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Fencer Vulns
critical 9.8

CVE-2021-26855 kev+ ransomware in the critical feed json

Microsoft Exchange Server Remote Code Execution Vulnerability

published 2021-03-02 5 y ago modified 2026-08-19 2 mo ago also EUVD-2021-13639

cvss
9.8
version 3.1, CNA
cisa kev
Exploited
added 2021-11-03, used in ransomware
epss
100.0%
EPSS 100.0% chance of exploitation in 30 days, percentile 99.9%
public exploits
7
Metasploit, Exploit-DB, plus a Nuclei check

Summary

Microsoft Exchange Server Remote Code Execution Vulnerability

Patch / fix

cisa required action Apply updates per vendor instructions.

  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 15.00.1395.012
  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 15.00.1473.006
  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 15.00.1497.012
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 15.01.1531.012
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 15.01.1591.018
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 15.01.1713.010
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 13 15.01.1779.008
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 14 15.01.1847.012
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 15 15.01.1913.012
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 16 15.01.1979.008
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 17 15.01.2044.013
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 18 15.01.2106.013
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 15.01.2176.009
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 8 15.01.1415.010
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 9 15.01.1466.016
  • Microsoft Microsoft Exchange Server 2019 15.02.0221.018
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 1 15.01.1979.006
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 2 15.02.0397.011
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 3 15.02.0464.015
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 4 15.02.0659.012
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 5 15.02.0595.008
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 6 15.02.0529.013
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 7 15.02.0721.013
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 15.02.0792.010

Timeline

  1. CISA SSVC: exploitation active
  2. 0-day patched (Project Zero)
  3. First public exploit (Metasploit auxiliary/gather/exchange_proxylogon_collector)
  4. Published
  5. Added to CISA KEV
  6. CISA patch deadline

Known exploited

Added
5 y ago
Patch by
2022-05-03 (4 y ago)
Ransomware
Known to be used in ransomware campaigns
Required action
Apply updates per vendor instructions.
Vendor, product
Microsoft, Exchange Server
CISA name
Microsoft Exchange Server Remote Code Execution Vulnerability

Zero-day

Detected in real attacks while still unknown to the vendor and the public, per Google Project Zero's 0day In-the-Wild tracker.

Patched
2021-03-02
Details
project zero root cause analysis

Ransomware

Groups reported to have exploited it in intrusions.

  • Conti ransomware gang
  • Cuba ransomware gang
  • AvosLocker ransomware gang

reported by tenable.comtrendmicro.comtrendmicro.com

Ransomware Vulnerability Matrix by BushidoUK (github.com/BushidoUK/Ransomware-Vulnerability-Matrix), reformatted per CVE. CC BY 4.0

ATT&CK techniques

How it is exploited and what it leads to, in MITRE ATT&CK terms.

  • T1133 External Remote Services exploitation
  • T1090 Proxy primary impact
  • T1005 Data from Local System secondary impact
  • T1505.003 Web Shell secondary impact

kev release 2025-07-28, att&ck v16.1. KEV to ATT&CK mappings by the MITRE Center for Threat-Informed Defense (Mappings Explorer, Apache-2.0). © 2026 The MITRE Corporation. This work is reproduced and distributed with the permission of The MITRE Corporation. Terms of use

CISA SSVC

Exploitation
Active, exploited in the wild
Automatable
yes
Technical impact
total
Assessed
5 y ago

Exploit prediction

100.00% chance of exploitation in the next 30 days, higher than 99.9% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

CVSS scores

assigned by version score vector
CNA 3.1 9.1 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
CNA 3.1 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
CNA 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Affected products

  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 21
  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 22
  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 10
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 11
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 12
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 13
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 14
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 15
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 16
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 17
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 18
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 19
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 8
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 9
  • Microsoft Microsoft Exchange Server 2019
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 1
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 2
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 3
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 4
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 5
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 6
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 7
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 8

Classification

Weakness
  • CWE-918 Server-Side Request Forgery
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.