In September 2020, we began investigating a Microsoft Exchange server at a Kuwaiti organization that a threat group compromised as part of a continued xHunt campaign. This investigation resulted in the discovery of two new backdoors called TriFive and Snugy, which we discussed in a prior blog, as well as a new webshell that we call BumbleBee that we will explain in greater detail in this blog. We use this name because the color scheme of the BumbleBee webshell includes white, black and yellow, as seen in Figure 1.
The actor used the BumbleBee webshell to upload and download files to and from the compromised Exchange server, but more importantly, to run commands that the actor used to discover additional systems and to move laterally to other servers on the network. We found BumbleBee hosted on an internal Internet Information Services (IIS) web server on the same network as the compromised Exchange server, as well as on two internal IIS web servers at two other Kuwaiti organizations. As mentioned in our prior xHunt Campaign blog, we still do not know the initial infection vector used to compromise the Exchange server, as this appears to have occurred prior to the logs we were able to collect.
We observed the actor interacting directly with the BumbleBee webshell on the compromised Exchange server of the Kuwaiti organization, as this server was accessible from the internet. The actor used Virtual Private Networks (VPNs) provided by Private Internet Access when directly accessing BumbleBee on internet-accessible servers. The actor would frequently switch between different VPN servers to change the external IP address of the activity that the server would store in the logs. Specifically, the actor changed the IP address to appear to be from different countries, including Belgium, Germany, Ireland, Italy, Luxembourg, the Netherlands, Poland, Portugal, Sweden and the United Kingdom. We believe this is an attempt to evade detection and make analysis of the malicious activities more difficult. We also observed the actor switching between different operating systems and browsers, specifically Mozilla Firefox or Google Chrome on Windows 10, Windows 8.1 or Linux systems. This suggests the actor has access to multiple systems and uses this to make analysis of the activities more difficult, or that there are multiple actors involved, who have differing preferences for operating systems and browsers.
In addition to using VPNs, the actor used SSH tunnels to interact with BumbleBee webshells hosted on internal IIS web servers that are not accessible directly from the internet at all three Kuwaiti organizations. The commands executed on the servers via BumbleBee suggest that the actor used the PuTTY Link (Plink) tool to create SSH tunnels to access services internal to the compromised network. We observed the actor using Plink to create an SSH tunnel for TCP port 3389, which suggests that the actor used the tunnel to access the system using Remote Desktop Protocol (RDP). We also observed the actor creating SSH tunnels to internal servers for TCP port 80, which suggests the actor used the tunnel to access internal IIS web servers. We believe that the actor accessed these additional internal IIS web servers to leverage file uploading functionality in internal web applications to install BumbleBee as a method of lateral movement.