Security Experts:

Dropbox Abused in Targeted Attacks Using PlugX RAT With "Time Bomb"

When analyzing a targeted attack against a government agency in Taiwan, security researchers from Trend Micro said they came across a variant of the PlugX remote access tool (RAT) that abuses the popular file hosting service Dropbox.

According to the security firm, the RAT downloads its command and control (C&C) settings from Dropbox, most likely in an effort to avoid raising any suspicion by disguising malicious traffic. Because Dropbox is often used by organizations to store files for legitimate purposes, security systems might not flag communications with the website as a potential threat.

The security company notified Dropbox of the incident, but experts highlight the fact that the attackers are not exploiting any vulnerabilities in the service. While it's not uncommon for cybercriminals to abuse legitimate file sharing services, this is the first time Dropbox has been used to store C&C settings as part of a targeted attack, the researchers said.

The samples analyzed by Trend Micro have been identified as BKDR_PLUGX.ZTBF-A and TROJ_PLUGX.ZTBF-A. Once it infects a system, the backdoor enables the attackers to log keystrokes, perform port maps and execute arbitrary commands.  An interesting technique employed by this particular PlugX RAT variant is a trigger date, which the malware steps into play only at a specified date to prevent victims from noticing the malicious activities. 

BKDR_PLUGX.ZTBF-A is a "type 2" PlugX variant, a category of PlugX RATs described at the BlackHat Asia 2014 security conference by Takahiro Haruyama and Hiroshi Suzuki of Internet Initiative Japan. This type of threat was first spotted in the third quarter of 2013 and compared to "type 1" PlugX RATs, it incorporates anti-forensic techniques, a different encryption algorithm, extended configuration, and more protocols and functions.

In the operation analyzed by Trend Micro, the attackers leveraged various tools to move throughout the network without being detected, including tools for password recovery, remote administration, proxies, networking utilities and port scanners. They've also used a tool called Htran, which enables attackers to hide their source IP and throw incident responders off their tracks by bouncing through several connections.

"Like so many smart businesses around the world, cybercriminals are increasingly jumping on the cloud bandwagon," Christopher Budd, Trend Micro's global manager of threat communications, told SecurityWeek in an emailed statement.

"Keep in mind, this isn't a problem with Dropbox per se: it appears these cybercriminals have signed up for legitimate accounts but are using them for malicious ends. There are two takeaways from this. First, cybercriminals recognize the business benefits of cloud services and will likely continue to migrate from self-hosted (or compromised-server-hosted) attacks to cloud services. Second, for CISOs and security managers, it increasingly makes sense to block access to any cloud-based services where there is no legitimate business need." 

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.