Authored by Fernando Ruiz 

McAfee Mobile Research Team identified an Android backdoor implemented with Xamarin, an open-source framework that allows building Android and iOS apps with .NET and C#. Dubbed Android/Xamalicious it tries to gain accessibility privileges with social engineering and then it communicates with the command-and-control server to evaluate whether or not to download a second-stage payload that’s dynamically injected as an assembly DLL at runtime level to take full control of the device and potentially perform fraudulent actions such as clicking on ads, installing apps among other actions financially motivated without user consent.

The second stage payload can take full control of the infected device due to the powerful accessibility services that were already granted during the first stage which also contains functions to self-update the main APK which means that it has the potential to perform any type of activity like a spyware or banking trojan without user interaction. However, we identified a link between Xamalicious and the ad-fraud app “Cash Magnet” which automatically clicks ads, installs apps, and other actions to fraudulently generate revenue while users that installed it may earn points that are supposed to be redeemable as a retail gift card. This means that the developers behind these threats are financially motivated and drive ad-fraud therefore this might be one of the main payloads of Xamalicious.

The usage of the Xamarin framework allowed malware authors to stay active and without detection for a long time, taking advantage of the build process for APK files that worked as a packer to hide the malicious code. In addition, malware authors also implemented different obfuscation techniques and custom encryption to exfiltrate data and communicate with the command-and-control server. 

We’ve identified about 25 different malicious apps that carry this threat. Some variants have been distributed on Google Play since mid-2020. The apps identified in this report were proactively removed by Google from Google Play ahead of our reporting. McAfee is a member of the App Defense Alliance and an active partner in the malware mitigation program, which aims to quickly find Potentially Harmful Applications (PHAs) and stop them before they ever make it onto Google Play. Android users are protected by Google Play Protect, which can warn users of identified malicious apps on Android devices. McAfee Mobile Security detects this threat as Android/Xamalicious.  

Based on the number of installations these apps may have compromised at least 327,000 devices from Google Play plus the installations coming from third-party markets that continually produce new infections based on the detection telemetry of McAfee clients around the world. This threat remains very active. 

 

Figure 1. “Count Easy Calorie Calculator” was available on Google Play on August 2022 and carries Android/Xamalicious 

Android/Xamalicious trojans are apps related to health, games, horoscope, and productivity. Most of these apps are still available for download in third-party marketplaces.  

Previously we detected malware abusing Xamarin framework such as the open-sourced AndroSpy and forked versions of it, but Xamalicious is implemented differently. Technical details about Xamarin architecture are well documented and detail how .NET code is interpreted by Android using Mono. 

Obtaining Accessibility Services

Let’s use the app “Numerology: Personal horoscope & Number predictions” as an example. Once started it immediately requests the victim to enable accessibility services for “correct work” and provides directions to activate this permission:  

 

Figure 2. Tricking users into granting accessibility services permission 

Users need to manually activate the accessibility services after several OS warnings such as the following on the accessibility options: 

Figure 3. Accessibility services configuration prompt highlights the risks of this permission. 

Where is the malicious code? 

This is not the traditional Java code or native ELF Android application, the malware module was written originally in .NET and compiled into a dynamic link library (DLL). Then it is LZ4 compressed, and it might be embedded into a BLOB file, or directly available in the /assemblies directory on the APK structure. This code is loaded then by a native library (ELF) or by the DEX file at runtime level. In simple words, this means that in some samples the reversing of the DLL assemblies is straightforward while in others it requires extra steps to unpack them. 

The malicious code is usually available in two different assembly files in the /assemblies directory on the apk. Usually, file names are core.dll and a <package-specific>.dll.

Some malware variants has obfuscated the DLL assemblies to avoid analysis and reversing of the malicious code while others keep the original code available.  

 

Figure 4. Core.dll and GoogleService.dll contain malicious code. 

Communication with the command-and-control server

Once accessibility permissions are granted the malware initiates communication with the malicious server to dynamically load a second-stage payload.  

Figure 5. App execution and communication with the malicious server 

Collect Device Information 

Android/Xamalicious collects multiple device data including the list of installed applications obtained via system commands to determine if the infected victim is a good target for the second stage payload. The malware can collect location, carrier, and network information among device rooting status, adb connectivity configuration, for instance, if the device is connected via ADB or is rooted, the C2 will not provide a second-stage payload DLL for download. 

Method/Command  Description 
DevInfo  Hardware and device information that includes: 

  • Android Id 
  • Brand, CPU, Model, Fingerprint, Serial 
  • OS Version, release, SDK 
  • Language 
  • Developer Option status 
  • Language 
  • SIM Information (operator, state, network type, etc) 
  • Firmware, firmware version 
GeoInfo  Location of the device based on IP address, the malware contacts services such as api.myip.com to verify the device location and ISP data. 

