What’s Vulnerable › Vulnerabilities

CVE-2021-24099: Skype for Business and Lync Denial of Service Vulnerability

• Severity: medium (CVSS 4.0 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 3.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-02-25 · updated 2026-06-17

Description

Skype for Business and Lync Denial of Service Vulnerability

Affected products

• Microsoft Lync Server: 2013
• Microsoft Skype for Business Server 2019 CU7: 2015-cu8, 2019-cu2
• Microsoft Lync Server 2013: before publication — fixed in publication
• Microsoft Skype for Business Server 2019 CU2: from 7.0.0 before publication — fixed in publication
• Microsoft Skype for Business Server 2015 CU 8: from 2015 CU 8 before publication — fixed in publication

How to fix

Upgrade to a fixed version: Microsoft Lync Server 2013 publication; Microsoft Skype for Business Server 2019 CU2 publication; Microsoft Skype for Business Server 2015 CU 8 publication. Check the vendor advisory for mitigations.

References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-24099
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-24099

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