Cloud Threats Memo: Defining the Latest Dropbox Threat

Who said that cloud services are only exploited by opportunistic cybercriminals? Researchers from Cybereason have recently discovered a new highly targeted campaign, dubbed Operation GhostShell targeting the Aerospace and Telecommunications industries mainly in the Middle East, with additional victims in the U.S., Russia, and Europe.

The campaign leverages a previously undocumented RAT (Remote Access Trojan) dubbed ShellClient, employed as the primary espionage tool. Even though this RAT has been under development since at least 2018, the threat actors have progressively introduced new functionalities to make it even stealthier. And guess what? The latest version deployed in this campaign has replaced the traditional command and control infrastructure with a well-known cloud service: Dropbox.

This familiar cloud service is exploited to both send commands to the ShellClient RAT and also to store the exfiltrated data, and the reasons are always the same: evasion and operational simplicity as the researchers themselves also point out. This architectural choice makes it harder to detect the communication since the network traffic would appear legitimate to security analysts as well as most security solutions, and additionally, this makes it difficult to trace the attackers’ infrastructure.

And It looks like state-sponsored actors have a particular predisposition for Dropbox. Earlier in July 2021 IndigoZebra, a Chinese state-sponsored actor, was spotted using the same technique in a campaign targeting the Afghan National Security Council. Less recently, the Molerats (aka The Gaza Cybergang) deployed the DropBook backdoor in a cyber espionage campaign against targets in the Middle East, not to mention DRBControl, an APT actor targeting gambling companies in Southeast Asia unearthed at the beginning of 2020. 

Researchers from Netskope Threat Labs have also created Saasi_boy, a proof-of-concept malware (for solely educational purposes) using Dropbox as its command and control infrastructure.

For the record, Dropbox has also been recently exploited to deliver the Dridex banking trojan.

How Netskope mitigates this threat

Dropbox is one of the thousands of services where the Netskope Next Gen SWG can provide granular access control and one of the dozens for which instance detection is also available. In similar cases where Dropbox is exploited to host the command and control, it is possible to configure a policy that prevents potentially dangerous activities (such as upload and download) from non-corporate instances. It is possible to make the scope of the policy broader, blocking these activities for any unneeded cloud storage service that can potentially be exploited for command and control or malware distribution.

Customers with the advanced threat protection license can leverage the machine learning engine to detect Office documents containing malicious macros, regardless of where they are downloaded from, whether it’s a traditional domain or a legitimate cloud service exploited by the threat actors to bypass legacy defenses.

Stay safe!