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CVE-2021-42305: Microsoft Exchange Server Spoofing Vulnerability

• Severity: medium (CVSS 4.3 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 7.9% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-11-10 · updated 2026-08-19

Description

Microsoft Exchange Server Spoofing Vulnerability

Affected products

• Microsoft Exchange Server: 2013-cumulative_update_23, 2016-cumulative_update_21, 2016-cumulative_update_22, 2019-cumulative_update_10, 2019-cumulative_update_11
• Microsoft Exchange Server 2013 Cumulative Update 23: from 15.00.0 before 15.00.1497.026 — fixed in 15.00.1497.026
• Microsoft Exchange Server 2016 Cumulative Update 21: from 15.01.0 before 15.01.2308.020 — fixed in 15.01.2308.020
• Microsoft Exchange Server 2016 Cumulative Update 22: from 15.0.0 before 15.01.2375.017 — fixed in 15.01.2375.017
• Microsoft Exchange Server 2019 Cumulative Update 10: from 15.02.0 before 15.02.0792.019 — fixed in 15.02.0792.019
• Microsoft Exchange Server 2019 Cumulative Update 11: from 15.02.0 before 15.02.0986.014 — fixed in 15.02.0986.014

How to fix

Upgrade to a fixed version: Microsoft Exchange Server 2013 Cumulative Update 23 15.00.1497.026; Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020; Microsoft Exchange Server 2016 Cumulative Update 22 15.01.2375.017; Microsoft Exchange Server 2019 Cumulative Update 10 15.02.0792.019; Microsoft Exchange Server 2019 Cumulative Update 11 15.02.0986.014. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-42305
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42305

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