bypassing ms windows memory protections shahriyar jalayeri [email protected] 1 st iranian...

Bypassing MS Windows Memory Protections

Shahriyar [email protected]

1st Iranian Confrence on Cybespace Security Incidents and Vulnerabilities

Who am I?

• Security researcher since 2005• Senior Member of Snoop Security Researching

community• Writer and Technical Editor of Snoop Magazine ( First

Persian Technical Information Security Magazine )• University student

What is this talk about?

• Overview of Softwares Security Flaws• Deep looking at OS protection mechanismes• Explaining ways to bypass some of OS PM

Security Flaws

• Stack Based Buffer Overflow• Heap Based Buffer Owerflow• Format String• etc

Stack Based Buffer Overflow

• an old story – Programer Copy Buffer entry into stack without

bound checking– Stack corrupted and FramePointer+Saved Return

Address overwritten– The BAD ret Instruction

Stack Based Buffer Overflow ( Cont. )

– Stack.c

#include <stdio.h>Int main( int argc, char **argv){

char buff[100];strcpy(buff,argv[1]);printf(“Hello , %s!\n”,buff);return 0;

}

Overflow occured here


Args--------------------

--Saved Ret

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

Saved FP--------------------

--BUFFER

Stac

k La

yout

0xHighMemAddr0xLowStackAddr

Args--------------------

--Saved Ret

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

Saved FP--------------------

--BUFFER

0xLowMemAddr0xHighStackAddr

AAAAAAAAAAAA

Overflow


– The BAD ret instruction ( and also leave :D ) 004012DC |. 83C4 10 ADD ESP,10004012DF |. C9 LEAVE004012E0 \. C3 RETN

Args--------------------

--Saved Ret

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

Saved FP--------------------

--BUFFER

EIP

Boom

AAAAAAAAAAAA

Overflow

Heap Overflow

– Heap Structure • LIST_ENTRY Structure ( WinNT.h )

• FreeList array– Pointer to free heap blocks

typedef struct _LIST_ENTRY { struct _LIST_ENTRY *Flink; struct _LIST_ENTRY *Blink;} LIST_ENTRY, *PLIST_ENTRY, *RESTRICTED_POINTER PRLIST_ENTRY;

Heap Overflow ( cont. )

– Heap.c

HANDLE h = HeapCreate(0, 0, 0);DWORD vulner(LPVOID str) {

LPVOID mem = HeapAlloc(h, 0, 128);// <..>strcpy(mem, str);// <..>LPVOID mem2 = HeapAlloc(h, 0, 128);// <..>return 0;

}

Overflow

Affected Memory

Heap Overflow ( cont. )

– Freeing Scenario and Pointers ( without Safeunlinking )

BOOLEAN RemoveEntryList(IN PLIST_ENTRY Entry){ PLIST_ENTRY Blink; PLIST_ENTRY Flink; Flink = Entry->Flink; // what Blink = Entry->Blink; // where Blink->Flink = Flink; // *(where) = what Flink->Blink = Blink; // *(what+4) = where return (BOOLEAN)(Flink == Blink);}

Heap Overflow ( Cont. )

– Freeing Scenario and Pointers ( without Safeunlinking )

• Disass...

– Exploitation Scenario

mov dword ptr [ecx],eax mov dword ptr [eax+4],ecxEAX - Flink ECX - Blink

ECX = Function PointerEAX = Instruction which transfer execution to shellcode

Heap Overflow ( Cont. )

– Function pointers which can overwrite during unlinking ( ECX )• Unhandled Exception Filter• Vector Exception Handling ( VEH )• PEB Pointers• TEB Pointers• Your own pointer !

Format String

– Using format specified functions in wrong way• Printf(buffer)

– We are able to overwrite arbitrary address• Mentioned pointers can be good target• Saved Return Address !!!

Memory Protections

• Protections against buffer overflow– Guard Stack ( a.k.a /GS )– Data Execution Prevention ( a.k.a DEP or

/NXCOMPAT )– Safe SEH Table ( a.k.a /SafeSEH )– SEH Overwrite Protection ( a.k.a SEHOP )– Address Space Layout Randomization ( a.k.a ASLR

or /DYNAMICBASE )

Guard Stack ( /GS ) Protection

– Compiler Insert Cookie after Function Stack Frame– There is also a copy of cookie in .data section– Before function return cookie check with .data

saved cookie– If dose not match , Ret instruction never execute– In overflow situation we have to overwrite cookie

befor reaching “Saved Return Address”– Now what ?

