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In this case, attackers have been offering a trojanized update for a legitimate online banking app distributed by South Korea's NH Nonghyup Bank. The Android app is used by up to 10 million people.
Running the malicious app triggers a screen asking users to enter their account details. "Should the user comply, their information would be sent to a remote malicious server controlled by the cybercriminal," said Peter Yan, a Trend Micro mobile security engineer, in a blog post. In other words, people who fall for the attack would be likely targets for cybercriminals trying to drain their bank accounts.
The trojanized banking app "shows just how dangerous the abuse of the master key vulnerability is to Android users," Yan said. "Furthermore, since it involves [tampering with] an app that is already in the device, the effect might not be at all noticeable to the user, until it is too late."
[ Want more Android vulnerability news? Read Android One-Click Google Apps Access Cracked. ]
The trojanized banking app isn't the first known attempt to exploit the master key vulnerability for malicious purposes. That accolade goes to a legitimate app used for making a doctor's appointments in China, for which Symantec reported finding trojanized versions two weeks ago. In that case, the attacker behind the revamped app had added code that allowed the device to be remotely controlled, and which could siphon the phone's international mobile equipment identity (IMEI) number and phone numbers stored on the device, as well as dial premium-rate numbers, thus draining the smartphone user's account and enriching attackers.
What both of those malicious apps have in common, besides abusing the same master-key vulnerability, is that they're only available from third-party app stores. That's a reminder to all Android users to download and install apps only from reputable sources. That means using the likes of Google Play or the Amazon Appstore for Android, or else an in-house, pre-vetted enterprise app store.
Unfortunately, few devices currently run Android 4.2.2, which is patched against the vulnerability. But Google has partially mitigated the flaw in older devices via its Verify Apps feature, which warns users when they try to install an app that's been flagged as malicious. While the feature was originally released for Android 4.2, it's since been automatically distributed to every device that runs Android version 2.3 and newer -- encompassing 95% of all Android devices -- that's configured to use Google Play.
While those are welcome protections, consumers in some countries -- such as China -- are blocked from accessing Google Play altogether, and must rely on third-party app stores. Their only recourse is to run mobile antivirus software, which will help detect and remove malware that attempts to abuse the master-key vulnerability.
What would automatically mitigate the vulnerability for a large swath of users, however, is if all handset manufacturers and carriers pushed timely Android updates or security patches for their devices. To date, of course, updates from some carriers and manufacturers have been patchy, sometimes taking months -- if ever -- to appear.
For example, Blue Box CTO Jeff Forristal discovered the vulnerability in February and privately reported it to Google, which quickly patched the bug in the Android Open Source Project and alerted business partners in March. More than four months later, however, only a handful of devices -- Samsung Galaxy S4 and HTC One devices running Android 4.2.2 or above -- have received a patch. That's the case even after other security researchers last month managed to identify the vulnerability, based on a non-technical preview of Forristal's Black Hat presentation, which he delivered Thursday.
If a full-on fix is still outstanding for most Android devices, Forristal said that the vulnerability had at least raised users' awareness of the glacial pace with which many carriers and handset manufacturers release Android updates. "Regardless of whether or not you feel users were truly at risk, [it] got users' attention, it got users asking the right questions," said Forristal last week in a Black Hat press conference. "I get inquiries from end users saying, who can I contact to ask for a device update? To me, that's a great education win ... and a great first step in helping to address what has largely been identified as a fracture in security in the Android device ecosystem."