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Vulnerabilities

Vulnerability · CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability

Published Mar 2, 2021 · catalog updated Oct 1, 2026 · Microsoft Exchange Server

Patch now

CVE-2021-26858 is in the CISA known-exploited catalog, due May 3, 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
High7.8
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
93.65%
Remediation
Remediation guidance
Affected products
cpe microsoft/exchange server · vendor Microsoft Exchange Server

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

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
  • vendor Microsoft Exchange Serverversion unspecified

How to fix CVE-2021-26858

What to do

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

Should you patch?

CVE-2021-26858 is in the CISA known-exploited catalog, due May 3, 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. Apply updates per vendor instructions.

What should an analyst verify?

No weakness class is stored. EPSS 93.65%. 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. Mar 2, 2021 · Catalog

    Detected

    Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

  2. Nov 3, 2021 · CISA KEV

    Known exploited

    Apply updates per vendor instructions.

  3. May 3, 2022 · CISA KEV

    Remediation due

    Apply updates per vendor instructions.

  4. Oct 1, 2026 · Catalog

    Updated

    Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Sources

  • CISA KEVConfirmed exploitationCVE-2021-26858

    Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

    cisa.gov
  • CVE.orgVulnerability recordCVE-2021-26858

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2021-26858

    30-day exploitation probability 93.7%

    first.org
  • NIST NVDVulnerability recordCVE-2021-26858

    Microsoft Exchange Server Remote Code Execution Vulnerability

    nvd.nist.gov
  • VulnCheck KEVConfirmed exploitationCVE-2021-26858

    Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

    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.