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CVE-2021-26855CISA KEV

Microsoft Exchange Server Remote Code Execution Vulnerability

Severity unknownCVSS unknownCheck the vendor advisory for fixed versions

CISA lists this Microsoft vulnerability as known exploited. Its MSRC release detail is outside this installation's collected history.

CISA KEVCheck MSRC

CISA catalog record

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

Product
Exchange Server
Added to KEV
3 Nov 2021
Federal remediation due
3 May 2022
Known ransomware use
Yes
Public detection template
PublicNuclei template · template membership does not establish exploit availability or applicability to your systems.

What changed

  1. Detection template: Nuclei template listed
  2. Detection template: Nuclei template listed
Page history

Changes are dated when recorded; day views use the last saved version from that UTC day, and citations stay fixed.

Cite this version
Field-level log (12)

Up to 100 recent field edits are shown; older edits may be outside retention.

  1. 13 Nov 2026 · 12:10:20 UTC Source coverage: public-exploit-codeIncomplete → Current
  2. 13 Nov 2026 · 11:07:20 UTC Source coverage: public-exploit-codeCurrent → Incomplete
  3. 6 Nov 2026 · 17:32:22 UTC Source coverage: public-exploit-codeIncomplete → Current
  4. 6 Nov 2026 · 16:31:02 UTC Source coverage: public-exploit-codeCurrent → Incomplete
  5. 2 Nov 2026 · 17:30:39 UTC Source coverage: public-exploit-codeIncomplete → Current
  6. 2 Nov 2026 · 16:27:39 UTC Source coverage: public-exploit-codeCurrent → Incomplete
  7. 29 Oct 2026 · 20:19:26 UTC Public detection templateNuclei template listed
  8. 28 Oct 2026 · 05:10:29 UTC Public exploit codeNuclei template listed
  9. 28 Oct 2026 · 05:10:29 UTC Source coverage: public-exploit-codeCurrent
  10. 28 Oct 2026 · 05:10:29 UTC Link: Nuclei templatehttps://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-26855.yaml
  11. 23 Oct 2026 · 09:33:49 UTC Source coverage: cisa-kevIncomplete → Current
  12. 23 Oct 2026 · 08:31:50 UTC Source coverage: cisa-kevCurrent → Incomplete

Explore global source statements, affected packages and advisory history for CVE-2021-26855. The catalog has a separate collection scope from Microsoft release records.

More advisories, affected versions and source history

Related advisories and sources

Publishers link these advisories through the identifiers shown below. A shared identifier or date alone does not tell you whether your software is affected or whether the advisories describe the same cause.

CVE-2021-26855 · CVE overview and exploitation status

cve-program · CVE-2021-26855

Related changes

Linked by CVE identifier
ChangeIntel

An IT change radar: releases, security, known issues, retirements, documentation changes, and service status from public sources. Every item links to supporting evidence; dates and statuses can change after they are read.

Sources read 13 Nov 18:29 UTC · 274 of 275 readable · 273 fully read · documentation 469/469 current

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