The company said on Wednesday AEDT the attacks would enable the attackers in question to access email accounts and also allow installation of malware to ensure long-term residence in victims' systems.

"Microsoft Threat Intelligence Centre 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," the company's advisory said.

It said the four vulnerabilities being exploited — CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065 — had all been patched on Tuesday US time.

Two companies, Volexity and Dubex, were credited with reporting different parts of the attack chain.

Four chained zero days are being exploited in the wild against Exchange Server, aka Outlook Web App.

*Patches available now, action required to apply*

Full remote code execution, without authentication. https://t.co/SPBbzT2iY9

— Kevin Beaumont (@GossiTheDog) March 2, 2021

In a blog post, Volexity researchers Josh Grunzweig, Matthew Meltzer, Sean Koessel, Steven Adair and Thomas Lancaster said the attacks appeared to have begun from 6 January onwards.

They said they had detected the attacks due to anomalies spotted on two of their customers' Exchange servers, with masses of data being sent to IP addresses that were not believed to be associated with legitimate users.

In these two cases, the Volexity researchers they had determined the attacker was exploiting CVE-2021-26855, a zero-day server-side request forgery vulnerability.

"This vulnerability has been confirmed to exist within the latest version of Exchange 2016 on a fully patched Windows Server 2016 server," they wrote.

"Volexity also confirmed the vulnerability exists in Exchange 2019, but has not tested against a fully patched version, although it believes they are vulnerable. It should also be noted that is vulnerability does not appear to impact Office 365."