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Vulnerabilities

Vulnerability · CVE-2021-42321

Microsoft Exchange Server Remote Code Execution Vulnerability

Published Nov 9, 2021 · catalog updated Aug 19, 2026 · Microsoft Exchange Server

Patch now

CVE-2021-42321 is in the CISA known-exploited catalog, due Dec 1, 2021. 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
High8.8
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
91.74%
Remediation
Remediation guidance
Affected products
cpe microsoft/exchange server · vendor Microsoft Exchange · vendor Microsoft Exchange Server

An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

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

How to fix CVE-2021-42321

What to do

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

Should you patch?

CVE-2021-42321 is in the CISA known-exploited catalog, due Dec 1, 2021. 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 2016; microsoft/exchange server 2019. Apply updates per vendor instructions.

What should an analyst verify?

CWE-184; CWE-502: Deserialization of Untrusted Data. EPSS 91.74%. The CISA KEV listing is the exploitation evidence. A public exploit reference is linked in Sources. That is not the same as confirmed exploitation. 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. Nov 9, 2021 · Catalog

    Detected

    An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

  2. Nov 17, 2021 · CISA KEV

    Known exploited

    Apply updates per vendor instructions.

  3. Dec 1, 2021 · CISA KEV

    Remediation due

    Apply updates per vendor instructions.

  4. Aug 19, 2026 · Catalog

    Updated

    An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

Sources

  • CISA KEVConfirmed exploitationCVE-2021-42321

    An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

    cisa.gov
  • CVE.orgVulnerability recordCVE-2021-42321

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2021-42321

    30-day exploitation probability 91.7%

    first.org
  • Metasploit ModulesPublic exploit moduleexploit_windows/http/exchange_chainedserializationbinder_rce

    Public module reference (excellent). Module source is not copied into this catalog.

    rapid7.com
  • NIST NVDVulnerability recordCVE-2021-42321

    Microsoft Exchange Server Remote Code Execution Vulnerability

    nvd.nist.gov
  • Packet Storm SecurityCited referencehttp://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html

    Packet Storm link already published on the finding.

    packetstormsecurity.com
  • Packet Storm SecurityCited referencep://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html

    Packet Storm link already published on the finding.

    packetstormsecurity.com
  • VulnCheck KEVConfirmed exploitationCVE-2021-42321

    An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

    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.