MalBus Actor Changed Market from Google Play to ONE Store

By and on Apr 09, 2020

McAfee Mobile Research team has found another variant of MalBus on an education application, developed by a South Korean developer. In the previous Malbus case, the author distributed the malware through Google Play, but new variants are distributed via the ONE Store in much the same way. ONE Store is a joint venture by the country’s three major telecom companies and is a preinstalled app on most Android phones selling in South Korea. It has 35 million users (close to 70% of South Korea’s population) and has already surpassed Apple’s app store sales from the end of 2018.

The application in question is distributed via Google Play and the ONE Store at the same time. The malicious application downloads and runs an encrypted payload with malicious functions.

McAfee Mobile Security detects this threat as Android/Malbus and alerts mobile users if it is present, while protecting them from any data loss.

Figure 1. Screen capture from the application page on the ONE Store

The Campaign

We found malicious code injected by an attacker, via the developer’s account, into versions 27 and 28 of the application distributed through the ONE Store. The App Signature Certificate for versions 26 through 29 distributed from the One Store are the same. No other application developed by the same author was found on the ONE Store. The ONE Store is now servicing version 29 which does not contain malicious code. Google Play still offers version 26, though this is also clear of infection.

Figure 2. Infected version history of the application

The overall flow of this application, focusing on the malicious function, is explained below:

Figure 3. Overview of malicious behavior

After the malware is installed, the malicious code has a latent period of 10 hours to avoid being discovered by dynamic analysis.

Figure 4. Using LastUpdateTime to check latent period

After the latent period, it starts two threads. The first one loads native library “libmovie.so” and calls one of its exported functions, “playMovie”, with a phone number as an argument while the second one creates a Java server socket for communication with another native library.

Figure 5. The malicious native library embedded in the APK

The first loaded library, libmovie.so, contains a curl binary and URLs for secondary payloads in XOR encoded data which are decoded at runtime. The XOR value is 0x8E and it is globally used in this library. All decoded URLs appear to have been hacked and the decoded URLs drop RC4 encrypted ELF files.

 

Table 1. Contents in libmovie.so

Simply put, libmovie.so is a downloader and executer. It downloads the next payload from a hacked web server by using a dropped curl binary, decrypts it and loads the library. Once the library is loaded, the downloaded file is deleted to avoid detection. Lastly, the downloaded code starts from exported function name “Libfunc”.

Figure 6. The main flow of libmovie.so

As for the RC4 cryptographic library, encryption is the most common way to hide or protect important things. Accordingly, it is assumed that there is some important in this file.

Table 2. Additional information of downloaded file

The file sizes and data for szServer_XX_1 and szServer_XX_2 are the same as shown in Table 2. But szServer_XX_3 has several functions that are added, removed or modified a little bit. However, it does not affect the overall process.

“doMainProc” is the core function called by “Libfunc”. The first job of the “doMainProc” is selecting the C2 server randomly.

Table 3. C2 server list

After selecting the C2 server, a randomly created TUID is sent to the server. Guessing from its usage, the TUID might be a target device ID to manage contaminated targets. Now the application is working as a spy agent, waiting for actions from the selected server and ready to execute commands. We discovered the following available commands:

 

Table 4. Available command list

Among the malicious commands, an eye-catching feature is SMS and MMS capturing. SMS and MMS are saved in the “/data/data/<package name>/files/” directory as file name “sms.txt” and “mms.txt” respectively.

Figure 7. SMS and MMS are saved in txt files

This feature can be activated by registering the Android receiver.

Figure 8. RegisterReceiver

This malicious app opens TCP port 1111 locally to communicate with the loaded native library. Below is manually interpreted Java code:

public void run()

{

CommunicationThread commThread;

Socket socket = null;

serverSocket = new ServerSocket(sock_port); // sock_port = 1111

if (serverSocket) {

while ((!Thread.currentThread().isInterrupted())) {

commThread = new CommunicationThread(this, serverSocket.accept());

new Thread(commThread).start();

}

}

}

Figure 9. com.joojang.CharacterClassic.MainService

The SMS/MMS capture feature is enabled when receiving a “SET” string on local TCP 1111 port and disabled by receiving “FREE”.

Figure 10. SET or FREE

The loaded native library connects when the “SD_SetSMSCapture” command receives and sends “SET”

Figure 11. SD_SetSmsCapture

Below is interpreted as C language.

client = socket(AF_INET, SOCK_STREAM, 0);

addr.sin_family = AF_INET;

addr.sin_port = htons(1111);

addr.sin_addr.s_addr= inet_addr(“127.0.0.1”);

One other function we have not seen before is “SD_LoadSoFile”. This loads a new native library and executes a specific function in it. This function seems to change the running native library to a newer one when the current binary has a problem, or to add new features.

Figure 12. SD_LoadSoFile

Compared to Malbus

This newly discovered malicious code has many similarities compared to Malbus, such as using the same malicious function name starting with “SD_”, file name, XOR’ed strings to hide original strings, embedded files in libraries, command ids, the same version of compiler and so on. It also has the differences mentioned above: downloading a malicious library directly instead of installing a plugin APK and no sensitive keyword list such as ‘North Korea’, ‘National Defense’ and so on.

Figure 13. The identical function names from the malicious library

Figure 14. The same version of compiler strings

Conclusion

Malware can be distributed through all manner of third-party app stores, not only official ones such as Google Play. This malware is carefully prepared – the final payload of the malware is the file that was hacked and uploaded to the vulnerable server before malware distribution. We believe the authors of this malware will continue to buy or hack trusted developer accounts to update malicious functionality, infiltrate and distribute through official app stores. As with previous cases, users should verify the applications they install, even if they download them from official stores. McAfee is working with Korean law enforcement agencies to help with the takedown of the attack campaign.

 

Hashes (SHA-256)

Initial Downloader (APKs)

  • 5e57bc8d83a372bf4d046c272cd43db9000036c9b32d8eecead1af75f4958c57
  • 1613b35c73c6497730490d7712ac015c2b42931446aed149e1292e2ba77d0ff4

Encrypted Trojan (additional payload)

  • d328373cd67c467485b9c96349a0ee08fc3b58fe2c11fb19f4dcb9ea6c7a0dae
  • c5bff68022ead6302f710f1ce1c3d5682a8cd3610b1f8ed2563098d7ac4e1909
  • c410cacbb0be8f649f082148c91f4cef27f101b8db3ce64a02882506c9b51a63
  • 178dddf38ec232d540bd88320521d8134644da1e7af19e7ae295b2d614e3ab56

Decrypted Trojan (additional payload)

  • 9fc914545fbb99b7e0d4a5207f5a2b32a8a127a36caa9159d4feeac445c509f7
  • df651ac1bfd60cd29cea85cc410002b933552260c2439fe86a4f32486abd0828
  • 63d10c9cd105c7b17effef18d31d571fe4c9c999966cc09bdb40df07c1b6baa8
  • f99212b70729942923fe26b996791cdd8eb561f8ae017e1d71202fbb97f7d245

About the Author

Chanung Pak

Chanung is a Security Researcher on McAfee's Mobile Research Team. Previously focused on finding new vulnerabilities in both software and hardware. He specializes in mobile threats and malware, and focuses on reverse-engineering of embedded devices and IOT.

Read more posts from Chanung Pak

Sang Ryol Ryu

Ryu, SangRyol is a mobile malware researcher at McAfee, where he specializes in the analysis of mobile threats and Android malware. He performs both static and dynamic analysis of suspicious applications to support McAfee’s Mobile Security. SangRyol is also skilled at finding new threats on embedded devices.

Read more posts from Sang Ryol Ryu

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