SARA / Free Tools / CVE / CVE-2021-34473

CVE-2021-34473 — Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS

9.1 CRITICAL

EPSS

94.27% (top 0.10%)

CWE

—

Published

2021-07-14T17:54:03.000Z

Last modified

2025-10-21T23:25:40.913Z

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C

CISA KEV — Actively Exploited

Added to KEV

2021-11-03

BOD 22-01 due

2021-11-17

Ransomware use

Known

View KEV detail

01What is this vulnerability?

Microsoft Exchange Server Remote Code Execution Vulnerability

Vendor	Product	Versions
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 23	15.00.0
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 9	15.02.0
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 20	15.01.0
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 19	15.01.0
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 8	15.02.0

Yes — actively exploited. Added to the CISA KEV catalog on 2021-11-03. Ransomware use: Known.

Patch to a fixed version listed in the vendor advisory (see references below).
Mitigate with WAF rules, network egress filters, or feature flags where the patch is not yet available.
Hunt historical logs for exploitation indicators — see Detection signatures below.

For the full vendor write-up, exploit chains, and reference implementations, see the references list in section 09.

Open the Sigma generator with a pre-filled prompt for this CVE to draft a starting detection in your stack of choice:

No directly-cited follow-up CVEs in the KB record for this advisory. The references list in section 09 carries the vendor cross-references.

portal.msrc.microsoft.com — https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-3447…
www.zerodayinitiative.com — https://www.zerodayinitiative.com/advisories/ZDI-21-821/
packetstormsecurity.com — http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote…

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