Frida - access to a class attribute that has the desired type

user_12984381 0
Viewed 2707

I have an android program that has been obfuscated. And in this program classes have attributes with the same name. Decompiled code like this

public class d implements c
{
    public int a;
    public Cache$Entry a;
    public Cache a;
    public volatile a a;
    public e a;
    public ByteArrayOutputStream a;
    public volatile AtomicBoolean a;

or smali code like this

# interfaces
.implements Le/a/x/c;
# instance fields
.field public a:I
.field public a:Lanetwork/channel/cache/Cache$Entry;
.field public a:Lanetwork/channel/cache/Cache;
.field public volatile a:Ld/a/w/a;
.field public a:Le/a/x/e;
.field public a:Ljava/io/ByteArrayOutputStream;
.field public volatile a:Ljava/util/concurrent/atomic/AtomicBoolean;

I create a hook to one method asd() and i need to access to attribute "a" of this class. But I need attribute "a" with type "e.a.x.e"

Java.perform(function () {
   var var_ddd = Java.use("e.a.x.d");
    var_ddd.asd.implementation = function() {
       this.asd();
       console.log("e.a.x.d.asd()",Java.cast(this.a.value,Java.use("e.a.x.e")));
    };
});

When I try to write this.a.value - I get a wrong attribute. When I write Java.cast(this.a.value,Java.use("e.a.x.e")) I get message

TypeError: cannot read property 'hasOwnProperty' of undefined

Please tell me how to get the right attribute with the right type

frida android security javascript java
2 Answers

In case there is a conflict between a method and a field of the same name Frida has built in workaround: Prepend the field name with an underscore: _a.

If there is a name collision, method & member has the same name, an underscore will be added to member.

But I am not sure if this information is still valid. The current Frida Java bridge code does not like it would rename fields with colliding field names: https://github.com/frida/frida-java-bridge/blob/master/lib/class-factory.js#L301

I also do't see a way to access the fields in Frida in a way that don't base on it's name.

The only chance I see is accessing the field via Java reflection:

const eaxe = Java.use("e.a.x.e");
for (f of eaxe.class.getDeclaredFields()) {
    if (f.getType().getName() == "e.a.x.e") {
        f.setAccessible(true);
        var fieldValue = f.get(this);
        console.log("Field of type e.a.x.e has value: " + fieldValue);
    }
}

Note: The code above has not been tested in Frida, therefore it may need some more refinement before it actually works.

user_1509780

Thanks to Robert, a solution was found.The code made minor corrections

var lo_fld_eaxe;
var lv_found = false;
var lt_fields = this.getClass().getDeclaredFields();
for (var i = 0; i < lt_fields.length && lv_found == false; i++) {
    if(lt_fields[i].getName().toString() == 'a' &&  lt_fields[i].getType().getName().toString() == 'e.a.x.e' ){
       lo_fld_eaxe = lt_fields[i];
       lv_found = true; 
  }
}
if(lv_found == true) {
   lo_fld_eaxe.setAccessible(true);
   try{ 
          var       lv_e_a_x_e = lo_fld_eaxe.get(this);   
   }
   catch(err){
          console.log("Error:"+err);
   }
 }
user_129843810
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