Guard Stack ( /GS ) Protection ( cont. )• Memory Layout

Parameters

Return Address

Frame Pointer

Locals

Parameters

Return Address

Frame Pointer

Exception Handler Frame

Locals

Parameters

Return Address

Frame Pointer

Exception Handler Frame

Locals

Safe Parameters

CookieCookie

(1)

(2)

(3)

(1) Without Cookie(2) MSVC++ 2003(3) MSVC++ 2005

Guard Stack ( /GS ) Protection ( cont. )

– Q : What is Safe Parameters ?– A : Function pointer overwrite prevention trick

– Prevention is better than cure :D

Para

met

ers

Retu

rn A

ddre

ss

Fram

e Poin

ter

Exce

ption

Handle

r Fra

me

Loca

lsSa

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Para

met

ers

Cook

ie

#include <windows.h>int main(int argc, char *argv[]){ HMODULE Lib=NULL; unsigned int FuncAddress = 0; int err = 0; Lib = LoadLibrary("msvcrt.dll"); FuncAddress = GetProcAddress(Lib,"printf"); err = DoStuff(argv[1],FuncAddress); return 0;}int DoStuff(char *buf, FARPROC fnk){ char buffer[20]=""; strcpy(buffer,buf); (fnk)(buffer); __asm add esp,4 return 0;}


– When /GS dose not protect your Code ?• The optimization (O) option is not enabled• The function does not contain a stack buffer• The function is marked with naked in C++• The function has a variable argument list• The function begins with inline assembly code• The compiler determines that the function’s variables

are used only in ways that are less likely to be exploitable


– Implementation• ___security_init_cookie()• Protected function prologue changes• Protected function epilogue changes• __security_check_cookie()• ___report_gsfailure()

Guard Stack ( /GS ) Protection Reversing in Action

Just trigger IDA

•/G

S Reversing–

___security_init_cookie()

Security Init Cookievoid __cdecl __security_init_cookie(void){

UINT_PTR cookie; if (__security_cookie != DEFAULT_SECURITY_COOKIE && (__security_cookie & 0xFFFF0000) != 0 ){ __security_cookie_complement = ~__security_cookie; return; }

GetSystemTimeAsFileTime(&systime.ft_struct);cookie = systime.ft_struct.dwLowDateTime;

cookie ^= systime.ft_struct.dwHighDateTime; cookie ^= GetCurrentProcessId(); cookie ^= GetCurrentThreadId(); cookie ^= GetTickCount(); QueryPerformanceCounter(&perfctr);

cookie ^= perfctr.LowPart; cookie ^= perfctr.HighPart;

if (cookie == DEFAULT_SECURITY_COOKIE){ cookie = DEFAULT_SECURITY_COOKIE + 1; }

else if ((cookie & 0xFFFF0000) == 0){ cookie |= cookie << 16; }

__security_cookie = cookie; __security_cookie_complement = ~cookie;}

UINT_PTR cookie_temp_storage,cookie_global;cookie_temp_storage = 0xCCCCCCCC;cookie_temp_storage = __security_cookie;// cookie_temp_storage = cookie_temp_storage ^ EBP;// cookie_global = cookie_temp_storage;__asm {

xor cookie_temp_storage, ebpmov dword prt [ebp-4],cookie_temp_storage

}

•/G

S Reversing–

Protected function prologue changesPrologue

cookie_temp_storage = cookie_global;// cookie_temp_storage = cookie_temp_storage ^ EBP;__asm{

xor cookie_temp_storage, ebp}__security_check_cookie();

•/G

S Reversing–

Protected function eplilogue changesEpilogue

void __security_check_cookie(){if ( cookie_temp_storage == __security_cookie )

return;__report_gsfailure();

}

•/G

S Reversing–

__security_check_cookie()Memory Protections

void __report_gsfailure(){// rise an fake exception // saving the contex record codes skiped :D GS_ContextRecord.ContextFlags = CONTEXT_CONTROL; GS_ExceptionRecord.ExceptionAddress = (PVOID)(ULONG_PTR)GS_ContextRecord.Eip;

GS_ExceptionRecord.ExceptionCode = STATUS_STACK_BUFFER_OVERRUN; GS_ExceptionRecord.ExceptionFlags = EXCEPTION_NONCONTINUABLE;

cookie[0] = __security_cookie; cookie[1] = __security_cookie_complement;

if ( IsDebuggerPresent() )_CRT_DEBUGGER_HOOK(_CRT_DEBUGGER_GSFAILURE);

