---
id: "CVE-2021-33766"
kind: "vulnerability"
aliases: ["CVE-2021-33766"]
title: "Microsoft Exchange Server: Microsoft Exchange Server Information Disclosure"
severity: "high"
cvss: 7.5
epss: 0.98176
exploited: true
status: "open"
category: "vendor"
products: [{"name": "Microsoft Exchange Server", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["email", "exchange-server", "exploited", "on-premises-exchange", "vendor"]
not_affected_if: ["exchange-online", "microsoft-365-cloud-only"]
published_at: "2021-07-14T18:15:10+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-33766", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-33766", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-33766", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# Microsoft Exchange Server: Microsoft Exchange Server Information Disclosure

Microsoft Exchange Server has an information disclosure flaw that lets an unauthenticated attacker read email traffic from the server. It is listed in CISA's known-exploited catalog and has a very high EPSS score, so attacks are happening.

_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, as CISA requires.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-33766
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-33766
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-33766

## From the sources

Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

CISA required action: Apply updates per vendor instructions.

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