Microsoft Exchange Server

CVE-2021-26857: Microsoft Exchange Server Remote Code Execution Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 246 days after the CVE record was published.

How likely is exploitation in the next 30 days?

95.8%

EPSS score, percentile 99.9 of all scored records

How severe?

7.8 High

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 3 May 2022. Apply updates per vendor instructions.

Advisories and patches (1)

Which versions are affected?

Affected

from 14.0.0.0 to before 14.03.0513.000
from 15.00.0 to before 15.00.1395.012
from 15.00.0 to before 15.00.1473.006
from 15.00.0 to before 15.00.1497.012
from 15.00.0 to before 15.00.0847.064
from 15.01.0 to before 15.01.1531.012

and 14 more in the table below

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

Required action, set by CISA

Apply updates per vendor instructions.

CISA due date for this action: 3 May 2022.

Advisories and patches

Microsoft Exchange Server Remote Code Execution Vulnerability (Vendor advisory, Patch)

Linked from CISA's catalog entry

Affected products and versions

As listed in the CVE record.

Microsoft Microsoft Exchange Server 2010 Service Pack 3
Status	Versions
Affected	from 14.0.0.0 to before 14.03.0513.000

Microsoft Microsoft Exchange Server 2013 Cumulative Update 21
Status	Versions
Affected	from 15.00.0 to before 15.00.1395.012

Microsoft Microsoft Exchange Server 2013 Cumulative Update 22
Status	Versions
Affected	from 15.00.0 to before 15.00.1473.006

Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
Status	Versions
Affected	from 15.00.0 to before 15.00.1497.012

Microsoft Microsoft Exchange Server 2013 Service Pack 1
Status	Versions
Affected	from 15.00.0 to before 15.00.0847.064

Microsoft Microsoft Exchange Server 2016 Cumulative Update 10
Status	Versions
Affected	from 15.01.0 to before 15.01.1531.012

Microsoft Microsoft Exchange Server 2016 Cumulative Update 11
Status	Versions
Affected	from 15.01.0 to before 15.01.1591.018

Microsoft Microsoft Exchange Server 2016 Cumulative Update 12
Status	Versions
Affected	from 15.01.0 to before 15.01.1713.010

Microsoft Microsoft Exchange Server 2016 Cumulative Update 13
Status	Versions
Affected	from 15.01.0 to before 15.01.1779.008

Microsoft Microsoft Exchange Server 2016 Cumulative Update 14
Status	Versions
Affected	from 15.01.0 to before 15.01.1847.012

Microsoft Microsoft Exchange Server 2016 Cumulative Update 15
Status	Versions
Affected	from 15.01.0 to before 15.01.1913.012

Microsoft Microsoft Exchange Server 2016 Cumulative Update 16
Status	Versions
Affected	from 15.01.0 to before 15.01.1979.008

Microsoft Microsoft Exchange Server 2016 Cumulative Update 17
Status	Versions
Affected	from 15.01.0 to before 15.01.2044.013

Microsoft Microsoft Exchange Server 2016 Cumulative Update 18
Status	Versions
Affected	from 15.01.0 to before 15.01.2106.013

Microsoft Microsoft Exchange Server 2016 Cumulative Update 19
Status	Versions
Affected	from 15.01.0 to before 15.01.2176.009

Microsoft Microsoft Exchange Server 2016 Cumulative Update 8
Status	Versions
Affected	from 15.01.0 to before 15.01.1415.010

Microsoft Microsoft Exchange Server 2016 Cumulative Update 9
Status	Versions
Affected	from 15.01.0 to before 15.01.1466.016

Microsoft Microsoft Exchange Server 2019
Status	Versions
Affected	from 15.02.0 to before 15.02.0221.018

Microsoft Microsoft Exchange Server 2019 Cumulative Update 1
Status	Versions
Affected	from 15.02.0 to before 15.01.1979.006

Microsoft Microsoft Exchange Server 2019 Cumulative Update 2
Status	Versions
Affected	from 15.02.0 to before 15.02.0397.011

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

CISA added CVE-2021-26857 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
CISA marks it as known to be used in ransomware campaigns.
The due date CISA sets for the required action is 3 May 2022.
The CVE record was published on 2 Mar 2021, 246 days before it was added to KEV.
The CVE record carries a CVSS v3.1 base score of 7.8 (High), given by its CNA.
FIRST's EPSS model puts the probability of exploitation in the next 30 days at 95.8% (percentile 99.9), as of 28 Oct 2026.
CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

CWE-502: Deserialization of Untrusted Data

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

Date	Event
2 Mar 2021	CVE record published
3 Nov 2021	Added to CISA KEV
3 May 2022	CISA due date for the required action
19 Aug 2026	CVE record last updated

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