[DETECTION] JiaguK packer (classes.dex) detection

[ReBensk]

Sample
Sample1.zip
Sample2.zip

Describe the detection issue
The apk samples classes.dex files are packed but this is not detected by APKiD 2.1.5
From the classes.dex file structure
-> data_size(offset 0x68) + data_off(offset 0x6C) > file_size (offset 0x20)
packer code starts after the end of the data directory.
The packer using xor key 0x69 to decrypt data

APKiD current results...

[+] APKiD 2.1.5 :: from RedNaga :: rednaga.io
[*] /root/Desktop/Samples/Jiagu/Sample1.apk!classes.dex
 |-> compiler : unknown (please file detection issue!)
[*] /root/Desktop/Samples/Jiagu/Sample1.apk!assets/l16f56f57_a64.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample1.apk!assets/l16f56f57_a32.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample1.apk!assets/l16f56f57_x64.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample1.apk!assets/l16f56f57_x86.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample2.apk!classes.dex
 |-> compiler : unknown (please file detection issue!)
[*] /root/Desktop/Samples/Jiagu/Sample2.apk!assets/l676efb5b_a64.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample2.apk!assets/l676efb5b_a32.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample2.apk!assets/l676efb5b_x64.so
 |-> protector : Virbox
[*] /root/Desktop/Samples/Jiagu/Sample2.apk!assets/l676efb5b_x86.so
 |-> protector : Virbox

Detection rule:

rule jiagu_k : packer
{
  meta:
    description = "Jiagu K"
    sample1     = "aa666b75ffb3588dd41c8e546d53e353cda67cf278b167c7737b1169262856bb"
    sample2     = "d9baf66e7ac116a8c68599ef16fae5397ac4fd0847e2fcfe3ee2c155ecf4f850"

  strings:
    
    $attachBaseContextOpcodes = {

      7502 0100 1700   //invoke-super/range {v23, v24}, Landroid/app/Application;.attachBaseContext:(Landroid/content/Context;)V // method@0001
      6901 ????        //sput-object v1, Lv45e7a802/l45e7a802;.i:Landroid/content/Context; // field@000c
      7401 ???? 1800   //invoke-virtual/range {v24}, Landroid/content/Context;.getFilesDir:()Ljava/io/File; // method@0008
      0c03             //move-result-object v3
      6e10 ???? 0300   //invoke-virtual {v3}, Ljava/io/File;.getAbsolutePath:()Ljava/lang/String; // method@001b
      0c03             //move-result-object v3
      2204 ????        //new-instance v4, Ljava/io/File; // type@0015
      7020 ???? 3400   //invoke-direct {v4, v3}, Ljava/io/File;.<init>:(Ljava/lang/String;)V // method@0018
      6e10 ???? 0400   //invoke-virtual {v4}, Ljava/io/File;.exists:()Z // method@001a
      0a05             //move-result v5
      3905 0500        //if-nez v5, 0021 // +0005
      6e10 ???? 0400   //invoke-virtual {v4}, Ljava/io/File;.mkdir:()Z // method@001c
      2204 ????        //new-instance v4, Ljava/lang/StringBuilder; // type@0025
      7010 ???? 0400   //invoke-direct {v4}, Ljava/lang/StringBuilder;.<init>:()V // method@003c
      6e20 ???? 3400   //invoke-virtual {v4, v3}, Ljava/lang/StringBuilder;.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; // method@003d
      1243             //const/4 v3, #int 4 // #4
      2335 ????        //new-array v5, v3, [B // type@0036
      2605 ???? 0000   //fill-array-data v5, 0000075a // +0000072e
      7110 ???? 0500   //invoke-static {v5}, Lv45e7a802/l45e7a802;.h:([B)Ljava/lang/String; // method@0067
      0c05             //move-result-object v5
      6e20 ???? 5400   //invoke-virtual {v4, v5}, Ljava/lang/StringBuilder;.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; // method@003d
      6e10 ???? 0400   //invoke-virtual {v4}, Ljava/lang/StringBuilder;.toString:()Ljava/lang/String; // method@003e
      0c04             //move-result-object v4
      2205 ????        //new-instance v5, Ljava/io/File; // type@0015
      7020 ???? 4500   //invoke-direct {v5, v4}, Ljava/io/File;.<init>:(Ljava/lang/String;)V // method@0018
      6e10 ???? 0500   //invoke-virtual {v5}, Ljava/io/File;.exists:()Z // method@001a
      0a06             //move-result v6
      3906 0500        //if-nez v6, 0048 // +0005
      6e10 ???? 0500   //invoke-virtual {v5}, Ljava/io/File;.mkdir:()Z // method@001c
      7401 0500 1700   //invoke-virtual/range {v23}, Landroid/app/Application;.getPackageName:()Ljava/lang/String; // method@0005
      0c05             //move-result-object v5  
    }
 
    /**    
        public static String h(byte[] bArr) {
            for (int i2 = 0; i2 < bArr.length; i2++) {
                 bArr[i2] = (byte) (bArr[i2] ^ 105);
            }
            return new String(bArr, 0, bArr.length);
        }
    */

    $xor_key = {
      1200             //const/4 v0, #int 0 // #0 
      1201             //const/4 v1, #int 0 // #0
      2132             //array-length v2, v3
      3521 0c00        //if-ge v1, v2, 000f // +000c
      4802 0301        //aget-byte v2, v3, v1
      df02 0269        //xor-int/lit8 v2, v2, #int 105 // #69
      8d22             //int-to-byte v2, v2
      4f02 0301        //aput-byte v2, v3, v1
      d801 0101        //add-int/lit8 v1, v1, #int 1 // #01
      28f4             //goto 0002 // -000c
      2201 ????        //new-instance v1, Ljava/lang/String; // type@0024
      2132             //array-length v2, v3
      7040 ???? 3120   //invoke-direct {v1, v3, v0, v2}, Ljava/lang/String;.<init>:([BII)V // method@0035
      1101             //return-object v1
    }
  
  condition:
  
    is_dex and all of them
}

[enovella]

Please format the code properly ( I edited your post, please edit and observe how to format code properly)

[enovella]

This rule could match many other apps. Could you please find unique patterns with this packer?

[enovella]

Can you pull-request this rule?

[ReBensk]

This rule could match many other apps. Could you please find unique patterns with this packer?

Yes, I agree, These apps are using a unique xor key value 0x69 and also i noticed this string "Virbox" in all the samples (classes.dex), if you want to add this string I will, so the detection logic becomes ( jiagu packer + virbox protector ) Is it ok ?

[ReBensk]

Created pull-request for the rule - jiagu_k : packer
#375

[enovella]

This rule could match many other apps. Could you please find unique patterns with this packer?

Yes, I agree, These apps are using a unique xor key value 0x69 and also i noticed this string "Virbox" in all the samples (classes.dex), if you want to add this string I will, so the detection logic becomes ( jiagu packer + virbox protector ) Is it ok ?

Unique fingerprints are crucial to avoid false positives, if you always encountered the string Virbox or whateverStringYouWantToThinkOf please combine it with your opcodes in the rules.

[ReBensk]

Included the class name "Lvirbox/StubApp"