Exploited CVEs / Microsoft / CVE-2021-26858

CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted, and the vulnerability is known to be used in ransomware campaigns. Federal agencies were required to remediate by 2022-05-03 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Microsoft Security Update Guide (MSRC), linked below.

Risk summary

UnratedCISA KEVRansomware

CVSS base

Not scored

EPSS exploit probability

Unrated

Severity

2021-11-03

Added to KEV

2022-05-03 (past due)

CISA due date

Actively exploited Microsoft Exchange Server vulnerability. Added to the CISA KEV catalog 2021-11-03.

Senserva AI read on CVE-2021-26858 CISA required action Official fix for CVE-2021-26858 Admin questions

Senserva AI Opinion for CVE-2021-26858

One of the ProxyLogon-chain Exchange flaws, this remote code execution bug rode alongside the others in widespread attacks and ransomware campaigns. Exchange servers that faced the internet without the 2021 fixes are the concern, since the chain gave attackers full server takeover. Years later it still surfaces on forgotten or shadow-IT Exchange instances.

Do this: Inventory every Exchange server, including decommission candidates, and confirm each carries the updates that close CVE-2021-26858.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-26858

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-26858, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-26858.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2021-26858: Microsoft Exchange Server Remote Code Execution Vulnerability
- Affected: Microsoft Exchange Server
- Severity: Unrated
- Added to CISA KEV: 2021-11-03; federal remediation due 2022-05-03
- Ransomware-linked: YES
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch immediately

Exploited in the wild and used in ransomware campaigns.

The fix ships from the vendor: Microsoft Security Update Guide (MSRC).

Do this now: Move this ahead of everything else in the queue, then verify the fix actually landed on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2021-26858

Published: 2021-11-03
Last revised: 2021-11-03
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

CISA required action

Federal (BOD 22-01) remediation due date: 2022-05-03 (past due).

Official fix for CVE-2021-26858

The authoritative fix and affected-version list are published by the vendor. Find the Exchange Server advisory on Microsoft Security Update Guide (MSRC).

Microsoft Security Update Guide (MSRC)CISA KEV entry

Common questions about CVE-2021-26858

Is CVE-2021-26858 actively exploited?

Yes. CVE-2021-26858 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03, which means exploitation in the wild has been confirmed, not predicted. CISA also reports it is known to be used in ransomware campaigns.

What is the CISA deadline and required action for CVE-2021-26858?

CISA set the federal (BOD 22-01) remediation due date at 2022-05-03, which has passed; treat any unremediated system as overdue.

What fixes CVE-2021-26858?

The authoritative fix is in the vendor advisory: see Microsoft Security Update Guide (MSRC) for the patched Exchange Server versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-26858?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-26858's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited Microsoft CVEs: CVE-2008-4250 CVE-2009-1537 CVE-2010-0249 CVE-2010-0806 CVE-2026-41091 CVE-2026-45498 CVE-2026-33825 CVE-2009-0238
All Microsoft exploited CVEs on one page: Microsoft vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog Microsoft MSRC FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.