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Android Trojan Intercepts Voice Call-Based 2FA: Symantec

Android.Bankosy Trojan Uses Call Forwarding to Steal Voice Call-Based Two-Factor Authentication Codes: Symantec

A new financial Trojan designed to deceive voice call-based two-factor authorization (2FA) systems is targeting Android devices, Symantec security researchers warn.

The new threat, detected by Symantec as Android.Bankosy, is an information stealing malware that includes typical functions of already known financial Trojans, while also packing call forwarding capabilities. As a result, it can intercept two-factor authorization codes even when they are transmitted via voice calls, a system already employed by numerous banks, as Symantec’s Dinesh Venkatesan explains in a blog post .

The voice call-based two-factor authorization system is meant to avoid having one-time passcodes (OTPs) sent to users’ registered mobile numbers via through short messaging service (SMS). Most malware designed to steal information from users’ mobile devices can intercept and read incoming SMS containing the OTP, and some banks have attempted to avoid this threat by delivering the OTP via voice calls.

However, the new Android.Bankosy has been designed to take advantage of this system by creating a backdoor communication connection with the command and control (C&C) server. Once installed on a compromised device, the malware opens the backdoor, collects a list of system-specific information, and sends it to the C&C server to register the device and get a unique identifier for it.

The Trojan also has the ability to intercept incoming SMS, delete SMS messages, wipe  data, and other commands supported by financial Trojans, in addition to the ability to enable call forwarding on the infected device.

What’s more, the backdoor includes support for enabling and disabling silent mode and for locking the device, which means that the victims are not even alerted during an incoming call. The function is especially useful on infected devices in the Asia Pacific region, where most carriers support a special service code to enable or disable call forwarding, Symantec said.

Once call forwarding has been set on the victim’s device, the attacker can initiate transactions, especially if they already have stolen the victim’s credentials, which is the first factor in two-factor authentication and authorization. As soon as the system demands that the victim enters the second factor, the attacker intercepts the call via call forwarding and can then complete the transaction.

To stay protected from this threat, users are advised to keep the software on their devices updated at all times, and should also refrain from downloading applications from third-party app stores or from unfamiliar sites.

Android Trojans have frequently made headlines over the past few months, though this is the first known to include call forwarding capabilities.

Just today, FireEye warned that a campaign involving the Android banking Trojan dubbed “SlemBunk” is ongoing and continues to evolve. First analyzed by FireEye in mid-December, the security firm noted at the time that it had identified 170 SlemBunk samples designed to target the users of 33 applications offered by financial institutions and service providers in North America, Europe and the Asia-Pacific region.

Back in November, Chinese mobile security Cheetah Mobile warned of a Trojan preloaded on Android tablets sold via Amazon, which allowed malicious actors to remotely control infected devices.

In early December, a new information-stealing Android Trojan called Rootnik was found to root infected devices via a commercial root tool called Root Assistant. In mid-December, Russian antivirus firm Doctor Web warned of ZBot, a piece of malware designed to steal sensitive information from user conducting online malware from their smartphones.

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