Five years ago, researchers made a grim discovery—a legitimate Android app in the Google Play market that was surreptitiously made malicious by a library the developers used to earn advertising revenue. With that, the app was infected with code that caused 100 million infected devices to connect to attacker-controlled servers and download secret payloads.
Now, history is repeating itself. Researchers from the same Moscow, Russia-based security firm reported Monday that they found two new apps, downloaded from Play 11 million times, that were infected with the same malware family. The researchers, from Kaspersky, believe a malicious software developer kit for integrating advertising capabilities is once again responsible.
Clever tradecraft
Software developer kits, better known as SDKs, are apps that provide developers with frameworks that can greatly speed up the app-creation process by streamlining repetitive tasks. An unverified SDK module incorporated into the apps ostensibly supported the display of ads. Behind the scenes, it provided a host of advanced methods for stealthy communication with malicious servers, where the apps would upload user data and download malicious code that could be executed and updated at any time.
The stealthy malware family in both campaigns is known as Necro. This time, some variants use techniques such as steganography, an obfuscation method rarely seen in mobile malware. Some variants also deploy clever tradecraft to deliver malicious code that can run with heightened system rights. Once devices are infected with this variant, they contact an attacker-controlled command-and-control server and send web requests containing encrypted JSON data that reports information about each compromised device and application hosting the module.
The server, in turn, returns a JSON response that contains a link to a PNG image and associated metadata that includes the image hash. If the malicious module installed on the infected device confirms the hash is correct, it downloads the image.
The SDK module “uses a very simple steganographic algorithm,” Kaspersky researchers explained in a separate post. “If the MD5 check is successful, it extracts the contents of the PNG file—the pixel values in the ARGB channels—using standard Android tools. Then the getPixel method returns a value whose least significant byte contains the blue channel of the image, and processing begins in the code.”
The researchers continued:
If we consider the blue channel of the image as a byte array of dimension 1, then the first four bytes of the image are the size of the encoded payload in Little Endian format (from the least significant byte to the most significant). Next, the payload of the specified size is recorded: this is a JAR file encoded with Base64, which is loaded after decoding via DexClassLoader. Coral SDK loads the sdk.fkgh.mvp.SdkEntry class in a JAR file using the native library libcoral.so. This library has been obfuscated using the OLLVM tool. The starting point, or entry point, for execution within the loaded class is the run method.
Follow-on payloads that get installed download malicious plugins that can be mixed and matched for each infected device to perform a variety of different actions. One of the plugins allows code to run with elevated system rights. By default, Android bars privileged processes from using WebView, an extension in the OS for displaying webpages in apps. To bypass this safety restriction, Necro uses a hacking technique known as a reflection attack to create a separate instance of the WebView factory.
This plugin can also download and run other executable files that will replace links rendered through WebView. When running with the elevated system rights, these executables have the ability to modify URLs to add confirmation codes for paid subscriptions and download and execute code loaded at links controlled by the attacker. The researchers listed five separate payloads they encountered in their analysis of Necro.
The modular design of Necro opens myriad ways for the malware to behave. Kaspersky provided the following image that provides an overview.
The researchers found Necro in two Google Play apps. One was Wuta Camera, an app with 10 million downloads to date. Wuta Camera versions 6.3.2.148 through 6.3.6.148 contained the malicious SDK that infects apps. The app has since been updated to remove the malicious component. A separate app with roughly 1 million downloads—known as Max Browser—was also infected. That app is no longer available in Google Play.
The researchers also found Necro infecting a variety of Android apps available in alternative marketplaces. Those apps typically billed themselves as modified versions of legitimate apps such as Spotify, Minecraft, WhatsApp, Stumble Guys, Car Parking Multiplayer, and Melon Sandbox.
People who are concerned they may be infected by Necro should check their devices for the presence of indicators of compromise listed at the end of this writeup.
