---
id: "CVE-2021-31207"
kind: "vulnerability"
aliases: ["CVE-2021-31207"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Security Feature Bypass Vulnerability"
severity: "medium"
cvss: 6.6
epss: 0.99782
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exchange-server", "exploited", "infrastructure", "on-premises", "ransomware", "windows"]
not_affected_if: ["exchange-online", "microsoft-365"]
published_at: "2021-05-11T19:15:10+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-2021-31207", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-31207", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-31207", "first_seen_at": "2026-10-23T21:57:34+00:00"}]
---

# Microsoft Exchange Server: Microsoft Exchange Server Security Feature Bypass Vulnerability

Microsoft Exchange Server has a security feature bypass flaw that CISA lists as exploited in the wild, and ransomware use is tagged. The sources do not describe exactly what the bypass grants, so the worst case below is inferred from the ProxyLogon-era chain it belongs to, not stated.

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

## Affected

- vendor:Microsoft Exchange Server affected ? fixed ?

## How to check

Check the installed Exchange build version against Microsoft's security update list for CVE-2021-31207 (sources do not give a command).

## Fix

Apply the Microsoft Exchange Server security update for CVE-2021-31207 per vendor instructions (CISA required action); exact fixed build unknown from sources.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-31207
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-31207
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-31207

## From the sources

Microsoft Exchange Server contains an unspecified vulnerability that allows for security feature bypass.

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_