Microsoft has urged its customers to patch their on-premises Exchange servers as soon as possible, as unpatched servers may provide a way for malicious actors to breach an organization’s network.

“Attackers looking to exploit unpatched Exchange servers are not going to go away. There are too many aspects of unpatched on-premises Exchange environments that are valuable to bad actors looking to exfiltrate data or commit other malicious acts,” the company explained in a blog post. “First, user mailboxes often contain critical and sensitive data. Second, every Exchange server contains a copy of the company address book, which provides a lot of information that is useful for social engineering attacks, including organizational structure, titles, contact info, and more. And third, Exchange has deep hooks into and permissions within Active Directory, and in a hybrid environment, access to the connected cloud environment.”

To defend servers against attacks exploiting known vulnerabilities administrators are advised to install the latest supported Cumulative Updates (CU12 for Exchange Server 2019, CU23 for Exchange Server 2016, and CU23 for Exchange Server 2013, January 2023 SU).

According to a recent research, Microsoft Exchange vulnerabilities top the list of the most commonly exploited flaws by threat actors. These include CVE-2021-31206, the infamous “ProxyShell” (CVE-2021-34523, CVE-2021-31207 and CVE-2021-34473), and “ProxyLogon” vulnerabilities. Furthermore, in early January researchers reported that nearly 60,000 Exchange servers have been found to be vulnerable to a pair of Exchange flaws collectively nicknamed “ProxyNotShell.”

Vulnerable Exchange servers are valuable targets for cybercriminals, who often use compromised servers as a way to break into organizations’ networks. For instance, FIN7, a well-known, financially motivated group focused on targeting businesses worldwide, developed an auto-attack system, which scans for multiple Microsoft Exchange remote code execution and privilege elevation vulnerabilities like CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207.