CVE-2021-34473
Microsoft Exchange Server Remote Code Execution Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 17 November 2021). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
- CVSS 3.1
- 9.1 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- EPSS
- 100.00% probability · 100th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 3 November 2021 · due 17 November 2021 · used in ransomware campaigns
- Weakness
- CWE-918
- Affected
- microsoft/exchange server
- Source
- secure@microsoft.com
EPSS 100.0% since 26 October 2026; no change recorded yet. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
CISA notes
Apply updates per vendor instructions. https://nvd.nist.gov/vuln/detail/CVE-2021-34473
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34473Patch, Vendor Advisory
- http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.htmlExploit, Third Party Advisory, VDB Entry
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34473Patch, Vendor Advisory
- https://www.zerodayinitiative.com/advisories/ZDI-21-821/Third Party Advisory, VDB Entry
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34473US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.