CVE-2021-42321: in the CISA KEV catalogue
Microsoft Exchange Server Remote Code Execution Vulnerability. CISA added CVE-2021-42321 to the Known Exploited Vulnerabilities catalogue on 2021-11-17 with a default due date of 2021-12-01. Known ransomware campaign use: Known. Forensic triage flag: No.
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution. (CISA description)
- Vendor / product
- Microsoft Exchange
- CWE
- CWE-184, CWE-502
- CISA required action
- Apply updates per vendor instructions.
- EPSS
- 92%, percentile 99 (scored 2026-11-03)
EPSS puts about 92 in 100 odds (92%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 99% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.
Vendor and reference links (from CISA)
Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.
Frequently asked questions
Is CVE-2021-42321 in the CISA KEV catalogue?
Yes. CISA added CVE-2021-42321 (Microsoft Exchange Server Remote Code Execution Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2021-11-17. CISA's default due date for it is 2021-12-01.
Is CVE-2021-42321 used in ransomware attacks?
CISA marks known ransomware campaign use as Known for CVE-2021-42321.
What should I do about CVE-2021-42321?
CISA's required action: Apply updates per vendor instructions.
Sources
- CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
- FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
- NVD record for CVE-2021-42321