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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 1 Feb 2022.
Exchange ServerApplication · Microsoft

CVE-2021-33766

HIGH · 7.3 CVSS v3.1 Developing Published 2021-07-14
Patch available
A vendor patch is available. No clean upgrade release — apply the published patch.
See remediation →
100/100
Remediation priority · Urgent
In the wild High EPSS Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Microsoft Exchange Server Information Disclosure Vulnerability

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · low confidence
Developing — this CVE was published recently and its authoritative NVD entry is still being established, so we're holding a technical summary until the source data settles rather than publish something unreliable. Re-checking after 2026-08-24.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Exchange ServerApplication
Affected:= 2013= 2016= 2019

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Patch available Apply the vendor patch
Vendor patch portal.msrc.microsoft.com →
Recommended fix High confidence

Exchange Server 2013 CU23 + July 2021 Security Update; Exchange Server 2016 CU21 + July 2021 Security Update; Exchange Server 2019 CU10 + July 2021 Security Update

  1. 1. Identify current Exchange Server version by running: Get-ExchangeServer | Format-List Name,AdminDisplayVersion
  2. 2. Determine the required Cumulative Update (CU) for your version (2013 requires CU23, 2016 requires CU21, 2019 requires CU10 or later)
  3. 3. Download the appropriate Cumulative Update from the Microsoft Update Catalog or Microsoft Download Center
  4. 4. Put the Exchange Server in maintenance mode before updates: . MaintenanceMode -Start
  5. 5. Install the Cumulative Update on the Exchange Server (this may require server restart)
  6. 6. After CU installation, download and install the corresponding Security Update (SU) for CVE-2021-33766 from Microsoft Update Catalog
  7. 7. Verify the update installed successfully: Get-ExchangeServer | Format-List Name,AdminDisplayVersion
  8. 8. Take the server out of maintenance mode: . MaintenanceMode -Stop
Caveat Cumulative Updates for Exchange Server require downtime and may have prerequisites; test thoroughly in non-production environment before applying to production

Generated from the published advisory — verify against the referenced sources before acting.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-33766 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data