Microsoft Exchange Server Remote Code Execution Vulnerability

cisa required action Apply updates per vendor instructions.

1. 2021-04-06 First public exploit (Metasploit exploit/windows/http/exchange_proxyshell_rce)
2. 2021-07-14 Published
3. 2021-11-03 Added to CISA KEV
4. 2021-11-17 CISA patch deadline
5. 2025-02-04 CISA SSVC: exploitation active

Added

5 y ago (2021-11-03)

Patch by

2021-11-17 (4 y ago)

Ransomware

Known to be used in ransomware campaigns

Required action

Apply updates per vendor instructions.

Vendor, product

Microsoft, Exchange Server

CISA name

Microsoft Exchange Server Remote Code Execution Vulnerability

Groups reported to have exploited it in intrusions.

• Conti ransomware gang
• Hive ransomware gang
• Cuba ransomware gang
• AvosLocker ransomware gang
• BlackCat ransomware gang
• Ghost/Cring ransomware gang

reported by news.sophos.comcisa.govsecurelist.comtrendmicro.comtrendmicro.comcisa.gov

Ransomware Vulnerability Matrix by BushidoUK (github.com/BushidoUK/Ransomware-Vulnerability-Matrix), reformatted per CVE. CC BY 4.0

How it is exploited and what it leads to, in MITRE ATT&CK terms.

• T1190 Exploit Public-Facing Application exploitation
• T1053.005 Scheduled Task primary impact
• T1048.003 Exfiltration Over Unencrypted Non-C2 Protocol secondary impact
• T1136 Create Account secondary impact
• T1486 Data Encrypted for Impact secondary impact

kev release 2025-07-28, att&ck v16.1. KEV to ATT&CK mappings by the MITRE Center for Threat-Informed Defense (Mappings Explorer, Apache-2.0). © 2026 The MITRE Corporation. This work is reproduced and distributed with the permission of The MITRE Corporation. Terms of use

Exploitation

Active, exploited in the wild

Automatable

yes

Technical impact

total

Assessed

21 mo ago (2025-02-04)

100.00% chance of exploitation in the next 30 days, higher than 99.9% of all scored CVEs (FIRST EPSS, 2026-11-08).

• Metasploit exploit/windows/http/exchange_proxyshell_rce Microsoft Exchange ProxyShell RCE metasploit exploit, excellent rank, 2021-04-06
• Nuclei template Exchange Server - Remote Code Execution nuclei detection template
• packetstormsecurity.com http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html referenced PoC, tagged "exploit" by a source, unverified

• Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
• Microsoft Microsoft Exchange Server 2016 Cumulative Update 19
• Microsoft Microsoft Exchange Server 2016 Cumulative Update 20
• Microsoft Microsoft Exchange Server 2019 Cumulative Update 8
• Microsoft Microsoft Exchange Server 2019 Cumulative Update 9

Weakness

• CWE-918 Server-Side Request Forgery

Technology feeds

• Microsoft Exchange rss critical rss

• msrc.microsoft.com /update-guide/vulnerability/CVE-2021-34473

  vendor-advisory, patch

• portal.msrc.microsoft.com /en-US/security-guidance/advisory/CVE-2021-34473

  patch, vendor-advisory

• packetstormsecurity.com /files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html

  exploit, third-party-advisory, vdb-entry

• zerodayinitiative.com /advisories/ZDI-21-821/

  third-party-advisory, vdb-entry

• cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34473

  government-resource, us-government-resource

• CVE List CVE-2021-34473
• NVD CVE-2021-34473

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.