CVE-2021-42321 — Microsoft Exchange Server Remote Code Execution Vulnerability

Executive Summary

None

Overview

8.8

CVSS HIGH

Important

MS Severity

Exploited

MS Exploit Status

Exploitation Detected

MS Exploit Likelihood

Category Remote Code Execution

Released Nov 9 2021

Last Updated Nov 9 2021

Publicly Disclosed No

CISA KEV Listed (added 2021-11-17)

Known Exploits None Known

EPSS Score 0.90388 — 0.9979 percentile

NVD CVSS 8.8 HIGH — matches MSRC

CVSS Vector

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

ATTACK VECTOR

Network

ATTACK COMPLEXITY

Low

PRIVILEGES REQUIRED

Low

USER INTERACTION

None

SCOPE

Unchanged

CONFIDENTIALITY

High

INTEGRITY

High

AVAILABILITY

High

EXPLOIT CODE MATURITY

Unproven

REMEDIATION LEVEL

Official Fix

REPORT CONFIDENCE

Confirmed

Temporal Score: 7.7

EPSS Score

0.90388

probability of exploitation in the next 30 days

0.9979 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

4 affected products

Product	KB Article	Severity	Impact	Restart Required
Microsoft Exchange Server 2016 Cumulative Update 21	`5007409 (Security Update)`	Important	Remote Code Execution	Yes
Microsoft Exchange Server 2016 Cumulative Update 22	`5007409 (Security Update)`	Important	Remote Code Execution	Yes
Microsoft Exchange Server 2019 Cumulative Update 10	`5007409 (Security Update)`	Important	Remote Code Execution	Yes
Microsoft Exchange Server 2019 Cumulative Update 11	`5007409 (Security Update)`	Important	Remote Code Execution	Yes

Patches

1 patch

Article	Type	Restart
`5007409`	Security Update	Yes

Exploits & PoC

3 public PoCs

Unverified third-party code

Public proof-of-concept repositories aggregated from PoC-in-GitHub. They are not reviewed and may be incomplete, non-functional, or malicious — inspect the code before running anything.

Repository	Stars	Published	Description
DarkSprings/CVE-2021-42321	83	2021-11-23	Microsoft Exchange Server Poc
7BitsTeam/exch_CVE-2021-42321	10	2022-10-08
timb-machine-mirrors/testanull-CVE-2021-42321_poc.py	0	2022-01-05	Clone from gist

Detection Rules

Detection available

Community detection & vulnerability-scanning rules aggregated from Sigma and Nuclei templates. Validate and tune to your environment before deploying.

Sigma rules 1

Possible Exploitation of Exchange RCE CVE-2021-42321 high

Acknowledgments

zcgonvh @ 360 noah lab
Yuhao Weng & Zhiniang Peng & Feng Dong with Sangfor
Microsoft Threat Intelligence Center (MSTIC)
Microsoft Security Response Center

References

MSRC Advisory MITRE NVD