CVE-2021-34523

Microsoft Exchange Server Elevation of Privilege Vulnerability

First PoC 2026-04-22
same day →Metasploit 2026-04-22
178 days →First cited attacker 2026-10-17

Exploited in the wild: Yesadded 2021-11-03 · due 2021-11-17 · known ransomware use. An observation, not a forecast.
EPSS · 30-day forecast: 100.0%Chance of exploitation in the next 30 days, 100th percentile of all CVEs. A forecast; KEV outranks it.
CVSS · NVD · CRITICAL: 9.0CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
Internet exposure: not stated by sourcesNo exposure census on this CVE's dispatches.

Exploits

1 known · links only

Metasploit1

Microsoft Exchange ProxyShell RCE ↗
This module exploits a vulnerability on Microsoft Exchange Server that allows an attacker to bypass the authentication (CVE-2021-31207), impersonate an arbitrary user (CVE-2021-34523) and write an arbitrary file (CVE-2021-34473) to achieve the RCE (Remote Code Execution). By taking advantage of this vulnerability, you can execute arbitrary commands on the remote Microsoft Exchange Server. This vulnerability affects Exchange 2013 CU23
ruby · seen 2026-10-18

On the wire

0 dispatches

No dispatch on our wire has named this CVE yet.

Cited as exploited by

blackbyte
1. “BlackByte ransomware threat actors exploit ProxyShell vulnerabilities (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207) found in Microsoft Exchange Server to gain initial access to the target network.”
  cited as exploiting · source: picussecurity.com ↗ · 2026-10-17
2. “In recent BlackByte attacks investigated by Symantec, the attackers exploited the ProxyShell ( CVE-2021-34473 , CVE-2021-34523 , and CVE-2021-31207 ) and ProxyLogon ( CVE-2021-26855 and CVE-2021-27065 ) vulnerabilities in Microsoft Exchange Servers to gain initial access.”
  cited as exploiting · source: security.com ↗ · 2026-10-17
APT35
1. “Magic Hound has exploited the Log4j utility (CVE-2021-44228), on-premises MS Exchange servers via "ProxyShell" (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207), and Fortios SSL VPNs (CVE-2018-13379).”
  cited as exploiting · source: attack.mitre.org ↗ · 2026-10-17
Witchetty
1. “the threat actors first gain initial access to a network by exploiting the Microsoft Exchange ProxyShell ( CVE-2021-34473 , CVE-2021-34523, and CVE-2021-31207) and ProxyLogon ( CVE-2021-26855 and CVE-2021-27065) attack chains.”
  cited as exploiting · named nearby · source: rewterz.com ↗ · 2026-10-17
2. “The attackers exploited the ProxyShell ( CVE-2021-34473 , CVE-2021-34523 , and CVE-2021-31207 ) and ProxyLogon ( CVE-2021-26855 and CVE-2021-27065 ) vulnerabilities to install web shells on public-facing servers”
  cited as exploiting · named nearby · source: symantec-enterprise-blogs.security.com ↗ · 2026-10-17
Worok
1. “During the ProxyShell ( CVE-2021-34523 ) vulnerability disclosure in early 2021, we observed activity from various APT groups . One exhibited characteristics common with TA428 :”
  cited as exploiting · named nearby · source: welivesecurity.com ↗ · 2026-10-17

Each actor is cited by the quoted source, linked under the quote. A citation is that source's claim, not our confirmation.

References

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Signals

Exploits

Metasploit1

On the wire

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