  • ISP Name 
  • Organization 
  • Services 

FraudScore: Self-protection to identify if the device is not a real user 

EmuInfo  It lists all adbProperties that in a real device are around 640 properties. This list is encoded as a string param in URL encoded format. 

This data may be used to determinate if the affected client is a real device or emulator since it contains params such as: 

  • CPU 
  • Memory  
  • Sensors 
  • USB Configuration 
  • ADB Status 
RootInfo  After trying to identify if the device is rooted or not with multiple techniques the output is consolidated in this command 
Packages  It uses the system commands “pm list packages -s” and “pm list packages -3” to list system and installed apps on the device. 
Accessibility  It provides the status if accessibility services permissions are granted or not 
GetURL  This command only provides the Android Id and it’s a request for the second-stage payload. The C2 evaluates the provided client request and returns a status and an encrypted assembly DLL. 

Data Encryption in JWT 

To evade analysis and detection, malware authors encrypted all communication and data transmitted between the C2 and the infected device, not only protected by HTTPS, it’s encrypted as a JSON Web Encryption (JWE) token using RSA-OAEP with a 128CBC-HS256 algorithm however the RSA key values used by the Xamalicious are hardcoded in the decompiled malicious DLL so decryption of transmitted information is possible if C2 infrastructure is available during the analysis. 

In the Send() function Android/Xamalicious first prepares the received object, usually a JSON structure calling the function encrypt() which creates the JWT using a hardcoded RSA key. So the data is exfiltrated fully encrypted to the malware host pointing to the path  “/Updater” via HTTP POST method. 

Then it waits for the C2 response and passes it to the decrypt() function which has a hardcoded RSA private key to properly decrypt the received command which might contain a second stage payload for the “getURL” command. 

Encrypt Method: 

Figure 6. Encrypt function with hardcoded RSA Key values as XML string 

The decryption method is also hardcoded into malware which allowed the research team to intercept and decrypt the communication from the C2 using the RSA key values provided as XML string it’s possible to build a certificate with the parameters to decrypt the JWE tokens content. 

C2 evaluation

Collected data is transmitted to the C&C to determine if the device is a proper target to download a second-stage payload. The self-protection mechanism of the malware authors goes beyond traditional emulation detection and country code operator limitations because in this case, the command-and-control server will not deliver the second stage payload if the device is rooted or connected as ADB via USB or does not have a SIM card among multiple other environment validations. 

DLL Custom Encryption 

With the getURL command, the infected client requests the malicious payload, if the C&C Server determines that the device is “Ok” to receive the malicious library it will encrypt a DLL with Advanced encryption standard (AES) in Cipher block chaining (CBC) using a custom key for the client that requested it based on the device id and other parameters explained below to decrypt the code since it’s a symmetric encryption method, the same key works for encryption and decryption of the payload. 

Delivers the Payload in JWT 

The encrypted DLL is inserted as part of the HTTP response in the encrypted JSON Web Token “JWT”. Then the client will receive the token, decrypt it, and then decrypt the ‘url’ parm with AES CBC and a custom key.  

The AES key used to decrypt the assembly is unique per infected device and its string of 32 chars of length contains appended the device ID, brand, model, and a hardcoded padding of “1” up to 32 chars of length. 

For instance, if the device ID is 0123456ABCDEF010 and the affected device is a Pixel 5, then the AES key is: “0123456ABCDEF010googlePixel 5111” 

This means that the DLL has multiple layers of encryption. 

  1. It’s a HTTPS protected.
  2. It’s encrypted as a JWE Token using RSA-OAEP with a 128CBC-HS256 algorithm.
  3. URL parameter that contains the DLL is encrypted with AES and encoded as base64 

All these efforts are related to hiding the payload and trying to stay under the radar where this threat had relative success since some variants might have been active years ago without AV detections. 

DLL Injected 

Xamalicious will name this DLL “cache.bin” and store it in the local system to finally dynamically load it using the Assembly.Load method. 

Figure 7. Loading of second stage payload using Assembly.Load method. 

Once the second stage payload has been loaded the device can be fully compromised because once accessibility permissions are granted, it can obverse and interact with any activity opening a backdoor to any type of malicious activity. 

During the analysis, the downloaded second stage payload contained a DLL with the class “MegaSDKXE” which was obfuscated and incomplete probably because the C2 didn’t receive the expected params to provide the complete malicious second stage that might be limited to a specific carrier, language, app installed, location, time zone or unknown conditions of the affected device, however, we can assure that this is a high-risk backdoor that leaves the possibility to dynamically execute any command on the affected device not limited to spying, impersonation or as a financially motivated malware. 

