CVE Watch—

CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS

—

No CVSS

EPSS

89.5%

p100

KEV

YES

Nov 3, 2021

Exploit Today

0-100

Published: — · Last modified: —

EPSS · 30d

89.5%EPSS · 30 days89.5%

2026-06-302026-07-16

KEV catalog record

Product

Microsoft / Exchange Server

Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Added to KEV

Nov 3, 2021

Remediate by

May 3, 2022

Known ransomware use

Yes

Summary description

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

Required action

Apply updates per vendor instructions.

Notes

Reference CISA's ED 21-02 (https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities) for further guidance and requirements. Note: The due date for addressing this vulnerability aligns with the requirements outlined in ED 21-02. https://nvd.nist.gov/vuln/detail/CVE-2021-26858

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