The FBI this month reportedly issued an alert to its private industry partners, warning that a probable nation-state hacking group had recently compromised the networks of two U.S. municipalities via unpatched, vulnerable Microsoft SharePoint servers.

According to the report, from ZDNet, the flaw the hackers reportedly abused was CVE-2019-0604, a remote code execution bug caused by a failure to check the source markup of an application package. Microsoft began issuing patches for this vulnerability in March 2019.

The culprit reportedly exfiltrated user information, elevated privileges and dropped web shells, enabling them to gain remote access and establish persistence.

The first unnamed municipality had its Active Directory database stolen and its admin credentials compromised. The attackers dropped four web shells onto the server, and went on to execute tools such as Mimikatz, PowerSploit and PSEXEC to further penetrate into the network. However, the actors were apparently detected while still performing reconnaissance.

The other municipality was reportedly attacked in October 2019. In this instance, the attackers were able to move laterally into the network’s DMZ segment, but the activity was detected after malicious command-and-control communications were discovered.

CVE-2019-0604 was a commonly abused vulnerability in 2019, the report notes, citing past Palo Alto research tying certain exploits of the vulnerability to reputed Chinese government-sponsored hacking group APT 27, aka Emissary Panda. In its alert, the FBI did not indicate that this particular APT group was responsible for the attacks on the two municipalities.