Researchers at Trend Micro say they have uncovered a scheme to use Dropbox to distribute command and control (C&C) updates as part of a targeted attack.

The situation was uncovered in an analysis of an attack against a Taiwanese government agency. According to Trend Micro, the malware downloads its C&C settings from Dropbox as part of an effort to mask malicious traffic by using a legitimate website. The firm found no vulnerability in Dropbox, and it informed Dropbox of the situation before mentioning it publicly.

The attackers are using variants of the PlugX remote administration tool (RAT). In a blog post, Trend Micro threat analyst Maersk Menrige explains that, when malware detected by Trend Micro as BKDR_PLUGX.ZTBF-A is executed, it performs a number of commands from a remote user, such as keystroke logging and remote shell. Typically, remote shell allows attackers to run any command on the infect4ed system to compromise its security, the researcher wrote.

Once the command and control communications are established, the threat actors move laterally into the network using a mix of malicious and legitimate tools to avoid being detected. These tools include password recovery tools, port scanners, and the HTran tool, which hides the attacker's source IP by bouncing TCP traffic through several connections. The password recovery tools are used to extract stored passwords in apps and the operating system found in registry and local drives, Menrige wrote.

"This is the first time we've seen this, but criminals are smart and copy proven tactics," Christopher Budd, global threat communications manager at Trend Micro, told us. "Just like we've seen malware hosting move into the cloud, we should expect to see more instances of C&C being hosted in the cloud."