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 "calc.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.