Microsoft Corp. today released software updates to plug four critical security holes that attackers have been using to plunder email communications at companies that use its Exchange Server products. The company says all four flaws are being actively exploited as part of a complex attack chain deployed by a previously unidentified Chinese cyber espionage group.

The software giant typically releases security updates on the second Tuesday of each month, but it occasionally deviates from that schedule when addressing active attacks that target newly identified and serious vulnerabilities in its products.

The patches released today fix security problems in Microsoft Exchange Server 2013, 2016 and 2019. Microsoft said its Exchange Online service — basically hosted email for businesses — is not impacted by these flaws.

Microsoft credited researchers at Reston, Va. based Volexity for reporting the attacks. Volexity President Steven Adair told KrebsOnSecurity it first spotted the attacks on Jan. 6, 2021.

Adair said while the exploits used by the group may have taken great skills to develop, they require little technical know-how to use and can give an attacker easy access to all of an organization’s email if their vulnerable Exchange Servers are directly exposed to the Internet.

“These flaws are very easy to exploit,” Adair said. “You don’t need any special knowledge with these exploits. You just show up and say ‘I would like to break in and read all their email.’ That’s all there is to it.”

Microsoft says the flaws are being used by a previously unknown Chinese espionage group that’s been dubbed “Hafnium,” which is known to launch its attacks using hosting companies based in the United States.

“Hafnium primarily targets entities in the United States across a number of industry sectors, including infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks, and NGOs,” Microsoft said. “HAFNIUM has previously compromised victims by exploiting vulnerabilities in internet-facing servers. Once they’ve gained access to a victim network, HAFNIUM typically exfiltrates data to file sharing sites like MEGA.”

According to Microsoft, Hafnium attackers have been observed combining all four zero-day flaws to target organizations running vulnerable Exchange Server products.

CVE-2021-26855 is a “server-side request forgery” (SSRF) flaw, in which a server (in this case, an on-premises Exchange Server) can be tricked into running commands that it should never have been permitted to run, such as authenticating as the Exchange server itself.

The attackers used CVE-2021-26857 to run code of their choice under the “system” account on a targeted Exchange server. The other two zero-day flaws — CVE-2021-26858 and CVE-2021-27065 — could allow an attacker to write a file to any part of the server.

After exploiting these vulnerabilities to gain initial access, Hafnium operators deployed web shells on the compromised server, Microsoft said. Web shells are essentially software backdoors that allow attackers to steal data and perform additional malicious actions that lead to further compromise.

Neither Microsoft nor Volexity is aware of publicly available code that would allow other cybercriminals to exploit these Exchange vulnerabilities. But given that these attackers are in the wild now, it may only be a matter of days before exploit code is publicly available online.

Microsoft stressed that the exploits detailed today were in no way connected to the separate SolarWinds-related attacks. “We continue to see no evidence that the actor behind SolarWinds discovered or exploited any vulnerability in Microsoft products and services,” the company said.