CISA KEV · Known exploitedHigh

CVE-2021-42321

Microsoft Exchange

CVE-2021-42321 affects Microsoft Exchange. Review the CVSS score, exploitation signals, affected versions, remediation status, and linked source material before making a change.

CVSS
8.8
EPSS
90.4%
99.8% percentile
CISA KEV
Listed
Published
2021.11.10
PRIORITY ASSESSMENT

Immediate review

CISA has listed this vulnerability in the Known Exploited Vulnerabilities catalog.

Known exploitationConfirmed by CISA KEV
Exploit probability90.4%
Technical severityCVSS 8.8

Vulnerability overview

CVE-2021-42321 affects Microsoft Exchange. Review the CVSS score, exploitation signals, affected versions, remediation status, and linked source material before making a change.

Affected product and versions

Product
Microsoft Exchange
Affected versions
15.01.0, 15.02.0, 15.0.0, 2016, 2019
Fixed versions
No verified fixed-version field is available yet

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide
CISA required action

Apply updates per vendor instructions.

Due date: 2021.12.01
  1. 1
    Identify

    Confirm that Microsoft Exchange and an affected version are present.

  2. 2
    Prioritize

    Combine exploitation signals with asset exposure and business criticality.

  3. 3
    Remediate

    Follow the vendor advisory or supported update path and preserve rollback options.

  4. 4
    Verify

    Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CWE
CWE-184, CWE-502
KEV added
2021.11.17
Ransomware use
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