Microsoft disclosed four actively exploited zero-day vulnerabilities being used to attack on-premises versions of Microsoft Exchange Server. The vulnerabilities identified are CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065, all of which affect Microsoft Exchange Server. Exchange Online is not affected. In the attacks 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. Microsoft Threat Intelligence Center (MSTIC) attributes this campaign with high confidence to HAFNIUM, a group assessed to be state-sponsored and operating out of China, based on observed victimology, tactics and procedures.

Rejetto HTTP File Server, up to and including version 2.3m, is vulnerable to a template injection vulnerability. This vulnerability allows a remote, unauthenticated attacker to execute arbitrary commands on the affected system by sending a specially crafted HTTP request. As of the CVE assignment date, Rejetto HFS 2.3m is no longer supported.