How severe is CVE-2021-33766?

High severity. CVSS v3 base score is 7.3 out of 10.

Is CVE-2021-33766 known to be exploited?

Yes. CVE-2021-33766 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-01-18), indicating it is being actively exploited. 19 public proof-of-concept repositories are indexed.

Vulnerability in Microsoft Exchange Server

CVE-2021-33766

Microsoft Exchange Server Information Disclosure Vulnerability

Published 2021-07-14 · last modified 2026-08-10.

CVSS v3 metric

CVSS v3 base score 7.3 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L.

EPSS exploit prediction

EPSS: 0.982 (99.9th percentile), scored 2026-10-18.

Very high probability of exploitation in the next 30 days — among the top 0.1% most-likely-to-be-exploited CVEs. 100th percentile — 99.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.981 · newest: 0.982 · change: +0.000

Affected products

Microsoft Exchange Server — versions 2013, 2016, 2019
Microsoft Exchange Server 2013 Cumulative Update 23 — versions 15.00.0
Microsoft Exchange Server 2016 Cumulative Update 19 — versions 15.01.0
Microsoft Exchange Server 2016 Cumulative Update 20 — versions 15.01.0
Microsoft Exchange Server 2019 Cumulative Update 8 — versions 15.02.0
Microsoft Exchange Server 2019 Cumulative Update 9 — versions 15.02.0

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2022-01-18. 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: 2022-02-01.

Required action: Apply updates per vendor instructions.

Public proof-of-concept exploits

References

af854a3a-2127-422b-91ae-364da2661108 (Patch, Vendor Advisory)
af854a3a-2127-422b-91ae-364da2661108 (Third Party Advisory, VDB Entry)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
msrc.microsoft.com/update-guide/vulnerability/CVE-2021-33766 (vendor-advisory, Patch, patch, Vendor Advisory)

Frequently asked questions

What is CVE-2021-33766?: CVE-2021-33766 is a high-severity vulnerability in Microsoft Exchange Server. CVSS score: 7.3/10. Published 2021-07-14.
How severe is CVE-2021-33766?: High severity. CVSS v3 base score is 7.3 out of 10.
Is CVE-2021-33766 known to be exploited?: Yes. CVE-2021-33766 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-01-18), indicating it is being actively exploited. 19 public proof-of-concept repositories are indexed.