In a rare sharing of information about vulnerabilities in a blog post, Microsoft this week urged customers to download software patches to Microsoft Exchange Server after it detected “multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks.”

According to Microsoft’s Threat Intelligence Center, “[W]e are sharing this information with our customers and the security community to emphasize the critical nature of these vulnerabilities and the importance of patching all affected systems immediately.” In the attacks Microsoft has observed, “the threat actor used these vulnerabilities to access on-premises Exchange servers which enabled access to email accounts, and allowed installation of additional malware to facilitate long-term access to victim environments.”

According to the blog post, the vulnerabilities being exploited were from state sponsored actors operating out of China.

The vulnerabilities being exploited were CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065. Microsoft issued a patch, which can be accessed here.


The post includes information on the threat actor, HAFNIUM, which has been behind numerous malicious exploits against “infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs.”

The vulnerabilities detected by Microsoft affect Microsoft Exchange Server 2013, 2016, and 2019. If your company is running any of these versions, please consult Microsoft’s instructions on patching.