Date: 24 August 2021
This Alert is intended for IT professionals and managers of notified organizations.
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.
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.
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:
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.
The Cyber Centre strongly encourages organizations to immediately apply the outstanding updates for these vulnerabilities and check for signs of anomalous activity.
 Windows KB Article – CVE-2021-34473
 Windows KB Article – CVE-2021-34523
 Windows KB Article – CVE-2021-31207
 Microsoft Security Advisory – April 2021 Monthly Rollup (Cyber Centre Advisory AV21-157)
 Microsoft Security Advisory – May 2021 Monthly Rollup (Cyber Centre Advisory AV21-219)
 Microsoft Exchange Team Blog - May 2021 Exchange Server Security Updates
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.
(The English version precedes)