CVE tracker / CVE-2026-78450

CVE-2026-78450

CVE-2026-78450 is the Windows Reliable Multicast Transport Driver (RMCAST) Remote Code Execution Vulnerability. Microsoft rates it Critical, CVSS 8.1. It is not in CISA's Known Exploited Vulnerabilities catalog. Published September 8, 2026.

Risk summary

HighRemote Code Execution

8.1

CVSS v3 base

0.7%

EPSS: chance of exploitation in 30 days

High

Severity

Critical

Microsoft's rating

Less Likely

Microsoft: exploitation

In CISA KEV

Microsoft describes it: “Use after free in Reliable Multicast Transport Driver (RMCAST) allows an unauthorized attacker to execute code over a network.”

How to fix it

Microsoft lists 8 affected products and the update that fixes each:

KB5122871 · fixed build 10.0.26100.33438
Windows Server 2025, Windows Server 2025 (Server Core installation)
KB5122876 · fixed build 10.0.17763.9245
Windows Server 2019 (Server Core installation), Windows Server 2019, Windows 10 Version 1809 for x64-based Systems, Windows 10 Version 1809 for 32-bit Systems
KB5122882 · fixed build 10.0.20348.5622
Windows Server 2022 (Server Core installation), Windows Server 2022

What Microsoft changed

September 8, 2026: Information published.

Details

Weakness: CWE-416: Use After Free
Publicly disclosed (Microsoft): No
Exploited (Microsoft): No
Known exploited (CISA KEV): Not listed
Published: September 8, 2026
Last revised: September 8, 2026

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References

Microsoft Security Update Guide NVD CVE record CWE

From Microsoft, CISA, NVD and FIRST data; the Microsoft details were read when this page was opened. Check the references above before you act.