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Fencer Vulns
medium 6.6

CVE-2021-31207 kev+ ransomware in the critical feed json

Microsoft Exchange Server Security Feature Bypass Vulnerability

published 2021-05-11 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-18120

cvss
6.6
version 3.1, CNA
cisa kev
Exploited
added 2021-11-03, used in ransomware
epss
99.8%
EPSS 99.8% chance of exploitation in 30 days, percentile 99.9%
public exploits
1
Metasploit

Summary

Microsoft Exchange Server Security Feature Bypass Vulnerability

Patch / fix

cisa required action Apply updates per vendor instructions.

  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 15.00.1497.018
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 9 15.02.0858.012
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 20 15.01.2242.010
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 15.01.2176.014
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 15.02.0792.015

Timeline

  1. First public exploit (Metasploit exploit/windows/http/exchange_proxyshell_rce)
  2. Published
  3. Added to CISA KEV
  4. CISA patch deadline
  5. CISA SSVC: exploitation active

Known exploited

Added
5 y ago
Patch by
2021-11-17 (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 Security Feature Bypass Vulnerability

Ransomware

Groups reported to have exploited it in intrusions.

  • Conti ransomware gang
  • Hive ransomware gang
  • Cuba ransomware gang
  • AvosLocker ransomware gang
  • BlackCat ransomware gang
  • Ghost/Cring ransomware gang

reported by news.sophos.comcisa.govsecurelist.comtrendmicro.comtrendmicro.comcisa.gov

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.

  • T1548.002 Bypass User Account Control exploitation
  • T1565 Data Manipulation primary 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
no
Technical impact
total
Assessed
21 mo ago

Exploit prediction

99.78% 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 6.6 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
CNA 3.1 6.6 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 6.5 AV:N/AC:L/Au:S/C:P/I:P/A:P

Affected products

  • Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 9
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 20
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 19
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 8

Classification

Weakness
  • CWE-434 Unrestricted Upload of File with Dangerous Type
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.