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CVE-2021-26421: Skype for Business and Lync Spoofing Vulnerability

• Severity: medium (CVSS 5.8 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-05-11 · updated 2026-06-17

Description

Skype for Business and Lync Spoofing Vulnerability

Affected products

• Microsoft Lync Server: 2013-cumulative_update_10
• Microsoft Skype for Business Server 2019 CU7: 2015-cumulative_update_11
• Microsoft Lync Server 2013 CU10: from 8308.0 before 8308.1144 — fixed in 8308.1144
• Microsoft Skype for Business Server 2015 CU11: from 2015 CU11 before 2046.369 — fixed in 2046.369

How to fix

Upgrade to a fixed version: Microsoft Lync Server 2013 CU10 8308.1144; Microsoft Skype for Business Server 2015 CU11 2046.369. Check the vendor advisory for mitigations.

References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26421
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26421

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