Cash Magnet Ad-Fraud and Xamalicious

One of the Xamalicious samples detected by McAfee Mobile generic signatures was “LetterLink” (com.regaliusgames.llinkgame) which was available on Google Play at the end of 2020, with a book icon. It was poorly described as a hidden version of “Cash Magnet”: An app that performs ad-fraud with automated clicker activity, apps downloads, and other tasks that lead to monetization for affiliate marketing. This application offers users points that are supposed to be redeemable by retail gift cards or cryptocurrency.

Figure 8a. LetterLink login page after running the app for the first time.

Figure 8b. LetterLink agreement for Cash Magnet

Originally published in 2019 on Google Play, “Cash Magnet” (com.uicashmagnet) was described as a passive income application offering users to earn up to $30 USD per month running automated ads. Since it was removed by Google the authors then infiltrated LetterLink and more recently “Dots: One Line Connector” (com.orlovst.dots) which are hidden versions of the same ad-fraud scheme.

Figure 9. LetterLink Icon that hides Cash Magnet

“LetterLink” performs multiple Xamalicious activities since it contains the “core.dll” library, it connects to the same C2 server, and it uses the same hardcoded private RSA certificate to build the JWE encrypted tokens which provide a non-repudiation proof that the developers of Cash Magnet are behind Xamalicious.

Figure 10. Cash Magnet infiltrated the app as a Game, available until the end of 2023

“Dots: One Line Connector” app is not a game, the screenshot published by Google Play does not correspond to the application behavior because once it is started it just asks for authentication credentials without any logo or reference to Cash Magnet. “Dots” does not contain the same DLLs as its predecessor, however the communication with the C2 is similar using the same RSA key parameters. We reported this app to Google and they promptly removed it from Google Play.

Affected Users 

Based on our telemetry we observed that more affected users are in the American continent with the most activity in the USA, Brazil, and Argentina. In Europe, clients also reported the infection, especially in the UK, Spain, and Germany. 

Figure 11. McAfee detections Android/Xamalicious around the world 

Conclusion 

Android applications written in non-java code with frameworks such as Flutter, react native and Xamarin can provide an additional layer of obfuscation to malware authors that intentionally pick these tools to avoid detection and try to stay under the radar of security vendors and keep their presence on apps markets. 

Avoid using apps that require accessibility services unless there is a genuine need for use. If a new app tries to convince you to activate accessibility services claiming that it’s required without a real and reasonable reason and requesting to ignore the operative system warning, then it’s a red flag. 

The second stage payload might take control of the device because accessibility permissions are granted so any other permission or action can then be performed by the malware if these instructions are provided in the injected code. 

Because it is difficult for users to actively deal with all these threats, we strongly recommend that users install security software on their devices and always keep up to date. By using McAfee Mobile Security products, users can further safeguard their devices and mitigate the risks linked with these kinds of malware, providing a safer and more secure experience. 

Android/Xamalicious Samples Distributed on Google Play: 

Package Name  App Name  Installs 
com.anomenforyou.essentialhoroscope  Essential Horoscope for Android       100,000  
com.littleray.skineditorforpeminecraft  3D Skin Editor for PE Minecraft       100,000  
com.vyblystudio.dotslinkpuzzles  Logo Maker Pro       100,000  
com.autoclickrepeater.free  Auto Click Repeater          10,000  
com.lakhinstudio.counteasycaloriecalculator  Count Easy Calorie Calculator          10,000  
com.muranogames.easyworkoutsathome  Sound Volume Extender            5,000  
com.regaliusgames.llinkgame  LetterLink            1,000  
com.Ushak.NPHOROSCOPENUMBER  NUMEROLOGY: PERSONAL HOROSCOPE &NUMBER PREDICTIONS            1,000  
com.browgames.stepkeepereasymeter  Step Keeper: Easy Pedometer                500  
com.shvetsStudio.trackYourSleep  Track Your Sleep                500  
com.devapps.soundvolumebooster  Sound Volume Booster                100  
com.Osinko.HoroscopeTaro  Astrological Navigator: Daily Horoscope & Tarot                100  
com.Potap64.universalcalculator  Universal Calculator                100  

