Updated January 18, 2022. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Exploited CVEs / Microsoft / CVE-2021-33766

CVE-2021-33766

Microsoft Exchange Server Information Disclosure. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2022-02-01 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Microsoft Security Update Guide (MSRC), linked below.

Risk summary

UnratedCISA KEV

CVSS v3 base

Not scored

EPSS exploit probability

Unrated

Severity

2022-01-18

Added to KEV

2022-02-01 (past due)

CISA due date

Actively exploited Microsoft Exchange Server vulnerability. Added to the CISA KEV catalog 2022-01-18.

Senserva AI read on CVE-2021-33766 CISA required action Official fix for CVE-2021-33766 Admin questions

Senserva AI Opinion for CVE-2021-33766

An unauthenticated attacker can use this Microsoft Exchange Server flaw to steal email traffic from a target, no credentials required. Mail content is exactly what attackers want for fraud, credential harvesting, and reconnaissance, and CISA's KEV listing confirms it was exploited. On-premises Exchange remains one of the most attacked server products anywhere.

Do this: Bring on-premises Exchange servers fully current on security updates and review web server logs from the exposure window.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-33766

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-33766, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-33766.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (morning and afternoon US Central)):
- CVE-2021-33766: Microsoft Exchange Server Information Disclosure
- Affected: Microsoft Exchange Server
- Severity: Unrated
- Added to CISA KEV: 2022-01-18; federal remediation due 2022-02-01
- Ransomware-linked: no
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Full tracking and change history for CVE-2021-33766

Published: 2022-01-18
Last revised: 2022-01-18
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

CISA required action

Federal remediation due date: 2022-02-01 (past due).

Official fix for CVE-2021-33766

The authoritative fix and affected-version list are published by the vendor. Find the Exchange Server advisory on Microsoft Security Update Guide (MSRC).

Microsoft Security Update Guide (MSRC)CISA KEV entry

Common questions about CVE-2021-33766

Is CVE-2021-33766 actively exploited?

Yes. CVE-2021-33766 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2022-01-18, which means exploitation in the wild has been confirmed, not predicted.

What is the CISA deadline and required action for CVE-2021-33766?

CISA set the federal remediation due date at 2022-02-01, which has passed; treat any unremediated system as overdue.

What fixes CVE-2021-33766?

The authoritative fix is in the vendor advisory: see Microsoft Security Update Guide (MSRC) for the patched Exchange Server versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-33766?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-33766's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (morning and afternoon US Central).

Scope and sources

More exploited Microsoft CVEs: CVE-2019-1068 CVE-2008-4250 CVE-2009-1537 CVE-2010-0249 CVE-2010-0806 CVE-2026-41091 CVE-2026-45498 CVE-2026-33825
All Microsoft exploited CVEs on one page: Microsoft vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog Microsoft MSRC FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.