft.com
- NVD mirror (vulners) vulners.com
- CVE/CNA record (vulners) vulners.com
- Strix CVE summary www.strix.ai
- EUVD mirror (vulners) vulners.com
- CWE-453 description (CVEdetails) www.cvedetails.com
- Threat intelligence aggregator cve.imfht.com
- Patch Tuesday coverage (Zecurit) zecurit.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, 19:38.
Original description (NVD)
Insecure default variable initialization in Microsoft QUIC allows an unauthorized attacker to bypass a security feature over a network.
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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