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CVE-2021-26855

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS 3.1 Critical 9.1Known exploited (CISA KEV)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

Official description from the CVE record, published by microsoft. Copyright © 1999-2026 The MITRE Corporation. See /security/cve/legal for the CVE license. View the record on cve.org

Key data

CVSS 3.1 Critical 9.1

Score published by microsoft in the CVE record.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C

  • Attack vector: Network

    The vulnerable component can be reached over a network, possibly from the Internet.

  • Attack complexity: Low

    No special conditions are needed. The attack can be repeated at will.

  • Privileges required: None

    No account or privileges are needed.

  • User interaction: None

    No action from any user is needed.

  • Scope: Unchanged

    The impact stays within the vulnerable component.

  • Confidentiality impact: High

    Serious or total effect on the secrecy of data.

  • Integrity impact: High

    Serious or total effect on the trustworthiness of data.

  • Availability impact: None

    No effect on availability.

CVSS is owned and managed by FIRST.Org, Inc. (FIRST) and is used by permission. Each CVSS score on this site is shown together with its vector string, as published in the CVE record by the source named next to it. Learn more at first.org/cvss.

EPSS

Exploit Prediction Scoring System by FIRST: an estimate of the probability that exploitation activity will be observed in the next 30 days.
Score
0.99996 (99.99% probability)
Percentile
99.98%
Scores of
Nov 10, 2026
Model
v2026.06.15

CISA Known Exploited Vulnerabilities

This CVE is listed in the CISA KEV catalog.
Name in the catalog
Microsoft Exchange Server Remote Code Execution Vulnerability
Added to the catalog
Nov 3, 2021
Federal due date
May 3, 2022
Ransomware campaigns
Known use

CISA assessment as of Mar 1, 2021

Stakeholder-Specific Vulnerability Categorization (SSVC) decision points added by CISA.
Exploitation
Recorded by CISA as active
Automatable
yes
Technical impact
total

Weakness (CWE)

CWE catalog version 5.0.
CWE names: Copyright © 2006-2026, The MITRE Corporation. See /security/cve/legal for the CWE license.

Affected products

Products listed in the CVE record, as the CNA published them.

  • Microsoft Exchange Server 2013 Cumulative Update 21

    • affected: 15.00.0 up to before 15.00.1395.012

  • Microsoft Exchange Server 2013 Cumulative Update 22

    • affected: 15.00.0 up to before 15.00.1473.006

  • Microsoft Exchange Server 2013 Cumulative Update 23

    • affected: 15.00.0 up to before 15.00.1497.012

  • Microsoft Exchange Server 2016 Cumulative Update 10

    • affected: 15.01.0 up to before 15.01.1531.012

  • Microsoft Exchange Server 2016 Cumulative Update 11

    • affected: 15.01.0 up to before 15.01.1591.018

  • Microsoft Exchange Server 2016 Cumulative Update 12

    • affected: 15.01.0 up to before 15.01.1713.010

  • Microsoft Exchange Server 2016 Cumulative Update 13

    • affected: 15.01.0 up to before 15.01.1779.008

  • Microsoft Exchange Server 2016 Cumulative Update 14

    • affected: 15.01.0 up to before 15.01.1847.012

  • Microsoft Exchange Server 2016 Cumulative Update 15

    • affected: 15.01.0 up to before 15.01.1913.012

  • Microsoft Exchange Server 2016 Cumulative Update 16

    • affected: 15.01.0 up to before 15.01.1979.008

  • Microsoft Exchange Server 2016 Cumulative Update 17

    • affected: 15.01.0 up to before 15.01.2044.013

  • Microsoft Exchange Server 2016 Cumulative Update 18

    • affected: 15.01.0 up to before 15.01.2106.013

  • Microsoft Exchange Server 2016 Cumulative Update 19

    • affected: 15.01.0 up to before 15.01.2176.009

  • Microsoft Exchange Server 2016 Cumulative Update 8

    • affected: 15.01.0 up to before 15.01.1415.010

  • Microsoft Exchange Server 2016 Cumulative Update 9

    • affected: 15.01.0 up to before 15.01.1466.016

  • Microsoft Exchange Server 2019

    • affected: 15.02.0 up to before 15.02.0221.018

  • Microsoft Exchange Server 2019 Cumulative Update 1

    • affected: 15.02.0 up to before 15.01.1979.006

  • Microsoft Exchange Server 2019 Cumulative Update 2

    • affected: 15.02.0 up to before 15.02.0397.011

  • Microsoft Exchange Server 2019 Cumulative Update 3

    • affected: 15.02.0 up to before 15.02.0464.015

  • Microsoft Exchange Server 2019 Cumulative Update 4

    • affected: 15.02.0 up to before 15.02.0659.012

  • Microsoft Exchange Server 2019 Cumulative Update 5

    • affected: 15.02.0 up to before 15.02.0595.008

  • Microsoft Exchange Server 2019 Cumulative Update 6

    • affected: 15.02.0 up to before 15.02.0529.013

  • Microsoft Exchange Server 2019 Cumulative Update 7

    • affected: 15.02.0 up to before 15.02.0721.013

  • Microsoft Exchange Server 2019 Cumulative Update 8

    • affected: 15.02.0 up to before 15.02.0792.010

References

Links listed in the CVE record. They lead to other sites, which CubePath does not control.

Timeline

  1. ID reserved
  2. Record published
  3. Added to CISA KEV
  4. Record last updated

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Data from the CVE Program (MITRE), CISA KEV and FIRST EPSS. CVSS is owned by FIRST and used by permission. Not affiliated with or endorsed by MITRE, CISA or FIRST. Data as of Nov 10, 2026, 19:52 UTC. Sources and licenses