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Fencer Intel
high 8.8

CVE-2021-42321 kev+ ransomware in the critical feed json

Microsoft Exchange Server Remote Code Execution Vulnerability

published 5y ago modified 2mo ago also EUVD-2021-29296

cvss
8.8
version 3.1, CNA
cisa kev
Exploited
added 2021-11-17, used in ransomware
epss
91.7%
EPSS 91.7% chance of exploitation in 30 days, percentile 99.8%
public exploits
1
Metasploit

Summary

Microsoft Exchange Server Remote Code Execution Vulnerability

Patch / fix

cisa required action Apply updates per vendor instructions.

  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 21 15.01.2308.020
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 22 15.01.2375.017
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 10 15.02.0792.019
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 11 15.02.0986.014

Is CVE-2021-42321 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Timeline

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

Known exploited

Added
4y ago
Patch by
2021-12-01 (4y ago)
Ransomware
Known to be used in ransomware campaigns
Required action
Apply updates per vendor instructions.
Vendor, product
Microsoft, Exchange
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
2021-11-09
Details
project zero tracker

ATT&CK techniques

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

  • T1078 Valid Accounts exploitation
  • T1059 Command and Scripting Interpreter 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
2y ago

Exploit prediction

91.74% chance of exploitation in the next 30 days, higher than 99.8% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

CVSS scores

assigned by version score vector
CNA 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
CNA 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:L/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 2016 Cumulative Update 21
  • Microsoft Microsoft Exchange Server 2016 Cumulative Update 22
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 10
  • Microsoft Microsoft Exchange Server 2019 Cumulative Update 11

Classification

Technology feeds

References

Sources

Fencer Intel 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.