How severe is CVE-2021-42321?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2021-42321 known to be exploited?

Yes. CVE-2021-42321 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-17), indicating it is being actively exploited. 67 public proof-of-concept repositories are indexed.

Vulnerability in Microsoft Exchange Server

CVE-2021-42321

Microsoft Exchange Server Remote Code Execution Vulnerability

Published 2021-11-10 · last modified 2026-08-19.

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.917 (99.8th percentile), scored 2026-10-15.

High probability of exploitation in the next 30 days — patch promptly. 100th percentile — 99.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.917 · newest: 0.917 · change: 0.000

Affected products

Microsoft Exchange Server — versions 2016, 2019
Microsoft Exchange Server 2016 Cumulative Update 21 — versions 15.01.0
Microsoft Exchange Server 2016 Cumulative Update 22 — versions 15.0.0
Microsoft Exchange Server 2019 Cumulative Update 10 — versions 15.02.0
Microsoft Exchange Server 2019 Cumulative Update 11 — versions 15.02.0

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2021-11-17. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2021-12-01.

Required action: Apply updates per vendor instructions.

Known ransomware campaign use: yes.

Public proof-of-concept exploits

References

af854a3a-2127-422b-91ae-364da2661108 (Patch, Vendor Advisory)
af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
af854a3a-2127-422b-91ae-364da2661108 (Exploit, Third Party Advisory, VDB Entry)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
msrc.microsoft.com/update-guide/vulnerability/CVE-2021-42321 (vendor-advisory, Patch, patch, Vendor Advisory)

Frequently asked questions

What is CVE-2021-42321?: CVE-2021-42321 is a high-severity vulnerability in Microsoft Exchange Server. CVSS score: 8.8/10. Published 2021-11-10.
How severe is CVE-2021-42321?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2021-42321 known to be exploited?: Yes. CVE-2021-42321 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-17), indicating it is being actively exploited. 67 public proof-of-concept repositories are indexed.