---
id: "CVE-2021-34473"
kind: "vulnerability"
aliases: ["CVE-2021-34473"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability"
severity: "critical"
cvss: 10.0
epss: 0.99999
exploited: true
status: "open"
category: "package"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exchange-server", "exploited", "package", "ransomware", "self-hosted", "windows"]
not_affected_if: ["exchange-online", "microsoft-365-cloud"]
published_at: "2021-07-14T18:15:11+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-2021-34473", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-34473", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-34473", "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 (ProxyShell family, CVE-2021-34473) that is actively exploited, with ransomware use noted in the sources. An unauthenticated attacker on the internet can run code on the mail server.

_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 required action); exact patch build is unknown from the sources.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-34473
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-34473
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-34473

## From the sources

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution.

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_