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Vulnerabilities

Vulnerability · CVE-2021-33766

Microsoft Exchange Server Information Disclosure

Published Jul 14, 2021 · catalog updated Aug 10, 2026 · Microsoft Exchange Server

Patch now

CVE-2021-33766 is in the CISA known-exploited catalog, due Feb 1, 2022. Confirm Microsoft Exchange Server against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Gaps on this record

  • No version range is stored. A customer cannot check an installed version against this record.
Severity
High7.3
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
98.18%
Remediation
Remediation guidance
Affected products
cpe microsoft/exchange server · vendor Microsoft Exchange Server

Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target. Initial-access exploit intelligence.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe microsoft/exchange server2013
  • cpe microsoft/exchange server2016
  • cpe microsoft/exchange server2019
  • vendor Microsoft Exchange Serverversion unspecified

How to fix CVE-2021-33766

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

CVE-2021-33766 is in the CISA known-exploited catalog, due Feb 1, 2022. Confirm Microsoft Exchange Server against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Which versions are affected?

Compare what you have installed with microsoft/exchange server 2013; microsoft/exchange server 2016; microsoft/exchange server 2019. Apply updates per vendor instructions.

What should an analyst verify?

CWE-287: Improper Authentication. EPSS 98.18%. The CISA KEV listing is the exploitation evidence. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Jul 14, 2021 · Catalog

    Detected

    Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

  2. Jan 18, 2022 · CISA KEV

    Known exploited

    Apply updates per vendor instructions.

  3. Feb 1, 2022 · CISA KEV

    Remediation due

    Apply updates per vendor instructions.

  4. Aug 10, 2026 · Catalog

    Updated

    Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

Sources

  • CISA KEVConfirmed exploitationCVE-2021-33766

    Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

    cisa.gov
  • CVE.orgVulnerability recordCVE-2021-33766

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2021-33766

    30-day exploitation probability 98.2%

    first.org
  • NIST NVDVulnerability recordCVE-2021-33766

    Microsoft Exchange Server Information Disclosure Vulnerability

    nvd.nist.gov
  • VulnCheck KEVConfirmed exploitationCVE-2021-33766

    Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

    vulncheck.com

References

Freshness

Last verified in this catalog Oct 17, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.