SetUnhandledExceptionFilter(NULL); UnhandledExceptionFilter((EXCEPTION_POINTERS *)&GS_ExceptionPointers);

TerminateProcess(GetCurrentProcess(), STATUS_STACK_BUFFER_OVERRUN);}

•/G

S Reversing–

___report_gsfailure()Report Gs Failure

– Puting shellcode into buffer– Overwrite EXCEPTION_REGISTRATION• Next = jump to shellcode• Handler = call [ebp+30] or pop,pop,ret or ...

– Causing an exception before coockie check– BOOOOM!!!

Defeating /Gs Scenario

– When exception occured os call _except_handler() function– The location of handler function is store into stack by

EXCEPTION_REGISTRATION structure ( E_R )

– Os find address of first regsiter E_R by accessing to fisrt 4 byte of thread TEB

– OS access to TEB through fs register– Fs[0] alwayes point to first installed E_R in TEB

Exception Handling Overview

typedef struct EXCEPTION_REGISTRATION{EXCEPTION_REGISTRATION *next;PEXCETION_HANDLER *handler;

}EXCEPTION_REGISTRATION, *PEXCEPTION_REGISTRATION;

– There is chain of E_R structures in stack– If one hanlder faild to handle exception , chain

traversealed to find a proper handler

Exception Handling Overview ( cont. )

– We can control value of both next and handler elements of a registerd E_R

– Overwiting handle with shellcode address and cusing an exception ,then boom ??? No !!!• There is some problems

– If handler address is in stack , handler never execute– If handler address is in heap , handler execute

» Finding writeable heap address in stack overflow situation is usualy impossible • Now what ?

Abusing registered E_R

– Rules are implemnted just for handler, we still have control over next pointer

– Secund parameter of _except_handler() function which is EstablisherFrame point to curent EXCEPTION_REGISTRATION frame

– We can access to this address by poping off two value from stack and then returning to it : pop,pop,ret

– There is also other instructions which work same as pop,pop,ret sequence, such as : • Call dword ptr[ebp+30]• Jmp dword ptr [esp+8]

Abusing registered E_R ( cont. )

– Our attack scenario :– Puting shellcode into buffer– Overwriting handler with pop,pop,ret instructions– Overwriting next with jump to the buffer– Cuasing an exception– Boom !!!

– But there is another problem : /SafeSEH

Abusing registered E_R ( cont. )

– Create a table of safe handlers– Compare each handler value with table values– If handler value is in safe table , handler excecute– If not , go to treminate process with

STATUS_STACK_BUFFER_OVERRUN flag

/SafeSEH Protection

– If program doesnt compile with /SafeSEH every handlers are valid and execute

– If handler address is in module which doesnt handle with /SafeSEH , handler execute

– If handler address is outside of loaded module, handler execute

/SafeSEH Protection Weaknesses

Bypassing /GS , /SafeSEH attack scenario

Target : Vista SP1 EN XP SP3 EN

NOP

Shellcode

Return Address

0x41414141

Jmp to Jmp 0xbuffer

Handler

Jump to Bufferinstructions

Adress of Pop,Pop,Ret

orCall dword ptr [ebp+30]

instruction

Bypassing /GS , /SafeSEH Protection inAction

Just trigger debuuger

– In this case we can use 0x00 in data– If can’t ?• Find address for pop,pop,ret which dosen’t contain

0x00• Put shellcode after overwrited E_R• Overwrite next with jmp forward to shellcode• Boom!

/GS Null Byte problem

– Data Execution Prevention (DEP) is a system-level memory protection feature that is built into the operating system

– DEP enables the system to mark one or more pages of memory as non-executable

– DEP prevents code from being run from data pages such as the default heap, stacks, and memory pools

– If an application attempts to run code from a data page that is protected, a memory access violation exception occurs

– if the exception is not handled, the calling process is terminated

Data Execution Prevention

– Hardware-enforced DEP enables the NX bit on compatible CPUs, through the automatic use of PAE kernel in 32-bit Windows

– The NX bit ( XD/EVP), which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data

– Physical Address Extension (PAE) is a feature of x86 and x86-64 processors that enable the use of more than 4 gigabytes[1] of physical memory to be used in 32-bit systems

