Known exploited · Microsoft
CVE-2021-42321: Microsoft Exchange Server Remote Code Execution Vulnerability
What it is
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.
- Vendor
- Microsoft
- Product
- Exchange
- Added to KEV
- November 17, 2021
- CISA due date
- December 1, 2021
- Ransomware use
- Known
What CISA requires
Apply updates per vendor instructions.
Forensic triage
No
What to do now
- Find every instance of Exchange you run, including the forgotten ones, and check its version against the vendor advisory.
- Apply the vendor's fix or mitigation, before December 1, 2021 if you are bound by CISA's deadline, sooner if it faces the internet.
- Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.
- Until every instance is patched, block the attack at the edge: a WAF in blocking mode stops Java deserialization payloads.
References
The Dome
Block it at the edge while you patch
The Dome's WAF runs the OWASP Core Rule Set in blocking mode and stops Java deserialization payloads before they reach your application.
See the DomeT Academy
Train the engineers who fix it
Security engineering on real hosts in the browser, every objective checked on the machine. The first two lessons of Linux 1 are free.
Start freeSource: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.