KB5123300: what it fixes and who gets it
KB5123300 is a security update from Microsoft's September 2026 release for Skype for Business Server 2019 CU8.
Microsoft data read
Should you install it?
Yes. It closes holes Microsoft rates Critical, the most serious rating it uses. Windows Update installs it automatically on supported home PCs.
If the installation fails or the PC misbehaves afterwards, note the exact error code first: the fix depends on it. Our update troubleshooting guides are linked on this page.
Security fixes in KB5123300
| CVE | Type | CVSS | Severity |
|---|---|---|---|
| CVE-2026-66302Skype for Business Remote Code Execution Vulnerability | Remote Code Execution | 9.8 | Critical |
| CVE-2026-69646Skype for Business Spoofing Vulnerability | Spoofing | 8.3 | Important |
| CVE-2026-66304Skype for Business Information Disclosure Vulnerability | Information Disclosure | 7.5 | Important |
| CVE-2026-66307Skype for Business and Lync Denial of Service Vulnerability | Denial of Service | 7.5 | Important |
| CVE-2026-66305Skype for Business Spoofing Vulnerability | Spoofing | 7.1 | Important |
| CVE-2026-63523Skype for Business Spoofing Vulnerability | Spoofing | 6.5 | Important |
| CVE-2026-66303Skype for Business and Lync Denial of Service Vulnerability | Denial of Service | 6.5 | Important |
| CVE-2026-66306Skype for Business Information Disclosure Vulnerability | Information Disclosure | 6.5 | Important |
| CVE-2026-66308Skype for Business and Lync Denial of Service Vulnerability | Denial of Service | 6.5 | Important |
| CVE-2026-69642Skype for Business Spoofing Vulnerability | Spoofing | 6.5 | Important |
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