New Android Trojan Targets 100+ Banking Apps

A Russian-speaking malware writer has developed a dangerous new Android Trojan that features a Swiss army knife-like set of capabilities targeting banking apps, crypto currency wallets, online payment services, and e-commerce sites.

Security vendor Group-IB, which discovered the threat, Thursday described it as spreading via text messages and targeting customers of more than 100 banks globally including several US-based ones such as Bank of America, Wells Fargo, and Capital One.

In an advisory Thursday, the Russia-based Group-IB described the malware, dubbed "Gustuff," as also capable of targeting users of 32 cryptocurrency apps, numerous ecommerce sites, and payment services such as Western Union, BitPay, and PayPal.  

"Gustuff is a new generation of malware complete with fully automated features designed to steal both fiat and crypto currency from user accounts en masse," the security vendor warned.

Rustam Mirkasymov, head of dynamic analysis of the malware department at Group-IB, says Gustuff infects Android smartphones via SMS messages containing a link to a malicious Android Package Kit (APK) file. The APK file format is what Android uses to distribute and install mobile app on user devices.

Clicking on the link in this case downloads Gustuff on the user's device. "The content of SMS can vary, but in general, it prompts users to click on the link to start downloading the malicious app," Mirkasymov says.

Typically, Android prevents users from installing applications downloaded from unknown sources. "However, many people disable this security option, so the malicious application asks the user to grant a number of permissions, including the use of Accessibility Services," he says. Once on a system, the malware is designed to spread further using the contacts list on the infected device.

Android's accessibility services feature is designed to allow users with disabilities to more easily use Android apps and services. Attackers have even previously shown how the feature can be manipulated to enable malware downloads and other malicious activities that bypass security controls on the system. However, so far at least such misuse has been relatively rare, Mirkasymov says.

Exploiting Android Accessibility Services

According to Group-IB, Gustuff makes use of the Android Accessibility Service to interact with online banking apps, cryptowallets, applications for payment services, e-commerce sites and, other apps of interest to the attackers. Gustuff gives attackers a way to use the Accessibility Service to enter or to change the values of the text fields in banking apps and carry out other illicit transactions.

The malware is also designed to push out fake notifications that appear to be from a legitimate banking and other targeted apps that an Android user might have installed on their device.

When a user clicks on the fake notification, the malware downloads a Web page that appears to belong to the banking or other app that sent the notification. For example, if the fake notification purported to be from the user's mobile bank app, the malware serves up a Web page that appears to be that of the banks' account login page.

In other instances, when a user clicks on the fake notification, the associated legitimate app opens and Gustuff then uses Android's Accessibility Service to automatically manipulate payment and other fields for illicit transactions, Group-IB said.

"Accessibility Service allows Gustuff to focus on the object, click on the object, and change the object text," Mirkasymov says. The privileges associated with the accessibility service "allows the Trojan to access to and interact with push notifications as well as SMS."

Significantly, Gustaff uses an automatic transfer system (ATS) function in conjunction with the Accessibility System for auto filling fields in legitimate mobile banking and other apps.

"Gustuff is not the first Trojan to successfully bypass security measures against interactions with other apps’ windows using Android Accessibility Service," Mirkasymov noted. "The major difference is that the ATS function is implemented with the help of Accessibility Service, which both speeds and scales up thefts."

Gustuff is another cautionary tale of downloading Android apps from outside the Google Play app store. Unsanctioned and insecure third-party app sites continue to be by far the biggest source of Android malware.

Also important is the need for users to pay attention when granting extra permissions to applications, Mirkasymov says. "It is important to always install software updates, pay attention to downloaded files’ extensions, and of course avoid suspicious SMS links."

Group-IB said its researchers first observed Gustuff on hacker forums in April 2018. A Russian-speaking cybercriminal appears to have developed the malware but it is exclusively for use outside the country.

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