Symantec warns of Android Trojans that mutate with every download

A new Android Trojan employs server-side polymorphism to generate unique variants

Researchers from security vendor Symantec have identified a new premium-rate SMS Android Trojan horse that modifies its code every time it gets downloaded in order to bypass antivirus detection.

This technique is known as server-side polymorphism and has already existed in the world of desktop malware for many years, but mobile malware creators have only now begun to adopt it.

A special mechanism that runs on the distribution server modifies certain parts of the Trojan in order to ensure that every malicious app that gets downloaded is unique. This is different from local polymorphism where the malware modifies its own code every time it gets executed.

Symantec has identified multiple variants of this Trojan horse, which it detects as Android.Opfake, and all of them are distributed from Russian websites. However, the malware contains instructions to automatically send SMS messages to premium-rate numbers from a large number of European and former Soviet Union countries.

In some cases, especially when security products rely heavily on static signatures, detecting malware threats that make use of server-side polymorphism can be difficult.

"As with malware that affects traditional computing devices, the level of sophistication of the polymorphism used can affect how easy or difficult the threat is to detect," said Vikram Thakur, the principal security response manager at Symantec. "More complicated polymorphism requires more intelligent countermeasures."

In the case of Android.Opfake the level of polymorphism is not very high, as only some of the Trojan's data files are being modified by the distribution server.

"If antivirus vendors place their detection on the executable and non-changing sections, all files would be successfully detected," said Tim Armstrong, malware researcher at Kaspersky Lab. However, if the Trojan's executable code were also polymorphic, the challenge of detecting it would be more difficult, he said.

According to Armstrong, server-side polymorphism is not very widespread on the Android platform at the moment because most users get their apps through official channels and the current structure of the Android Market does not allow for a malware distribution scheme like this one.

However, he agrees that polymorphic Android malware could force antivirus vendors to step up their game in the future. "I think many of the features that are currently available on traditional platforms will start to arrive on these mobile platforms out of necessity as the criminals change their attack methods," Armstrong said.

There have been many new developments on the mobile threat landscape recently and increasing their attention towards smartphones is a logical move for malware writers, because they usually go where the money is, said Jamz Yaneza, research manager at antivirus company Trend Micro.

Users should become more aware of this fact and the capabilities of their mobile devices, which are now similar to those of mobile PCs, Yaneza said. "They should treat app downloads with the same caution as they do on desktops," and install or make use of whatever security add-ons they can as this creates another protective layer.