CVE-2021-33766 Microsoft Exchange Server Information Disclosure
Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.
From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.
Should You Worry?
- CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 18 Jan 2022.
- Rated High severity (CVSS 7.3, scored by Vendor).
- This product type is often exposed to the internet, which is where attackers scan for it first.
- It has been in the catalogue for 1,753 days. Systems still unpatched now have almost certainly been scanned for it.
What It Is
Microsoft Exchange Server Information Disclosure Vulnerability
Affected Versions
| Product | Versions |
|---|---|
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 to before 15.00.1497.015 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.0 to before 15.01.2176.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 20 | 15.01.0 to before 15.01.2242.008 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0 to before 15.02.0792.013 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 9 | 15.02.0 to before 15.02.0858.010 |
From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.
What To Do Now
- 1
Find it
Search your asset inventory and external attack surface for Microsoft Exchange Server. Include cloud images, appliances and anything a supplier runs for you.
Attack Surface Management - 2
- 3
Hunt for compromise
If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.
Incident Response - 4
Prove it is fixed
Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.
Penetration Testing
Vendor Advisories
The Compliance Clock
General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.
Not Sure If You Are Exposed?
Our attack surface and vulnerability teams can check your internet-facing estate for CVE-2021-33766 and every other exploited flaw on the list.