Authored by Fernando Ruiz 

McAfee Cell Analysis Workforce recognized an Android backdoor carried out with Xamarin, an open-source framework that permits constructing Android and iOS apps with .NET and C#. Dubbed Android/Xamalicious it tries to realize accessibility privileges with social engineering after which it communicates with the command-and-control server to guage whether or not or to not obtain a second-stage payload that’s dynamically injected as an meeting DLL at runtime stage to take full management of the system and probably carry out fraudulent actions similar to clicking on adverts, putting in apps amongst different actions financially motivated with out person consent.

The second stage payload can take full management of the contaminated system as a result of highly effective accessibility providers that have been already granted throughout the first stage which additionally accommodates features to self-update the primary APK which implies that it has the potential to carry out any sort of exercise like a adware or banking trojan with out person interplay. Nonetheless, we recognized a hyperlink between Xamalicious and the ad-fraud app “Money Magnet” which mechanically clicks adverts, installs apps, and different actions to fraudulently generate income whereas customers that put in it might earn factors which are speculated to be redeemable as a retail reward card. Because of this the builders behind these threats are financially motivated and drive ad-fraud due to this fact this is likely to be one of many important payloads of Xamalicious.

The utilization of the Xamarin framework allowed malware authors to remain lively and with out detection for a very long time, making the most of the construct course of for APK recordsdata that labored as a packer to cover the malicious code. As well as, malware authors additionally carried out completely different obfuscation strategies and customized encryption to exfiltrate information and talk with the command-and-control server. 

We’ve recognized about 25 completely different malicious apps that carry this risk. Some variants have been distributed on Google Play since mid-2020. The apps recognized on this report have been proactively eliminated by Google from Google Play forward of our reporting. McAfee is a member of the App Protection Alliance and an lively accomplice within the malware mitigation program, which goals to rapidly discover Probably Dangerous Functions (PHAs) and cease them earlier than they ever make it onto Google Play. Android customers are protected by Google Play Shield, which may warn customers of recognized malicious apps on Android gadgets. McAfee Cell Safety detects this risk as Android/Xamalicious.  

Based mostly on the variety of installations these apps could have compromised not less than 327,000 gadgets from Google Play plus the installations coming from third-party markets that frequently produce new infections based mostly on the detection telemetry of McAfee shoppers world wide. This risk stays very lively. 

Determine 1. “Rely Simple Calorie Calculator” was accessible on Google Play on August 2022 and carries Android/Xamalicious 

Android/Xamalicious trojans are apps associated to well being, video games, horoscope, and productiveness. Most of those apps are nonetheless accessible for obtain in third-party marketplaces.  

Beforehand we detected malware abusing Xamarin framework such because the open-sourced AndroSpy and forked variations of it, however Xamalicious is carried out in another way. Technical particulars about Xamarin structure are nicely documented and element how .NET code is interpreted by Android utilizing Mono. 

Acquiring Accessibility Companies

Let’s use the app “Numerology: Private horoscope & Quantity predictions” for example. As soon as began it instantly requests the sufferer to allow accessibility providers for “right work” and gives instructions to activate this permission:  

Determine 2. Tricking customers into granting accessibility providers permission 

Customers have to manually activate the accessibility providers after a number of OS warnings similar to the next on the accessibility choices: 

Determine 3. Accessibility providers configuration immediate highlights the dangers of this permission. 

The place is the malicious code? 

This isn’t the standard Java code or native ELF Android software, the malware module was written initially in .NET and compiled right into a dynamic hyperlink library (DLL). Then it’s LZ4 compressed, and it is likely to be embedded right into a BLOB file, or immediately accessible within the /assemblies listing on the APK construction. This code is loaded then by a local library (ELF) or by the DEX file at runtime stage. In easy phrases, which means that in some samples the reversing of the DLL assemblies is simple whereas in others it requires additional steps to unpack them. 

The malicious code is often accessible in two completely different meeting recordsdata within the /assemblies listing on the apk. Often, file names are core.dll and a <package-specific>.dll.

