Is CVE-2021-34473 in the CISA KEV catalog?

Yes. CVE-2021-34473 is on the CISA Known Exploited Vulnerabilities list, added 2021-11-03, and has known ransomware campaign use.

What is the CVSS severity of CVE-2021-34473?

CVE-2021-34473 has a CVSS v3 base score of 9.1 (CRITICAL).

What is the EPSS score for CVE-2021-34473?

CVE-2021-34473 has an EPSS score of 100.00% (100th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-34473?

Install KB5001779 for Microsoft Exchange Server 2013 Cumulative Update 23. Fixes are available for 5 affected products.

CVE & CISA-KEV Catalog

CVE-2021-34473

CRITICALCISA KEVEPSS 100th pctl

9.1

CVSS v3

NVD

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

CWE-918Server-Side Request Forgery (SSRF)

How to fix

Remediation Available

Microsoft Exchange Server 2013 Cumulative Update 23Microsoft

Install:KB5001779Microsoft

Microsoft Exchange Server 2016 Cumulative Update 19Microsoft

Install:KB5001779Microsoft

Microsoft Exchange Server 2016 Cumulative Update 20Microsoft

Install:KB5001779Microsoft

Microsoft Exchange Server 2019 Cumulative Update 8Microsoft

Install:KB5001779Microsoft

Microsoft Exchange Server 2019 Cumulative Update 9Microsoft

Install:KB5001779Microsoft

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Exploit Intelligence

100.00%probability of exploitation in 30 days

100thpercentile

Very high risk: more likely to be exploited than 100% of all known CVEs.

Known Exploited Vulnerability (CISA KEV)

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution.

Apply updates per vendor instructions.

Remediation due: 2021-11-17

Associated with ransomware campaigns

References

Patch1

portal.msrc.microsoft.com

Exploit1

packetstormsecurity.com

Third-Party Advisory1

zerodayinitiative.com

Government Resource1

cisa.gov

Related Vulnerabilities

Other CWE-918 (Server-Side Request Forgery (SSRF)) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2024-21893	High	8.2	100%	KEV + Ransom	-
CVE-2021-40438	Critical	9.0	100%	KEV	Fix
CVE-2021-26855	Critical	9.1	100%	KEV + Ransom	Fix
CVE-2021-21985	Critical	9.8	100%	KEV + Ransom	Fix
CVE-2022-41040	High	8.8	100%	KEV + Ransom	Fix
CVE-2021-22986	Critical	9.8	100%	KEV + Ransom	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-10-29.