On March 2nd, Microsoft has released emergency out-of-band security updates that address four zero-day issues (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065) in all supported Microsoft Exchange versions that are actively exploited in the wild.

The IT giant reported that at least one China-linked APT group, tracked as HAFNIUM, chained these vulnerabilities to access on-premises Exchange servers to access email accounts, and install backdoors to maintain access to victim environments. According to Microsoft, the Hafnium APT exploited these vulnerabilities in targeted attacks against US organizations. 

This week, the independent security researcher Nguyen Jang published on GitHub a proof-of-concept tool to hack Microsoft Exchange servers. The tool chains two of the ProxyLogon vulnerabilities recently addressed by Microsoft.

The availability of the proof-of-concept code was first reported by The Record.

“A Vietnamese security researcher has published today the first functional public proof-of-concept exploit for a group of vulnerabilities in Microsoft Exchange servers known as ProxyLogon, and which have been under heavy exploitation for the past week.” reads the post published by The Record. “The proof-of-concept code was published on GitHub earlier today. A technical write-up (in Vietnamese) is also available on blogging platform Medium.”

The availability of the exploit online was immediately noticed by several cyber security experts, including Marcus Hutchins.

A few hours after the publication, GitHub took down the PoC hacking tool because it posed a threat to Microsoft’s customers using the Microsoft Exchange solution. 

“We understand that the publication and distribution of proof of concept exploit code has educational and research value to the security community, and our goal is to balance that benefit with keeping the broader ecosystem safe,” the spokesperson said in an email sent to the Vice.. “In accordance with our Acceptable Use Policies, we disabled the gist following reports that it contains proof of concept code for a recently disclosed vulnerability that is being actively exploited.”

Expert publishes PoC exploit code for Microsoft Exchange flaws