Some malware variants has obfuscated the DLL assemblies to keep away from evaluation and reversing of the malicious code whereas others preserve the unique code accessible.  

Determine 4. Core.dll and GoogleService.dll comprise malicious code. 

Communication with the command-and-control server

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

Determine 5. App execution and communication with the malicious server 

Accumulate Gadget Data 

Android/Xamalicious collects a number of system information together with the listing of put in purposes obtained by way of system instructions to find out if the contaminated sufferer is an efficient goal for the second stage payload. The malware can acquire location, service, and community data amongst system rooting standing, adb connectivity configuration, for example, if the system is related by way of ADB or is rooted, the C2 is not going to present a second-stage payload DLL for obtain. 

Information Encryption in JWT 

To evade evaluation and detection, malware authors encrypted all communication and information transmitted between the C2 and the contaminated system, not solely protected by HTTPS, it’s encrypted as a JSON Net Encryption (JWE) token utilizing RSA-OAEP with a 128CBC-HS256 algorithm nonetheless the RSA key values utilized by the Xamalicious are hardcoded within the decompiled malicious DLL so decryption of transmitted data is feasible if C2 infrastructure is on the market throughout the evaluation. 

Within the Ship() operate Android/Xamalicious first prepares the acquired object, often a JSON construction calling the operate encrypt() which creates the JWT utilizing a hardcoded RSA key. So the info is exfiltrated totally encrypted to the malware host pointing to the trail  “/Updater” by way of HTTP POST methodology. 

Then it waits for the C2 response and passes it to the decrypt() operate which has a hardcoded RSA personal key to correctly decrypt the acquired command which could comprise a second stage payload for the “getURL” command. 

Encrypt Technique: 

Determine 6. Encrypt operate with hardcoded RSA Key values as XML string 

The decryption methodology can be hardcoded into malware which allowed the analysis staff to intercept and decrypt the communication from the C2 utilizing the RSA key values offered as XML string it’s potential to construct a certificates with the parameters to decrypt the JWE tokens content material. 

C2 analysis

Collected information is transmitted to the C&C to find out if the system is a correct goal to obtain a second-stage payload. The self-protection mechanism of the malware authors goes past conventional emulation detection and nation code operator limitations as a result of on this case, the command-and-control server is not going to ship the second stage payload if the system is rooted or related as ADB by way of USB or doesn’t have a SIM card amongst a number of different atmosphere validations. 

DLL Customized Encryption 

With the getURL command, the contaminated consumer requests the malicious payload, if the C&C Server determines that the system is “Okay” to obtain the malicious library it’ll encrypt a DLL with Superior encryption customary (AES) in Cipher block chaining (CBC) utilizing a customized key for the consumer that requested it based mostly on the system id and different parameters defined under to decrypt the code because it’s a symmetric encryption methodology, the identical key works for encryption and decryption of the payload. 

Delivers the Payload in JWT 

The encrypted DLL is inserted as a part of the HTTP response within the encrypted JSON Net Token “JWT”. Then the consumer will obtain the token, decrypt it, after which decrypt the ‘url’ parm with AES CBC and a customized key.  

The AES key used to decrypt the meeting is exclusive per contaminated system and its string of 32 chars of size accommodates appended the system ID, model, mannequin, and a hardcoded padding of “1” as much as 32 chars of size. 

As an example, if the system ID is 0123456ABCDEF010 and the affected system is a Pixel 5, then the AES key’s: “0123456ABCDEF010googlePixel 5111” 

Because of this the DLL has a number of layers of encryption. 

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

All these efforts are associated to hiding the payload and making an attempt to remain underneath the radar the place this risk had relative success since some variants might need been lively years in the past with out AV detections. 

DLL Injected 

Xamalicious will title this DLL “cache.bin” and retailer it within the native system to lastly dynamically load it utilizing the Meeting.Load methodology. 

