Stealth Backdoor “Android/Xamalicious” Actively Infecting Gadgets

Authored by Fernando Ruiz 

McAfee Cell Analysis Group 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 achieve accessibility privileges with social engineering after which it communicates with the command-and-control server to judge whether or not or to not obtain a second-stage payload that’s dynamically injected as an meeting DLL at runtime degree to take full management of the machine and doubtlessly carry out fraudulent actions resembling clicking on adverts, putting in apps amongst different actions financially motivated with out consumer consent.

The second stage payload can take full management of the contaminated machine because of the highly effective accessibility providers that have been already granted throughout the first stage which additionally accommodates features to self-update the principle APK which signifies that it has the potential to carry out any kind of exercise like a spy ware or banking trojan with out consumer interplay. Nonetheless, we recognized a hyperlink between Xamalicious and the ad-fraud app “Money Magnet” which robotically clicks adverts, installs apps, and different actions to fraudulently generate income whereas customers that put in it could earn factors which can be imagined to be redeemable as a retail reward card. Which means the builders behind these threats are financially motivated and drive ad-fraud subsequently this is perhaps one of many fundamental payloads of Xamalicious.

The utilization of the Xamarin framework allowed malware authors to remain lively and with out detection for a very long time, profiting from 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 knowledge 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 Doubtlessly Dangerous Functions (PHAs) and cease them earlier than they ever make it onto Google Play. Android customers are protected by Google Play Defend, which might warn customers of recognized malicious apps on Android units. McAfee Cell Safety detects this risk as Android/Xamalicious.  

Based mostly on the variety of installations these apps could have compromised a minimum of 327,000 units from Google Play plus the installations coming from third-party markets that regularly produce new infections based mostly on the detection telemetry of McAfee shoppers all over the world. This risk stays very lively. 

Determine 1. “Rely Straightforward Calorie Calculator” was out there 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 out there 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 a different 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 instance. As soon as began it instantly requests the sufferer to allow accessibility providers for “appropriate work” and offers 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 resembling 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 normal 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 perhaps embedded right into a BLOB file, or instantly out there 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 degree. In easy phrases, which means that in some samples the reversing of the DLL assemblies is easy whereas in others it requires additional steps to unpack them. 

The malicious code is normally out there 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 maintain the unique code out there.  

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 Machine Info 

Android/Xamalicious collects a number of machine knowledge together with the checklist of put in functions obtained through system instructions to find out if the contaminated sufferer is an effective goal for the second stage payload. The malware can gather location, service, and community data amongst machine rooting standing, adb connectivity configuration, for example, if the machine is related through ADB or is rooted, the C2 won’t present a second-stage payload DLL for obtain. 

Information Encryption in JWT 

To evade evaluation and detection, malware authors encrypted all communication and knowledge transmitted between the C2 and the contaminated machine, not solely protected by HTTPS, it’s encrypted as a JSON Internet 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 offered throughout the evaluation. 

Within the Ship() perform Android/Xamalicious first prepares the acquired object, normally a JSON construction calling the perform encrypt() which creates the JWT utilizing a hardcoded RSA key. So the information is exfiltrated absolutely encrypted to the malware host pointing to the trail  “/Updater” through HTTP POST methodology. 

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

Encrypt Methodology: 

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

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

C2 analysis

Collected knowledge is transmitted to the C&C to find out if the machine 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 won’t ship the second stage payload if the machine is rooted or related as ADB through 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 machine is “Okay” to obtain the malicious library it would 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 machine 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 Internet 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 machine and its string of 32 chars of size accommodates appended the machine ID, model, mannequin, and a hardcoded padding of “1” as much as 32 chars of size. 

As an illustration, if the machine ID is 0123456ABCDEF010 and the affected machine is a Pixel 5, then the AES secret is: “0123456ABCDEF010googlePixel 5111” 

Which means 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 below 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 machine could be absolutely compromised as a result of as soon as accessibility permissions are granted, it could actually obverse and work together with any exercise opening a backdoor to any kind of malicious exercise. 

In the course of 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 entire malicious second stage that is perhaps restricted to a particular service, language, app put in, location, time zone or unknown situations of the affected machine, nonetheless, we will guarantee that this can be a high-risk backdoor that leaves the likelihood to dynamically execute any command on the affected machine 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 out there on Google Play on the finish of 2020, with a guide 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 internet online affiliate marketing. This software affords customers factors which can be imagined to be redeemable by retail reward playing cards or cryptocurrency.

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

Determine 8b. LetterLink settlement for Money Magnet

Initially revealed 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 30 days operating automated adverts. Because it was eliminated by Google the authors then infiltrated LetterLink and extra just 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 non-public 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, out there till the tip of 2023

“Dots: One Line Connector” app just isn’t a sport, the screenshot revealed by Google Play doesn’t correspond to the appliance habits as a result of as soon as it’s began it simply asks for authentication credentials with none emblem 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 all over the world 

Conclusion 

Android functions written in non-java code with frameworks resembling Flutter, react native and Xamarin can present a further layer of obfuscation to malware authors that deliberately decide these instruments to keep away from detection and attempt to keep below the radar of safety distributors and maintain their presence on apps markets. 

Keep away from utilizing apps that require accessibility providers except 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 with out a actual and cheap cause and requesting to disregard the operative system warning, then it’s a purple flag. 

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

As a result of it’s troublesome for customers to actively take care of all these threats, we strongly advocate that customers set up safety software program on their units and at all times maintain updated. Through the use of McAfee Cell Safety merchandise, customers can additional safeguard their units 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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