Google has pulled from Google Play two malicious Android apps that were disguised as legitimate versions of popular Mario Brothers and Grand Theft Auto titles.

According to Symantec, which recently spotted the apps and alerted Google, they were first added to Google Play on June 24 and had since been downloaded between 50,000 and 100,000 times. One was packaged as "Super Mario Bros.," while the other was titled "GTA 3 Moscow City," which also offers a clue to the geographic location of targeted Android users.

In reality, both apps were a Trojan app known as Dropdialer. Once installed, Dropdialer downloads an additional package, "Activator.apk," via file-sharing website Dropbox. The secondary payload allows the malware to send messages to premium-rate SMS numbers in Eastern Europe, in a type of attack often referred to as toll fraud. The malware then uninstalls the secondary payload, helping to disguise what it's been up to.

[ A security researcher has discovered an Android botnet that hijacks Android devices and turns them into spam servers. Read about it here: Android Botnet Seen Spewing Spam. ]

"What is most interesting about this Trojan is the fact that the threat managed to stay on Google Play for such a long time, clocking up some serious download figures before being discovered," said Symantec security researcher Irfan Asrar in a blog post. "Our suspicion is that this was probably due to the remote payload employed by this Trojan."

By breaking up the malicious payload into pieces, the malware's developer was able to sneak it past the automated Google Bouncer malware-checking service that reviews apps before they can be added to Google Play. "The idea is simple: instead of having one payload that carries all of the malicious code for any given attack, break the threat into separate modules that can be delivered independently," said Asrar, in an overview of the technique he published last year.

Breaking malicious applications into pieces offers numerous additional benefits to attackers. "First, it obviates the telltale sign of a huge, overzealous permissions list accompanying the installation of the threat, which may alarm the user as to the intention of the malicious app," he said. "Secondly, smaller pieces are easier to hide and inject into other apps. Furthermore, dispersing the attack across separate apps complicates the integrated revocation processes from the service provider, marketplace, etc."

One mitigating factor with most Android malware is that users must first approve the app's installation, as well as its permission requests. Hence attackers, via a social-engineering attack, often disguise their malware as well-known software, and especially as "free" versions of these applications.

In the case of the Dropdialer apps recently discovered on Google Play, users had multiple opportunities to prevent the malicious software from executing. "After installation, the first application shows a notification in Russian informing the user that the application must be activated. The application mentions that by activating the application the user agrees to a set of rules. If a user chooses to read these rules--which are also in Russian--they provide very vague information regarding how much a user will be charged," read an analysis of the malware published by mobile security firm Lookout.

After the user agrees, the app downloads its secondary payload, which again triggers an Android warning. "A normal Android system installation message appears, asking if the user would like to install the application, which includes permissions to send SMS," said Lookout.

Occasionally, however, attackers do find ways to install malicious apps automatically. For example, the malicious Android application jSMSHider--targeting Chinese users--was designed to infect smartphones using custom ROMs, which are custom-built Android distributions. In particular, attackers targeted a vulnerability related to the Android Open Source Project (AOSP), which uses "publicly available private keys ... to sign the custom ROM builds," according to an analysis of the malware published by Lookout.

These keys are used to authorize updates for the custom ROM. In a move reminiscent of the Flame malware, which used a spoofed Windows certificate to automatically install itself on targeted PCs via the Windows Update mechanism, jSMSHider could be delivered to custom ROM builds disguised as a system update, allowing the software--which eavesdrops on SMS communications--to automatically install itself. Oftentimes, the malware could then install secondary payloads, adding further attack functionality.

Reached for comment, Google declined to offer specifics on the Dropdialer-in-disguise app takedowns. "We remove applications that violate our policies, such as apps that are illegal or that promote hate speech," said a Google spokesman via email. "We don’t comment on individual applications--however, you can check out our policies for more information."

In addition to enforcing those policies, Google last year--in the wake of security firms noting an alarming rise in Android malware--unveiled Bouncer. Bouncer, however, isn't foolproof. For starters, it's automated. In comparison, Apple employs teams of application reviewers, who hand-screen all iOS apps before they're allowed into the Apple App Store. To date, only one malicious app, "Find and Call,"--as well as one proof-of-concept attack authored by security researcher Charlie Miller--appears to have snuck by those reviewers.

By comparison, Google regularly needs to excise malicious apps--often aimed at perpetrating premium SMS toll fraud--from Google Play, but only after they've been spotted, downloaded, and apparently infected Android devices, thus generating fraudulent profits for attackers.

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