---
id: "CVE-2021-26857"
kind: "vulnerability"
aliases: ["CVE-2021-26857"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability"
severity: "critical"
cvss: 7.8
epss: 0.95762
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exchange-server", "exploited", "infrastructure", "ransomware", "self-hosted", "windows"]
not_affected_if: ["exchange-online"]
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-2021-26857", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-26857", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-26857", "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 in its code that allows remote code execution, and it is part of the ProxyLogon chain. Attackers can run code on the Exchange server, which is a known ransomware target, and it is actively exploited in the wild.

_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 this CVE per vendor instructions (CISA required action). Exact patched build numbers: unknown from sources.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-26857
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-26857
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-26857

## 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_