CVE-2021-26858

What is affected

Vendor	Microsoft
Product	Exchange Server

The vulnerability

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.

US federal civilian agencies were required to act by 3 May 2022 under BOD 22-01. The deadline is a compliance date, not a measure of severity — it says nothing about how dangerous the flaw is to anyone else.

Ransomware

CISA records this vulnerability as having been used in ransomware campaigns. That is a confirmed observation, not an assessment of likelihood.

Where this comes from

Every field on this page is taken from the CISA Known Exploited Vulnerabilities catalog, last synced 12 Nov 2026, 00:29. Nothing here is our own assessment. For the technical scoring, see the NVD record below.

NVD record ↗ CVE.org record ↗