In recent weeks, hackers have sought to exploit vulnerabilities that Microsoft has identified in its Exchange software that customers have loaded onto their Exchange Servers. These attacks started with a hacking group known as Hafnium and have since affected Microsoft users globally. Microsoft has also detected a new family of ransomware, known as DearCry, being deployed after the initial compromise of Exchange Servers.

To protect against future attacks, Microsoft is advising organizations to identify and update vulnerable on-premises Exchange Servers, and follow its mitigation and investigation advice without delay.

In early March, four Microsoft Exchange security vulnerabilities were identified. The four critical vulnerabilities are a server-side request forgery (CVE-2021-26855) used to authenticate as the Exchange Server, a unified messaging service (CVE-2021-26857) enabling the running of code as SYSTEM and two post-authentication arbitrary file writing vulnerabilities (CVE-2021-26858 and CVE-2021-27065) which allow attackers to write a file to any path on the server. After exploiting these vulnerabilities, attackers deploy a web shell in order to gain remote control of the server. Web shells can then potentially lead to data theft and additional malicious activity.

Since the initial report of the vulnerabilities, Microsoft has taken swift action and reports that more than 92 per cent of known worldwide Exchange IPs are now patched or mitigated. However, Microsoft acknowledges that patching a system does not necessarily cut off an attacker's access to any particular account. Furthermore, the exploitation of these vulnerabilities may not be felt immediately as attackers are installing web shells as broadly as possible for future exploitation. Organizations should remain extremely vigilant and adhere to Microsoft’s advice in mitigating these attacks.