---
id: "CVE-2021-26858"
kind: "vulnerability"
aliases: ["CVE-2021-26858"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability"
severity: "critical"
cvss: 7.8
epss: 0.93651
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exchange-server", "exploited", "infrastructure", "on-premises-exchange", "ransomware", "windows"]
not_affected_if: ["exchange-online", "microsoft-365-hosted"]
published_at: "2021-03-03T00:15:12+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:34+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-26858", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-26858", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-26858", "first_seen_at": "2026-10-23T21:57:34+00:00"}]
---

# Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server has an unspecified flaw that allows remote code execution, and it is part of the ProxyLogon chain. Attackers are exploiting it in the wild and it is tagged as used in ransomware campaigns.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Microsoft Exchange Server affected ? fixed ?

## Fix

Apply the Microsoft updates for Exchange Server per vendor instructions (CISA KEV required action); the specific build numbers are not in the sources.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-26858
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-26858
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-26858

## From the sources

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

CISA required action: Apply updates per vendor instructions.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_