What’s Vulnerable › Vulnerabilities

CVE-2021-31207: Microsoft Exchange Server Security Feature Bypass Vulnerability

• Severity: medium (CVSS 6.5 v2.0)
• Exploited in the wild: yes — in CISA KEV since 2021-11-03
• EPSS: 99.8% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-05-11 · updated 2026-06-17
• Weakness: CWE-434

Description

Microsoft Exchange Server Security Feature Bypass Vulnerability

Affected products

• Microsoft Exchange Server: 2013-cumulative_update_23, 2016-cumulative_update_19, 2016-cumulative_update_20, 2019-cumulative_update_8, 2019-cumulative_update_9
• Microsoft Exchange Server 2013 Cumulative Update 23: from 15.00.0 before 15.00.1497.018 — fixed in 15.00.1497.018
• Microsoft Exchange Server 2016 Cumulative Update 19: from 15.01.0 before 15.01.2176.014 — fixed in 15.01.2176.014
• Microsoft Exchange Server 2016 Cumulative Update 20: from 15.01.0 before 15.01.2242.010 — fixed in 15.01.2242.010
• Microsoft Exchange Server 2019 Cumulative Update 8: from 15.02.0 before 15.02.0792.015 — fixed in 15.02.0792.015
• Microsoft Exchange Server 2019 Cumulative Update 9: from 15.02.0 before 15.02.0858.012 — fixed in 15.02.0858.012

How to fix

Upgrade to a fixed version: Microsoft Exchange Server 2013 Cumulative Update 23 15.00.1497.018; Microsoft Exchange Server 2016 Cumulative Update 19 15.01.2176.014; Microsoft Exchange Server 2016 Cumulative Update 20 15.01.2242.010; Microsoft Exchange Server 2019 Cumulative Update 8 15.02.0792.015; Microsoft Exchange Server 2019 Cumulative Update 9 15.02.0858.012. Check the vendor advisory for mitigations.

References

• http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207
• https://www.zerodayinitiative.com/advisories/ZDI-21-819/
• http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207
• https://www.zerodayinitiative.com/advisories/ZDI-21-819/
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31207

Get alerted when CVE-2021-31207 or the next vulnerability affects your stack