Critical 9.0Known exploited (CISA KEV)Published Jul 14, 2021

CVE-2021-34523Microsoft Exchange Server Elevation of Privilege Vulnerability

Updated under an hour ago (Oct 27, 2026, 1:10 PM ET)

Does this affect you?

It affects Microsoft Exchange Server 2013 Cumulative Update 23 and 5 other products. Add what you run to find out whether that includes you.

CISA lists it as known exploited since Nov 3, 2021; federal agencies had to remediate by Nov 17, 2021, and it is known to be used in ransomware. Treat that date as your deadline.

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Every CVE for Microsoft Exchange Server 2013 Cumulative Update 23

Exploited in the last 30 days

Microsoft Exchange Server Elevation of Privilege Vulnerability

CVSS: 9.0 CRITICAL (v3.1, CNA)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
Weakness (CWE): Not given
Known exploited: Added to CISA KEV Nov 3, 2021; federal remediation due Nov 17, 2021; known ransomware useApply updates per vendor instructions.
EPSS (exploitation in the next 30 days): 99.99% (percentile 100)
Assigned by: microsoft
NVD analysis: Analyzed
Record last updated: Aug 10, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 microsoft:exchange_server_2013_cumulative_update_23 (from the name)	from 15.00.0 before 15.00.1497.015	CNA record
Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 microsoft:exchange_server_2016_cumulative_update_19 (from the name)	from 15.01.0 before 15.01.2176.012	CNA record
Microsoft Microsoft Exchange Server 2016 Cumulative Update 20 microsoft:exchange_server_2016_cumulative_update_20 (from the name)	from 15.01.0 before 15.01.2242.008	CNA record
Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 microsoft:exchange_server_2019_cumulative_update_8 (from the name)	from 15.02.0 before 15.02.0792.013	CNA record
Microsoft Microsoft Exchange Server 2019 Cumulative Update 9 microsoft:exchange_server_2019_cumulative_update_9 (from the name)	from 15.02.0 before 15.02.0858.010	CNA record
microsoft exchange_server microsoft:exchange_server	2013; 2016; 2016; 2019; 2019	NVD

References

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34523	vendor-advisory, patch
http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html	exploit, third-party-advisory, vdb-entry
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34523	patch, vendor-advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-822/	third-party-advisory, vdb-entry
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34523	us-government-resource

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.