CVE-2026-66307

Skype for Business and Lync Denial of Service Vulnerability

CVE-2026-66307 is fixed in 3 products on 1 platform, first by KB5123301, released September 8, 2026. It is not reported as exploited. Patch Radar priority: High.

High

CVSS

7.5

Important

Exploitation

None reported

Exploitation Less Likely

CISA KEV

Not listed

Not in the Known Exploited Vulnerabilities catalog

EPSS

0.0117

67th percentile

<p>Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.</p>

Where it's fixed

Product	Fix	Released
Microsoft
Skype for Business Server 2015 CU13	KB5123301	Sep 8, 2026
Skype for Business Server 2019 CU8	KB5123300	Sep 8, 2026
Skype for Business Server Subscription Edition CU1	KB5123287	Sep 8, 2026

Details

Published
September 8, 2026
Weakness
Integer Underflow (Wrap or Wraparound)
Vendor advisory
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66307

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