Authored by Dexter Shin
Abstract
Cybercriminals are continuously evolving their methods to bypass safety measures. Not too long ago, the McAfee Cellular Analysis Staff found malware campaigns abusing .NET MAUI, a cross-platform improvement framework, to evade detection. These threats disguise themselves as reputable apps, focusing on customers to steal delicate data. This weblog highlights how these malware function, their evasion methods, and key suggestions for staying protected.
Background
In recent times, cross-platform cellular improvement frameworks have grown in recognition. Many builders use instruments like Flutter and React Native to construct apps that work on each Android and iOS. Amongst these instruments, Microsoft offers a framework based mostly on C#, referred to as Xamarin. Since Xamarin is well-known, cybercriminals generally use it to develop malware. We’ve beforehand discovered malware associated to this framework. Nonetheless, Microsoft ended assist for Xamarin in Could 2024 and launched .NET MAUI as its substitute.
In contrast to Xamarin, .NET MAUI expands platform assist past cellular to incorporate Home windows and macOS. It additionally runs on .NET 6+, changing the older .NET Normal, and introduces efficiency optimizations with a light-weight handler-based structure as an alternative of customized renderers.
As expertise evolves, cybercriminals adapt as properly. Reflecting this pattern, we just lately found new Android malware campaigns developed utilizing .NET MAUI. These Apps have their core functionalities written solely in C# and saved as blob binaries. Which means in contrast to conventional Android apps, their functionalities don’t exist in DEX recordsdata or native libraries. Nonetheless, many antivirus options give attention to analyzing these parts to detect malicious habits. In consequence, .NET MAUI can act as a kind of packer, permitting malware to evade detection and stay energetic on units for a very long time.
Within the following sections, we’ll introduce two Android malware campaigns that use .NET MAUI to evade detection. These threats disguise themselves as reputable companies to steal delicate data from customers. We’ll discover how they function and why they pose a big threat to cellular safety.
Am I protected?
McAfee Cellular Safety already detects all of those apps as Android/FakeApp and protects customers from these threats. For extra details about our Cellular Product, go to McAfee Cellular Safety.
Technical Findings
Whereas we discovered a number of variations of those malicious apps, the next two examples are used to show how they evade detection.
First off, the place are customers discovering these malicious apps? Usually, these apps are distributed by way of unofficial app shops. Customers are usually directed to such shops by clicking on phishing hyperlinks made obtainable by untrusted sources on messaging teams or textual content messages. Because of this we advocate at McAfee that customers keep away from clicking on untrusted hyperlinks.
Instance 1: Faux Financial institution App
The first pretend app we discovered disguises itself as IndusInd Financial institution, particularly focusing on Indian customers. When a consumer launches the app, it prompts them to enter private and monetary particulars, together with their identify, telephone quantity, e mail, date of delivery, and banking data. As soon as the consumer submits this information, it’s instantly despatched to the attacker’s C2 (Command and Management) server.
Determine 1. Faux IndusInd Financial institution app’s display screen requesting consumer data
As talked about earlier, this isn’t a standard Android malware. In contrast to typical malicious apps, there aren’t any apparent traces of dangerous code in the Java or native code. As a substitute, the malicious code is hidden inside blob recordsdata positioned contained in the assemblies listing.
Determine 2. Blob accommodates malicious code
The next code snippet reveals how the app collects and transmits consumer information to the C2 server. Based mostly on the code, the app constructions the required data as parameters earlier than sending it to the C2 server.
Determine 3. C# code answerable for stealing consumer information and sending it to the C2 server
Instance 2: Faux SNS App
In distinction to the first pretend app, this second malware is much more tough for safety to investigate. It particularly targets Chinese language-speaking customers and makes an attempt to steal contacts, SMS messages, and pictures from their units. In China, the place entry to the Google Play Retailer is restricted, such apps are sometimes distributed by way of third-party web sites or various app shops. This enables attackers to unfold their malware extra simply, particularly in areas with restricted entry to official app shops. 
Determine 4. Distribution website and faux X app focusing on Chinese language-speaking customers
One of many key methods this malware makes use of to stay undetected is multi-stage dynamic loading. As a substitute of immediately embedding its malicious payload in an simply accessible format, it encrypts and hundreds its DEX recordsdata in three separate phases, making evaluation considerably tougher.
Within the first stage, the app’s major exercise, outlined in AndroidManifest.xml, decrypts an XOR-encrypted file and hundreds it dynamically. This preliminary file acts as a loader for the subsequent stage. Within the second stage, the dynamically loaded file decrypts one other AES-encrypted file and hundreds it. This second stage nonetheless doesn’t reveal the core malicious habits however serves as one other layer of obfuscation. Lastly, within the third stage, the decrypted file accommodates code associated to the .NET MAUI framework, which is then loaded to execute the primary payload.
Determine 5. Multi-stage dynamic loading
The principle payload is finally hidden inside the C# code. When the consumer interacts with the app, akin to urgent a button, the malware silently steals their information and sends it to the C2 server.
Determine 6. C# code answerable for stealing photographs, contacts, and SMS information
Past multi-stage dynamic loading, this malware additionally employs extra tips to make evaluation tougher. One method is manipulating the AndroidManifest.xml file by including an extreme variety of pointless permissions. These permissions embrace massive quantities of meaningless, randomly generated strings, which might trigger errors in sure evaluation instruments. This tactic helps the malware evade detection by disrupting automated scanners and static evaluation.
Determine 7. AndroidManifest.xml file with extreme random permissions
One other key method is encrypted socket communication. As a substitute of utilizing commonplace HTTP requests, that are simpler to intercept, the malware depends on TCP socket connections to transmit information. This method makes it tough for conventional HTTP proxy instruments to seize community site visitors. Moreover, the malware encrypts the info earlier than sending it, which means that even when the packets are intercepted, their contents stay unreadable.
Yet another necessary facet to notice is that this malware adopts numerous themes to draw customers. Along with the pretend X app, we additionally found a number of courting apps that use the identical methods. These apps had totally different background photographs however shared the identical construction and performance, indicating that they have been probably created by the identical developer because the pretend X app. The continual emergence of comparable apps means that this malware is being broadly distributed amongst Chinese language-speaking customers.
Determine 8. Varied pretend apps utilizing the identical method
Suggestions and Conclusion
The rise of .NET MAUI-based malware highlights how cybercriminals are evolving their methods to keep away from detection. A number of the methods described embrace:
- hiding code blobs inside assemblies
- multi-stage dynamic loading
- encrypted communications
With these evasion methods, the threats can stay hidden for lengthy intervals, making evaluation and detection considerably more difficult. Moreover, the invention of a number of variants utilizing the identical core methods means that this sort of malware is changing into more and more widespread.
Customers ought to all the time be cautious when downloading and putting in apps from unofficial sources, as these platforms are sometimes exploited by attackers to distribute malware. That is particularly regarding in international locations like China, the place entry to official app shops is restricted, making customers extra susceptible to such threats.
To maintain up with the speedy evolution of cybercriminal ways, customers are strongly suggested to put in safety software program on their units and preserve it updated always. Staying vigilant and making certain that safety measures are in place might help defend towards rising threats. Through the use of McAfee Cellular Safety, customers can improve their gadget safety and detect threats associated to this sort of malware in real-time.
Glossary of Phrases
Indicators of Compromise (IOCs)
APKs:
C2:
- tcp[://]120.27.233.135:1833
- https[://]onlinedeskapi.com
