Microsoft has detected multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks. In the attacks observed, threat actors used this vulnerability to access on-premises Exchange servers, which enabled access to email accounts, and install additional malware to facilitate long-term access to victim environments. Microsoft Threat Intelligence Center (MSTIC) attributes this campaign with high confidence to HAFNIUM.

Describes a cumulative update for Exchange Server 2016 that's dated December 15, 2020.