Tushu, Take Twoshu: Malicious SDK Reappears in Google Play

Researchers who analyzed a new pool of malicious Android applications found they contained a new version of the Tushu SDK, which was seen infecting apps on Google Play earlier this year. The Twoshu SDK, as they have dubbed this lookalike, was built with new evasive techniques.

The ad fraud primarily associated with the Tushu Software Development Kit was first seen in Crazy Brainstorming, an Android gaming app available in Google Play from January through March 2019. During those months, it was downloaded more than a million times, mostly in the United States, say researchers with the White Ops Threat Intelligence Team who found it.

Strings in the app's code indicated the SDK was developed by 1tu1shu[.]com, a China-based company self-described as a "data driven intelligent marketing" firm. Analysis showed 71 other applications with this SDK implemented in their code base. All of the extensions and apps observed during their analysis led to suspicious domains characterized as malware repositories.

"There were definitely a lot of very suspicious things going on with this particular SDK," says John Laycock, threat analyst with the White Ops Threat Intelligence Team.

The Tushu SDK had a few defining characteristics. It could display full-screen ads out of the app's context, meaning ads could appear even if the app wasn't running in the foreground. This let attackers monetize ads while the user attempted to interact with other apps. The SDK was also capable of delivering ads when the screen was locked. Ads could be triggered by network changes such as connecting or disconnecting to Wi-Fi, or plugging a device in to charge.

There were a few red flags that alerted the White Ops team to malicious activity, says Laycock. Its high download count, for one, was "significant and somewhat unusual for a single app," he explains — especially since it was the only app from a developer under the name Linda Wang, assumed to be a random persona. The app itself wasn't well done, he adds, and a number of user comments complained of slow processes and too many advertisements. Researchers published their analysis, and Crazy Brainstorming was taken down.

Roughly six months later, the same team investigated six HiddenAd apps shared by ESET Research. The apps were posted on Google Play in mid-August and taken down in September. Analysis revealed code similar to the original Tushu with obfuscation and anti-analysis tactics. The increase in sophistication showed researchers had disrupted the attackers' cash flow; as a result, they saw a need to improve their tactics before jumping back into the ad fraud space.

The lookalike "Twoshu" SDK contains single-byte XOR obfuscation, says Laycock, while the original Tushu was visible in plaintext. The intent is to slow down analysis, researchers explain. "Instead of storing important strings in clear text within the dex file, there is a call to a decoder function with one of the many statically-assigned byte arrays," according to a White Ops team blog post.

"What we're seeing right now with a lot of the Android packages is the different actors out there are trying to increase their sophistication," says Laycock. It used to be easy to open an app and look at code in plaintext; now attackers are upping their game. "They had repackaged everything and basically made it very difficult to read," he says of the Tushu developers.

For anti-analysis, Twoshu contains code from a Chinese open source project dubbed EasyProtector, which determines if the device is an emulator. The code enumerates all installed packages on a system and checks them against an internal list of antivirus tools, researchers report. It also checks to see if a target device is connected to known antivirus service set identifiers (SSID).

The code will not run on a device with fewer than 10 apps installed, or with more than three apps that have ".test." in their package name because it assumes it's an analysis system.

"It's a real fun cat-and-mouse game," says Laycock. Similar to the original Tushu SDK, Twoshu collects an "impressive" amount of data, including GPS coordinates, Wi-Fi SSIDs, and the International Mobile Equipment Identity (IMEI) of each device. White Ops plans to continue monitoring this SDK for continued attempts to evade detection.

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