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. 66 public proof-of-concept repositories are indexed.

Vulnerability in Microsoft Exchange Server 2016 Cumulative Update 21

CVE-2021-42321

Microsoft Exchange Server Remote Code Execution Vulnerability

EPSS: 0.936 (99.8th percentile) — read the EPSS interpretation.

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/E:U/RL:O/RC:C.

Affected products

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

portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321 (x_refsource_MISC)
packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Exec… (x_refsource_MISC)
packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializa… (x_refsource_MISC)

Frequently asked questions

What is CVE-2021-42321?: CVE-2021-42321 is a high-severity vulnerability in Microsoft Exchange Server 2016 Cumulative Update 21. 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. 66 public proof-of-concept repositories are indexed.