AlertsOngoing Exploitation of ProxyShell Exploit Chain

Number: AL21-017
Date: 24 August 2021

AUDIENCE

This Alert is intended for IT professionals and managers of notified organizations.

PURPOSE

An Alert is used to raise awareness of a recently identified cyber threat that may impact cyber information assets, and to provide additional detection and mitigation advice to recipients. The Canadian Centre for Cyber Security ("Cyber Centre") is also available to provide additional assistance regarding the content of this Alert to recipients as requested.

OVERVIEW

The Cyber Centre is aware of ongoing scanning and exploitation of the ProxyShell exploit chain affecting unpatched Microsoft Exchange Servers. Microsoft has previously released updates to remediate these vulnerabilities in April and May of 2021.

DETAILS

The Cyber Centre is aware of ongoing scanning and exploitation of the ProxyShell exploit chain affecting unpatched Microsoft Exchange Servers. There are three vulnerabilities involved in the ProxyShell exploit chain:

  • CVE-2021-34473 [1]
  • CVE-2021-34523 [2]
  • CVE-2021-31207 [3]

Microsoft released updates for these vulnerabilities as part of their April 2021 and May 2021 monthly security updates [4][5][6].

ProxyShell exploits a vulnerability in the Microsoft Exchange Autodiscover service to access an arbitrary backend URL leading to access as a privileged user on the system. A malicious actor can leverage ProxyShell to gain full control of the affected system.

The Cyber Centre is aware of publicly available exploit code.

MITIGATION

Microsoft released updates for these vulnerabilities as part of their April 2021 and May 2021 monthly security updates [4][5][6].

The Cyber Centre strongly encourages organizations to immediately apply the outstanding updates for these vulnerabilities and check for signs of anomalous activity.

REFERENCES

[1] Windows KB Article – CVE-2021-34473
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34473

[2] Windows KB Article – CVE-2021-34523
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34523  

[3] Windows KB Article – CVE-2021-31207
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-31207  

[4] Microsoft Security Advisory – April 2021 Monthly Rollup (Cyber Centre Advisory AV21-157)
https://cyber.gc.ca/en/alerts/microsoft-security-advisory-april-2021-monthly-rollup

[5] Microsoft Security Advisory – May 2021 Monthly Rollup (Cyber Centre Advisory AV21-219)
https://cyber.gc.ca/en/alerts/microsoft-security-advisory-may-2021-monthly-rollup

[6] Microsoft Exchange Team Blog - May 2021 Exchange Server Security Updates
https://techcommunity.microsoft.com/t5/exchange-team-blog/released-may-2021-exchange-server-security-updates/ba-p/2335209  

 

NOTE TO READERS

The Canadian Centre for Cyber Security (Cyber Centre) operates as part of the Communications Security Establishment. We are Canada's national authority on cyber security and we lead the government's response to cyber security events. As Canada's national computer security incident response team, the Cyber Centre works in close collaboration with government departments, critical infrastructure, Canadian businesses and international partners to prepare for, respond to, mitigate, and recover from cyber events. We do this by providing authoritative advice and support, and coordinating information sharing and incident response. The Cyber Centre is outward-facing, welcoming partnerships that help build a stronger, more resilient cyber space in Canada.
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