Whose Line Is It? When Voice Phishing Attacks Get Sneaky

What if social engineers, instead of calling victims with voice phishing attacks, intercepted phone calls their victims make to legitimate phone numbers? Malicious apps let cybercriminals do just that – a tactic that puts a subtle twist on traditional voice phishing.

Min-Chang Jang, manager at Korea Financial Security Institute and Korea University, began investigating these apps in September 2017 when he received a report of an app impersonating a financial firm. Analysis revealed a phone interception feature in the app, which intrigued him.

That's how Jang discovered a new type of voice phishing crime, which combines traditional voice phishing with malicious apps to trick unsuspecting callers into chatting with cybercriminals.

Here's how they work: An attacker must first convince a victim to download an app. The attacker may send a link to the victim, enticing the person with something like a low-interest loan, and prompt him to install the app for it. If the target takes the bait and later calls a financial company for loan consultation, the call is intercepted and connected to the attacker.

"The victims believe that they are talking to a financial company employee, but they aren't," Jang says. It's unlikely victims will know a scam is taking place, he says. Most of these attacks mimic apps from financial firms.

Unfortunately, when Jang and his research team first discovered malicious apps with the interception feature, they didn't have access to a live malicious app distribution server because it had already been closed by the time they received victim reports. In April 2018, Jang found a live distribution server – a pivotal point for their research into malicious phishing apps.

This particular distribution server had a very short operating cycle, ranging from a few hours to two days. "I found it while monitoring community sites for the information gathering," Jang explains. He discovered a post written to educate users to be careful of phishing sites; fortunately, it discussed the malicious applications they were hoping to investigate.

"I found a specific string in the Web page source code of a live malware distribution server," he says, "and I used the string for scanning to get more malware distribution servers." 

With access to one server, researchers could check which of its ports were open and access the Web page source code. Based on those strings of code from the first distribution server, they were able to create a real-time malicious app collection script, Jang explains. The automated system they created is able to collect malware distribution servers and apps in near real time.

Using this script, researchers have been able to find malicious app distribution servers and variant malicious apps. Following their discovery of the first live distribution server, they have collected about 3,000 malicious apps from various servers. The command-and-control (C2) server address was hard-coded inside malicious apps, Jang says, and could be easily extracted.

Their research continued to unfold. The team analyzed the C2 server, where they discovered a file containing the account data they needed to access it. This data helped the team gain the privileges of the Windows server admin of the distribution server and of the database admin of the C2 server. A Remote Desktop Protocol (RDP) connection to the server led to more information – the team confirmed this attacker was connecting to the Internet via the Point-to-Point Protocol over Ethernet (PPPoE), a sign the server's location was in Taiwan.

In a presentation at Black Hat Asia, entitled "When Voice Phishing Met Malicious Android App," Jang will disclose and discuss the findings of criminal traces in voice phishing analysis conducted by his research team over the past few months.

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