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CVE-2021-26858

High CVSS 7.8 Known exploited

Published 2021-03-03 · last changed 2026-08-19 · added to CISA KEV 2021-11-03

This one is being exploited in the wild.

CISA added CVE-2021-26858 to the Known Exploited Vulnerabilities catalog on 2021-11-03. Federal agencies are required to remediate it on a deadline. If you have affected machines, this is the class of finding that moves ahead of the rest of the queue.

The fix

We hold no Microsoft remediation data for CVE-2021-26858, which normally means it is not a Windows CVE. For non-Microsoft software the fix is the vendor's own updated release, and the affected versions are listed below.

What it is

Microsoft Exchange Server Remote Code Execution Vulnerability

Sources

CVSS vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H