As soon as the second stage payload has been loaded the system will be totally compromised as a result of as soon as accessibility permissions are granted, it may possibly obverse and work together with any exercise opening a backdoor to any sort of malicious exercise. 

Throughout the evaluation, the downloaded second stage payload contained a DLL with the category “MegaSDKXE” which was obfuscated and incomplete most likely as a result of the C2 didn’t obtain the anticipated params to supply the whole malicious second stage that is likely to be restricted to a selected service, language, app put in, location, time zone or unknown situations of the affected system, nonetheless, we are able to guarantee that this can be a high-risk backdoor that leaves the likelihood to dynamically execute any command on the affected system not restricted to spying, impersonation or as a financially motivated malware. 

Money Magnet Advert-Fraud and Xamalicious

One of many Xamalicious samples detected by McAfee Cell generic signatures was “LetterLink” (com.regaliusgames.llinkgame) which was accessible on Google Play on the finish of 2020, with a e book icon. It was poorly described as a hidden model of “Money Magnet”: An app that performs ad-fraud with automated clicker exercise, apps downloads, and different duties that result in monetization for affiliate internet marketing. This software gives customers factors which are speculated to be redeemable by retail reward playing cards or cryptocurrency.

Determine 8a. LetterLink login web page after working the app for the primary time.

Determine 8b. LetterLink settlement for Money Magnet

Initially printed in 2019 on Google Play, “Money Magnet” (com.uicashmagnet) was described as a passive revenue software providing customers to earn as much as $30 USD per thirty days working automated adverts. Because it was eliminated by Google the authors then infiltrated LetterLink and extra lately “Dots: One Line Connector” (com.orlovst.dots) that are hidden variations of the identical ad-fraud scheme.

Determine 9. LetterLink Icon that hides Money Magnet

“LetterLink” performs a number of Xamalicious actions because it accommodates the “core.dll” library, it connects to the identical C2 server, and it makes use of the identical hardcoded personal RSA certificates to construct the JWE encrypted tokens which offer a non-repudiation proof that the builders of Money Magnet are behind Xamalicious.

Determine 10. Money Magnet infiltrated the app as a Sport, accessible till the tip of 2023

“Dots: One Line Connector” app just isn’t a sport, the screenshot printed by Google Play doesn’t correspond to the appliance conduct as a result of as soon as it’s began it simply asks for authentication credentials with none brand or reference to Money Magnet. “Dots” doesn’t comprise the identical DLLs as its predecessor, nonetheless the communication with the C2 is analogous utilizing the identical RSA key parameters. We reported this app to Google they usually promptly eliminated it from Google Play.

Affected Customers 

Based mostly on our telemetry we noticed that extra affected customers are within the American continent with essentially the most exercise within the USA, Brazil, and Argentina. In Europe, shoppers additionally reported the an infection, particularly within the UK, Spain, and Germany. 

Determine 11. McAfee detections Android/Xamalicious world wide 

Conclusion 

Android purposes written in non-java code with frameworks similar to Flutter, react native and Xamarin can present an extra layer of obfuscation to malware authors that deliberately decide these instruments to keep away from detection and attempt to keep underneath the radar of safety distributors and preserve their presence on apps markets. 

Keep away from utilizing apps that require accessibility providers until there’s a real want to be used. If a brand new app tries to persuade you to activate accessibility providers claiming that it’s required and not using a actual and cheap purpose and requesting to disregard the operative system warning, then it’s a pink flag. 

The second stage payload would possibly take management of the system as a result of accessibility permissions are granted so every other permission or motion can then be carried out by the malware if these directions are offered within the injected code. 

As a result of it’s tough for customers to actively cope with all these threats, we strongly suggest that customers set up safety software program on their gadgets and at all times preserve updated. By utilizing McAfee Cell Safety merchandise, customers can additional safeguard their gadgets and mitigate the dangers linked with these sorts of malware, offering a safer and safer expertise. 

Android/Xamalicious Samples Distributed on Google Play: 

Indicators of Compromise 

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