Android banking Trojan defeats voice call-based 2FA

Bankosy is a banking Trojan that has been targeting Android users for a while now, but has only recently been updated with a new capability of note: the ability to deceive voice call-based two-factor authorization (2FA) systems.

Before this update, Bankosy did the usual things required of a banking Trojan: it would gather information about the victim and the mobile device, log login credentials for financial apps, intercept text messages from financial institutions that deliver a one-time passcode (OTP) as the second authentication factor, and send it all to a C&C server operated by cyber crooks.

The C&C server would react by assigning a unique ID to the infected device, which allowed it to send commands to the device – enable/disable silent mode, wipe data for the device, delete SMS messages, enable/disable call forwarding, etc.

But with the update, the Trojan has become capable of intercepting voice calls used by some financial organizations to deliver the OTP. The crooks do that by setting up call forwarding (to a phone number controlled by them) on the device.

“Once the unconditional call forwarding is set on the victim’s device, the attacker—who has already stolen the victim’s credentials (the first factor in two-factor authentication and authorization)—can then initiate a transaction,” Symantec researchers explain.

“As part of the design, when the system demands the victim to enter the second factor (i.e., the authorization token sent through a voice call), the attacker will get the call through call forwarding and enter the second factor as well to complete the transaction. The back door also has support for disabling and enabling silent mode, in addition to locking the device, so that the victim is not alerted during an incoming call.”

The Bankosy Trojan is usually installed on devices by the users themselves, as it often masquerades as a legitimate app. Users who want to avoid this and other similar threats should be very careful about the apps they choose to install/use.