Proof of Concept Reveals PlugX Trojan Intrusion of 6+ Years

Patient Zero

eSentire was conducting a proof of concept (POC) with a potential customer. We installed an eSentire MDR for Network sensor, which immediately triggered an alert in our Security Operations Center (SOC) for a Command and Control check in for a PlugX trojan. Based on this, eSentire received permission from the customer to install an endpoint sensor on the machine that triggered the alert of a PlugX trojan, a remote access tool (RAT) that uses modular plugins. It is a common tool used by multiple threat groups because it is complex and often evades typical security measures.

Investigation revealed that legitimate antivirus (AV) software had been used to install the malicious trojan without being detected by the AV, through a technique called dll hijacking. The trojan had a lot of complexity and encryption, so that it did not appear malicious. It included a “do not execute until 2013” time code, revealing that the machine had been infected for several years. Decrypting this was no easy task.

The trojan initially loads a dll that is a simple launcher. It looks for an encrypted, hidden “stage-one” payload, which then launches a “stage-two” payload, also hidden, encrypted and compressed. When unpacked, it launches the final, malicious dll file containing the trojan. Investigation showed two functionalities for the final payload: periodic screenshots of the infected machine and a keystroke logger. There was no evidence of any screenshots to be found. But, we decrypted a 30mb file of keystroke data with timestamps going back to 2014.

Threat Quickly Eliminated

eSentire isolated the affected machine when we installed the endpoint sensor. It is unknown when the initial infection occurred because there was no sensor on the endpoint to collect telemetry. The malware had a network channel back to the attacker, but we had no way to prove what they saw. Luckily, what we were able to find in the 30mb file was not the company’s critical data. The customer had an IT security company monitoring their network and they detected nothing for five-plus years. During the eSentire POC, eSentire detected, isolated, investigated and removed the longstanding threat in less than two weeks.

Summary of Events