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Early this month we blogged about the master key Android vulnerability – a vulnerability that allows cybercriminals to ‘update’ legitimate apps already installed in the user’s device and insert malicious code into them. We’ve been on the lookout to find threats that take advantage of it ever since, and we discovered one that targets users of the NH Bank online banking app.

NH Nonghyup Bank is one of South Korea’s biggest financial institutions. Their online banking app is in high circulation among mobile device owners, having been installed from five to ten million times.

Cybercriminals took advantage of the app’s popularity by offering a downloadable update for the app on third party app download websites. This update is of course malicious. It utilizes the master key Android vulnerability to insert a malicious file into the app, thus ‘trojanizing’ it.

The inserted malicious file, classes.dex has a smaller file size at 205 kb, than the legitimate version.

The cybercriminals responsible also offered an already trojanized version of the legitimate app, in case the user does not have the banking app installed on their device yet. Running the app triggers the display a spoofed page, one that asks the user to input their account information.

The page displayed when the user executes the trojanized app.

 Should the user comply, their information would be sent to a remote malicious server controlled by the cybercriminal.

This particular finding shows just how dangerous the abuse of the master key vulnerability is to Android users. The fact that it was used to “trojanize” a banking app makes the risk comparable to the online banking threats we know today, as it poses not just the risk of personal information leakage, but financial loss as well.

Furthermore, since it involves the tampering of an app that is already in the device, the effect might not be at all noticeable to the user until it is too late.

Users are advised to  download apps or app updates only from trusted sources, preferably from official sources or app stores. Trend Micro customers are protected from this threat via our Trend Micro Mobile Security App, as it can detect apps that abuse the master key vulnerability.