---
id: "CVE-2021-26855"
kind: "vulnerability"
aliases: ["CVE-2021-26855"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability"
severity: "critical"
cvss: 9.8
epss: 0.99996
exploited: true
status: "open"
category: "vendor"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exchange-server", "exploited", "on-premises", "ransomware", "vendor", "windows"]
not_affected_if: ["exchange-online", "microsoft-365"]
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-2021-26855", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-26855", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-26855", "first_seen_at": "2026-10-23T21:57:34+00:00"}]
---

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

Microsoft Exchange Server has a remote code execution flaw, part of the ProxyLogon chain, that is actively exploited in the wild and tied to ransomware use. An unauthenticated attacker who reaches the Exchange server over the network can run code on it.

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

## Affected

- vendor:Microsoft Exchange Server affected ? fixed ?

## Fix

Apply Microsoft's updates for CVE-2021-26855 per vendor instructions (CISA required action); the specific patched 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-2021-26855
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-26855
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-26855

## 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_