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. The announcement and fixes came a week ahead of the company's regular monthly updates.

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."

Commenting on the announcement, Satnam Narang, staff research engineer at security shop Tenable, said: "The fact that Microsoft chose to patch these flaws out-of-band rather than include them as part of next week's Patch Tuesday release leads us to believe the flaws are quite severe, even if we don't know the full scope of those attacks.

"While Microsoft says HAFNIUM primarily targets entities within the US, other researchers say they have seen these vulnerabilities being exploited by different threat actors targeting other regions.

"Based on what we know so far, exploitation of one of the four vulnerabilities requires no authentication whatsoever and can be used to potentially download messages from a targeted user's mailbox.

"The other vulnerabilities can be chained together by a determined threat actor to facilitate a further compromise of the targeted organisation's network.

"We expect other threat actors to begin leveraging these vulnerabilities in the coming days and weeks, which is why it is critically important for organisations that use Exchange Server to apply these patches immediately."