How severe is CVE-2021-34523?

CVE-2021-34523 has a CVSS 3.1 base score of 9 (CRITICAL). EPSS estimates a 100.0% probability of exploitation in the next 30 days. It is listed in the CISA Known Exploited Vulnerabilities catalog.

ProxyShell (CVE-2021-34523): Microsoft Exchange Server Elevation of

Q: What is ProxyShell?

ProxyShell is the common name for CVE-2021-34523. Part of the ProxyShell chain: this is the Exchange Server elevation of privilege flaw. CISA listed it with CVE-2021-34473 and CVE-2021-31207 as the ProxyShell vulnerabilities under active exploitation in August 2021.

CISA Known Exploited Vulnerabilities

Microsoft Exchange Server Privilege Escalation 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-2021-34523

Also Known as ProxyShell

Part of the ProxyShell chain: this is the Exchange Server elevation of privilege flaw. CISA listed it with CVE-2021-34473 and CVE-2021-31207 as the ProxyShell vulnerabilities under active exploitation in August 2021.

Name sources

Description

Microsoft Exchange Server Elevation of Privilege Vulnerability

Severity

CVSS 3.1 · CNA9 CRITICAL

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N/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-2021-34523 to the KEV catalog 112 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 (100.0%) is higher than that of 96% of CISA KEV entries.

Affected Products

Vendor	Product	Affected versions
Microsoft	Microsoft Microsoft Exchange Server 2013 Cumulative Update 23	15.00.0 to <15.00.1497.015
Microsoft	Microsoft Microsoft Exchange Server 2016 Cumulative Update 19	15.01.0 to <15.01.2176.012
Microsoft	Microsoft Microsoft Exchange Server 2016 Cumulative Update 20	15.01.0 to <15.01.2242.008
Microsoft	Microsoft Microsoft Exchange Server 2019 Cumulative Update 8	15.02.0 to <15.02.0792.013
Microsoft	Microsoft Microsoft Exchange Server 2019 Cumulative Update 9	15.02.0 to <15.02.0858.010

References

4 Links

Vendor advisories

Microsoft Exchange Server Elevation of Privilege Vulnerability

Other references

Record Details

Published

2021-07-14

Last updated

2026-08-10

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.