---
id: CVE-2021-26855
title: Microsoft Exchange Server Remote Code Execution Vulnerability
summary: Microsoft Exchange Server Remote Code Execution Vulnerability
severity: critical
cvss: 9.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N'
cwe:
  - CWE-918
  - CWE-918
vendor: microsoft
product: exchange_server
affected:
  - exchange_server = 2013
  - exchange_server = 2016
  - exchange_server = 2019
published: '2021-03-03'
updated: '2026-08-12'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2021-26855'
references:
  - url: >-
      http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
    label: secure@microsoft.com
  - url: >-
      http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
    label: secure@microsoft.com
  - url: >-
      http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
    label: secure@microsoft.com
  - url: >-
      http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
    label: secure@microsoft.com
  - url: >-
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
    label: secure@microsoft.com
  - url: >-
      http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - kev
  - in-the-wild
  - exploit-available
epss: 0.99996
epssPercentile: 0.99989
kev: true
kevDateAdded: '2021-11-03'
kevDueDate: '2022-05-03'
kevRansomware: true
exploited: true
exploitAvailable: true
zeroDay: true
ingestedAt: '2026-08-12T19:54:25.643Z'
exploits:
  exploitdb: true
  github: 51
  githubRepos:
    - 'https://github.com/sgnls/exchange-0days-202103'
    - 'https://github.com/soteria-security/HAFNIUM-IOC'
    - 'https://github.com/cert-lv/exchange_webshell_detection'
  metasploit:
    - auxiliary/gather/exchange_proxylogon_collector
    - auxiliary/scanner/http/exchange_proxylogon
    - exploit/windows/http/exchange_proxylogon_rce
  nuclei:
    - CVE-2021-26855
  checkedAt: '2026-09-19T16:22:51.230Z'
---

## Overview

Microsoft Exchange Server Remote Code Execution Vulnerability

## Affected

- `exchange_server = 2013`
- `exchange_server = 2016`
- `exchange_server = 2019`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.