How severe is CVE-2021-31207?

CVE-2021-31207 has a CVSS 3.1 base score of 6.6 (MEDIUM). EPSS estimates a 99.8% probability of exploitation in the next 30 days. It is listed in the CISA Known Exploited Vulnerabilities catalog.

ProxyShell (CVE-2021-31207): Microsoft Exchange Server Security

Q: What is ProxyShell?

ProxyShell is the common name for CVE-2021-31207. Part of the ProxyShell chain: this is the Exchange Server security feature bypass flaw. Tenable notes it was rated less likely to be exploited on its own, but chained with the other two it enables code execution.

CISA Known Exploited Vulnerabilities

Microsoft Exchange Server Security Feature Bypass Vulnerability is confirmed as actively exploited and is listed in the CISA KEV catalog.

Added to KEV

2021-11-03

Remediation due

2021-11-17

Ransomware

Known use

Required action
Apply updates per vendor instructions.

CISA notes
https://nvd.nist.gov/vuln/detail/CVE-2021-31207

Also Known as ProxyShell

Part of the ProxyShell chain: this is the Exchange Server security feature bypass flaw. Tenable notes it was rated less likely to be exploited on its own, but chained with the other two it enables code execution.

Name sources

Description

Microsoft Exchange Server Security Feature Bypass Vulnerability

Severity

CVSS 3.1 · CNA6.6 MEDIUM

CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

SSVC · CISA-ADP

Exploitationactive

Automatableno

Technical impacttotal

Exploitation Context

as of October 13, 2026

All KEV statistics→

CISA added CVE-2021-31207 to the KEV catalog 176 days after its CVE record was published.
It is one of 390 Microsoft entries in the CISA KEV catalog, and one of 83 added in 2021.
Its current EPSS score (99.8%) is higher than that of 90% of CISA KEV entries.

Affected Products

Vendor	Product	Affected versions
Microsoft	Microsoft Microsoft Exchange Server 2013 Cumulative Update 23	15.00.0 to <15.00.1497.018
Microsoft	Microsoft Microsoft Exchange Server 2019 Cumulative Update 9	15.02.0 to <15.02.0858.012
Microsoft	Microsoft Microsoft Exchange Server 2016 Cumulative Update 20	15.01.0 to <15.01.2242.010
Microsoft	Microsoft Microsoft Exchange Server 2016 Cumulative Update 19	15.01.0 to <15.01.2176.014
Microsoft	Microsoft Microsoft Exchange Server 2019 Cumulative Update 8	15.02.0 to <15.02.0792.015

References

3 Links

Other references

Record Details

Published

2021-05-11

Last updated

2025-10-21

Assigner (CNA)

microsoft

Credited to

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CVE record data © The MITRE Corporation, used under the CVE Terms of Use. Exploited-in-the-wild data from the CISA Known Exploited Vulnerabilities catalog (CC0). Exploit prediction scores by FIRST EPSS (first.org/epss). This site is not endorsed or certified by MITRE, NVD, CISA, or FIRST.