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 KEV Check 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

Detection template: Nuclei template listed
29 Oct 2026
Detection template: Nuclei template listed
28 Oct 2026

Page history

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Field-level log (12)

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

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Exploitation and risk

Source status

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CVE-2021-26855

Listed in CISA KEV · added 3 Nov 2021 · 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.

CISA source

No EPSS score is available here for this CVE.

Saved 23 Oct 2026 16:01:26 UTC · View this risk record (JSON)

Risk history

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CVE-2021-26855 · 23 Oct 2026 16:01:26 UTC

Advisory history

1 saved versions

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23 Oct 2026 15:48:27 UTC

Microsoft Exchange Server Remote Code Execution Vulnerability

cve-program · CVE-2021-26855 · Published