Online Sandboxing: A Stash for Exfiltrated Data?

Online sandboxing services are an extremely important resource for threat hunters to use to consolidate knowledge and collaborate about emerging threats. However, new research out this week shows that the tables can be turned on organizations using services like VirusTotal or Hybrid Analysis by using them as a way to exfiltrate data from heavily locked down systems. 

Published by researchers with SafeBreach, the report released today adds a new layer to previous work the team did on leaky sandbox services used by cloud-based antivirus platforms. Presented at BlackHat last summer, that earlier research demonstrated how researchers could exfiltrate data off of air-gapped and otherwise disconnected systems by tricking cloud-enhanced anti virus agents into sending stolen data onto a criminal's server. 

This time around the SafeBreach team used that same principle of hiding a wolf in sheep's clothing to offer a proof-of-concept for how online sandboxing services could be used as an outbound channel for stolen sensitive data. Unlike the previous research, this time around the hidden data never needs for an attacker to communicate out of the sandbox in order for them to find their stash.

Here is the way it works: a piece of custom malware is designed to be found on the system and uploaded to an online sandboxing service like VirusTotal, either by a human security analyst or a tool integrated with the online service. Unbeknownst to the victim, the stolen data is encoded and embedded in that malware. And so is a secret and searchable passphrase.

"One of the main objectives that needs to be addressed by the attacker is how to make the data that they incorporate into the malware visible to him - to be identified by him after the malware is uploaded to the server - but still be hidden enough that any other user cannot identify that the data is (taken) from an organization, or from any malicious contents inside it," says Dor Azouri, one of the primary researchers for SafeBreach on this project.

Azouri explains that this can be achieved by developing a "magic" string of text and either encrypting or encoding it in such a way that the attacker can make a simple query into the public sandboxing platform in order to pull up that specific piece of malware that's been uploaded to the database.

By passively collecting the data in this way, the attacker doesn't need to emit outbound network traffic or run an HTTP server or authoritative DNS server, according to Azouri, noting that the process is very low-profile and can be done on even extremely isolated systems. The downside, he says, is that the attacker must know - or at least bet heavily - that their targeted victim is using an online sandbox engine as part of their normal security processes.

According to a VirusTotal spokesperson, the SafeBreach research presented here isn't a bug in VirusTotal. It is just a way for attackers to misuse the same kind of feature you'd have in any number of services widely used by enterprises today that allow the uploading, publishing and retrieving of data. Of course, the point from SafeBreach is that this method is meant to be used as a last-mile connection between an attacker and an otherwise well-fortified system. A sensitive system like this wouldn't be using any other kind of SaaS-based information sharing platform, but the security team might not think twice about sending off a sample to VirusTotal from that system.

Nevertheless, VirusTotal's spokesperson contends that this isn't a likely or practical channel for attackers.

"The approach laid out by Safebreach does not make practical sense from an attacker perspective," the spokesperson says. "As an attacker, you don't want to use a channel that is being inspected by the whole security industry to exfiltrate data when there are thousands of other public internet services that could accomplish the same thing much more discreetly.”

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