Known exploited · Microsoft
CVE-2021-33766: Microsoft Exchange Server Information Disclosure
What it is
Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.
- Vendor
- Microsoft
- Product
- Exchange Server
- Added to KEV
- January 18, 2022
- CISA due date
- February 1, 2022
- Ransomware use
- Unknown
- Weakness
- CWE-287: Improper Authentication
What CISA requires
Apply updates per vendor instructions.
Forensic triage
No
What to do now
- Find every instance of Exchange Server you run, including the forgotten ones, and check its version against the vendor advisory.
- Apply the vendor's fix or mitigation, before February 1, 2022 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.
References
H2 Security
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Start freeSource: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.