Microsoft named Hafnium as the group responsible for the2021 Microsoft Exchange Server data breach, and alleged they were "state-sponsored and operating out of China".[3][4] According to Microsoft, they are based in China but primarily use United States–basedvirtual private servers,[6] and have targeted "infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks and NGOs".[7]
In July 2021, UK foreign secretaryDominic Raab said the attack had been performed by "Chinese state-backed groups" linked to theMinistry of State Security (MSS).[8][9] The Chinese government has denied responsibility for the 2021 Microsoft breach.[3]
The name "Hafnium" was assigned to the group by Microsoft, which publicly disclosed the group's activity on March 2, 2021. Microsoft described the group as "highly skilled and sophisticated".[10][11] Hafnium is closely connected toAPT40.[5]
Hafnium was linked to the creation of Tarrask, a defense evasion malware used on previous attacks. The malware was used on telecommunications, Internet service providers, and data service companies from August 2021 to February 2022. The malware uses scheduled task abuse to hide payloads delivered to servers.[12]
In July 2025, a hacker allegedly working with Hafnium, Xu Zewei, was arrested in Milan and faces extradition to the United States.[13][14] The same month, Silk Typhoon exploited a security flaw in SharePoint, affecting thousands of servers.[15][16]
In March 2021, it was reported the group had access to theChina Chopper web shell, which it has used in the 2021 Microsoft Exchange Server data breach to control hacked servers.[17][18][8]
