Is CVE-2021-26857 in the CISA KEV catalog?

Yes. CVE-2021-26857 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-2021-26857?

CVE-2021-26857 has a CVSS v3 base score of 7.8 (HIGH).

What is the EPSS score for CVE-2021-26857?

CVE-2021-26857 has an EPSS score of 94.01% (100th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-26857?

Install KB5000978 for Microsoft Exchange Server 2010 Service Pack 3. Fixes are available for 26 affected products.

CVE & CISA-KEV Catalog

CVE-2021-26857

HIGHCISA KEVEPSS 100th pctl

7.8

CVSS v3

NVD

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

CWE-502Deserialization of Untrusted Data

How to fix

Remediation Available

Microsoft Exchange Server 2010 Service Pack 3Microsoft

Install:KB5000978Microsoft

Microsoft Exchange Server 2013 Cumulative Update 21Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2013 Cumulative Update 22Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2013 Cumulative Update 23Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2013 Service Pack 1Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 10Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 11Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 12Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 13Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 14Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 15Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 16Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 17Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 18Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 19Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 8Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2016 Cumulative Update 9Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 1Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 2Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 3Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 4Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 5Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 6Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 7Microsoft

Install:KB5000871Microsoft

Microsoft Exchange Server 2019 Cumulative Update 8Microsoft

Install:KB5000871Microsoft

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 VectorLocal

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

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

Exploit Intelligence

94.01%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. This vulnerability is part of the ProxyLogon exploit chain.

Apply updates per vendor instructions.

Remediation due: 2022-05-03

Associated with ransomware campaigns

References

Patch1

portal.msrc.microsoft.com

Government Resource1

cisa.gov

Related Vulnerabilities

Other CWE-502 (Deserialization of Untrusted Data) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2023-0669	High	7.2	100%	KEV + Ransom	Fix
CVE-2021-44228	Critical	10.0	100%	KEV + Ransom	Fix
CVE-2021-35464	Critical	9.8	100%	KEV + Ransom	Fix
CVE-2023-29300	Critical	9.8	100%	KEV + Ransom	-
CVE-2025-53770	Critical	9.8	100%	KEV + Ransom	Fix
CVE-2022-41082	High	8.0	100%	KEV + Ransom	Fix

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Markdown

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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 2025-12-18.