Indicators of Compromise 

SHA256  Package Name 
63cb930ab83afe80d40ba620a0f2ed5e8a55cceec23bdad919bf9dfa3d8c6e5b  com.android.accessibility.service 
7149acb072fe3dcf4dcc6524be68bd76a9a2896e125ff2dddefb32a4357f47f6  com.android.accessibility.service 
a5de2dc4e6005e75450a0df0ea83816996092261f7dac30b5cf909bf6daaced0  com.android.accessibility.service 
22803693c21ee17667d764dd226177160bfc2a5d315e66dc355b7366b01df89b  com.android.callllogbacup 
efbb63f9fa17802f3f9b3a0f4236df268787e3d8b7d2409d1584d316dabc0cf9  com.android.dreammusic 
e801844333031b7fd4bd7bb56d9fb095f0d89eb89d5a3cc594a4bed24f837351  com.android.statementsandservices 
6316edebe5995fc3e2715a44b78dcb2ec4f0409234851ee5dbb20c0fb60d1bf0  com.android.tvresources 
d3833f608e476ed24382bf3991cec503ebc7124481758bdb5e46390b367c1210  com.android.ui.clock 
5fffb10487e718634924552b46e717bbcbb6a4f9b1fed02483a6517f9acd2f61  com.android.ui.clock 
81a9a6c86b5343a7170ae5abd15f9d2370c8282a4ed54d8d28a3e1ab7c8ae88e  com.android.ui.clock 
ac9bb11cf71b11d3d50620660b56f1612e10e4ee9f7fc5637c0c281a688ef978  com.android.venting 
9c646516dd189cab1b6ced59bf98ade42e19c56fc075e42b85d597449bc9708b  com.android.version.shared 
a4e7279daf2261d74e108be9ea46373777adb209de06d229cee1f77f7196bfdf  com.android.version.shared 
177ff9e281f96bf08ff9724e0c92cecc7538692cadc217cc2770c668b451f2dc  com.android.version.shared 
488942923780887114471dd0114ffb144deb8054773351b2b2aac3c974c569d8  com.android.wall.lifetick 
dfdca848aecb3439b8c93fd83f1fd4036fc671e3a2dcae9875b4648fd26f1d63  com.anomenforyou.essentialhoroscope 
e7ffcf1db4fb13b5cb1e9939b3a966c4a5a894f7b1c1978ce6235886776c961e  com.autoclickrepeater.free 
8927ff14529f03cbb2ebf617c298f291c2d69be44a8efa4e0406dea16e53e6f9  com.autoclickrepeater.free 
117fded1dc51eff3788f1a3ec2b941058ce32760acf61a35152be6307f6e2052  com.browgames.stepkeepereasymeter 
28a4ae5c699a7d96e963ca5ceec304aa9c4e55bc661e16c194bdba9a8ad847b7  com.devapps.soundvolumebooster 
b0b9a8e9ec3d0857b70464617c09ffffce55671b227a9fdbb178be3dbfebe8ed  com.kolomia.mineskineditor 
899b0f186c20fdbfe445b4722f4741a5481cd3cbcb44e107b8e01367cccfdda3  com.lakhinstudio.counteasycaloriecalculator 
e52b65fdcb77ed4f5989a69d57f1f53ead58af43fa4623021a12bc11cebe29ce  com.lakhinstudio.counteasycaloriecalculator 
e694f9f7289677adaf2c2e93ba0ac24ae38ab9879a34b86c613dd3c60a56992d  com.littleray.skineditorforpeminecraft 
19ffe895b0d1be65847e01d0e3064805732c2867ce485dfccc604432faadc443  com.muranogames.easyworkoutsathome 
6a3455ff881338e9337a75c9f2857c33814b7eb4060c06c72839b641b347ed36  com.Osinko.HoroscopeTaro 
e6668c32b04d48209d5c71ea96cb45a9641e87fb075c8a7697a0ae28929913a6  com.Potap64.universalcalculator 
6953ba04233f5cf15ab538ae191a66cb36e9e0753fcaeeb388e3c03260a64483  com.regaliusgames.llinkgame 
f52917321b1010c0b145f699531947c84b26d63e91a5a67bf5131ba8904e1326  com.shvetsstudio.newmapsforminecraftpe 
cb8d4ae27b64c56831a4e9bb5f1d53a3a72eaea369ab7b40851009ed577d2b14  com.shvetsStudio.trackYourSleep 
01c56911c7843098777ec375bb5b0029379b0457a9675f149f339b7db823e996  com.shvetsStudio.trackYourSleep 
f42e79c4dc6b1573731610a1135cb0a71152e869c598215f0db09958f2d761f5  com.skladainc.phonedetectiveclaptofind 
e55408ef4f10715e7a521e95df038114a75f865f5b2ac3c1dcfee2261724b5df  com.skladainc.phonedetectiveclaptofind 
cc439631c09763003b92b80bbea2ff0f5d764694575ed7a9be155fbc420753db  com.skladainc.phonedetectiveclaptofind 
3201785a7de8e37e5d12e8499377cfa3a5b0fead6667e6d9079d8e99304ce815  com.turovskyi.magicofnumbers 
acb5de2ed2c064e46f8d42ee82feabe380364a6ef0fbfeb73cf01ffc5e0ded6b  com.Ushak.NPHOROSCOPENUMBER 
9b4dc1e80a4f4c798d0d87a52f52e28700b5b38b38a532994f70830f24f867ba  com.Ushak.NPHOROSCOPENUMBER 
1bfc02c985478b21c6713311ca9108f6c432052ea568458c8bd7582f0a825a48  com.vyblystudio.dotslinkpuzzles 

 

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