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Security advisories — Aug 24, 2021

UPDATE: PoC Released, Active Exploitation of Exchange Vulnerabilities Observed

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eSentire has observed active exploitation of the ProxyShell vulnerabilities; external sources have confirmed attacks are ongoing. Multiple threat actor groups are exploiting the cluster of Exchange vulnerabilities known as ProxyShell: CVE-2021-31207, CVE-2021-34523, CVE-2021-34473. Exploitation of the ProxyShell vulnerabilities may result in Remote Code Execution (RCE). In observed attacks, threat actors have been identified deploying webshells to unpatched Exchange servers for later access.

Security patches for all three vulnerabilities have been available since May 2021. It is critical that organizations ensure security patches have been deployed. If the relevant security patches have not already been deployed and the vulnerable server is exposed to the Internet, organizations should assume exploitation has occurred.

What we’re doing about it

What you should do about it

Additional information

ProxyShell is comprised of three Exchange vulnerabilities chained together to execute malicious code on vulnerable servers:

The attack was demonstrated at Pwn2own 2021 in April 2021. Microsoft patched the vulnerabilities in their April and May 2021 security updates respectively. CVE-2021-34473 and CVE-2021-34523 were disclosed in July but originally patched in the April 2021 security updates.

Following researcher Orange Tsai’s technical reveal of the vulnerabilities at Black Hat 2021, attacker interest has grown. eSentire security teams observed Proof-of-Concept code posted to GitHub and Metasploit modules in mid-August, greatly facilitating attacks. Widespread exploitation in the wild was observed in external reports ,and eSentire security teams have responded to several incidents at our customers.

Incident Observables

In observed exploitation events, webshells are uploaded to vulnerable Exchange servers via ProxyShell vulnerabilities and remotely accessed via port 443 from the Internet.

Webshell Creation

Webshell Interaction

Aspx Webshell Content

IP Addresses Observed in Exploitation Events:



[1] https://us-cert.cisa.gov/ncas/current-activity/2021/08/21/urgent-protect-against-active-exploitation-proxyshell
[2] https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2021-31207
[3] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34523
[4] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34473
[5] https://www.zerodayinitiative.com/blog/2021/8/17/from-pwn2own-2021-a-new-attack-surface-on-microsoft-exchange-proxyshell
[6] https://twitter.com/DaveKleinatland/status/1429690829166235648

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