– In traditional 32-bit protected mode, x86 processors use a two-level page translation scheme, where the control register CR3 points to a single 4 KB long page directory, which is divided into 1024 × 4 byte entries

– The NX bit flag in the page directory, in bit 63, to mark pages as "No eXecute"

– Similat technologies : W^X , PAX , Exec Shield

Data Execution Prevention ( Implementation )

– Software DEP, while unrelated to the NX bit, is what Microsoft calls their enforcement of "Safe Structured Exception Handling“

– Software DEP/SafeSEH simply checks when an exception is thrown to make sure that the exception is registered in a function table for the application

– requires the program to be built with it

Data Execution Prevention ( Implementation Cont. )

– OptIn: DEP is enabled by default for limited system binaries and programs that "opt in". With this option, only Windows system binaries are covered by DEP

– OptOut: DEP is enabled by default for all processes. A list of specific programs that should not have DEP applied can be entered using the System dialog box in Control Panel.

– AlwaysOn: This setting provides full DEP coverage for the whole system. All processes always run with DEP applied. The exceptions list to exempt specific programs from DEP protection is not available.

– AlwaysOff: This setting does not provide any DEP coverage for any part of the system, regardless of hardware DEP support.

Data Execution Prevention (Configuration )

– The very easy way• Using newly added DEP related API

– SetProcessDEPPolicy Fucntion

– Thr easy way:• Filling buffer with shellcode • Calling NtSetInformationProcess with Desire parameters to disable

DEP for Current Process• Jumping into buffer ( jmp esp)

– The hard way :• Filling buffer with shellcode • Making a page RWX by calling VirtualAlloc function• Copying Shellcode to RWX memry region through calling memcpy• Jumping to RWX memory region

Bypassing DEP Scenario

– We can change DEP policy for spesific process at run-time.

– We will use SetProcessDEPPolicy with desire parameters

• setting dwFlags to 0 disables DEP for the process

– Jump back to shellcode

The very easy way

BOOL WINAPI SetProcessDEPPolicy( DWORD dwFlags );

Finall attack scenarioThe very easy way Target : XP SP3 EN

NOP

Shellcode

Return AddressSetProcessDEPPolicy

Shellcodeaddr

0x00000000

Bypassing DEP Protection ( very easy way ) in

ActionJust trigger debugger

– We can change DEP policy for spesific process at run-time.– LdrpCheckNXCompatibility determine whether or not NX support should

be enabled for the process– As a result of these checks, NX support is either enabled or disabled

through a new PROCESSINFOCLASS named ProcessExecuteFlags (0x22).– When we calling NtSetInformationProcess 4 byte bit-mask parameter

push to the buffer as parameter for MmSetExecuteOptions– MmSetExecuteOptions perform appropriate operation to set the

executio flag– Parameters which we can pass to NtSetInformationProcess are :

• MEM_EXECUTE_OPTION_ENABLE ( 0x1 )• MEM_EXECUTE_OPTION_DISABLE (0x2 )

– So ,all we have to do is calling NtSetInformationProcess with mentioned parameters for current process at the overflow time

The easy way

• Take a quick look at NtSetInformationProcess :

• We can find NtSetInformationProcess Calling with desire parameters in a few dlls , such as : ntdll.dll , acgenral.dll , ....

• In exploit code i use ntdll instruction sequence becaus :– is more interesting than direct call in acgenrall.dll ....– Maybe our program dosnt load acgenral.dll...

• For acgenral.dll call ,Just search this sequence of byte in selected dll by msfpescan:– Msfpescan –r "\x6A\x04\x8D\x45\x08\x50\x6A\x22\x6A\xFF“

The easy way ( cont. )

NtSetInformationProcess(NtCurrentProcess(), // (HANDLE)-1ProcessExecuteFlags, // 0x22&ExecuteFlags, // ptr to 0x2sizeof(ExecuteFlags)); // 0x4

push 4;lea eax, [ebp+arg_0];push eax;push 22h;push 0FFFFFFFFh

• IDA View :D

The easy way ( cont. )

Finall attack scenarioThe easy way Target : XP SP3 EN

NOP

Shellcode

Return AddressNtSetInformationProcess

Some Pad

NtSetInfoReturn Address Jump to Buffer

Bypassing DEP Protection ( easy way ) inAction

Just trigger debugger

– calling VirtualAlloc function as return address and allocate commited memory with RWX protection mode

– set flAllocationType to MEM_COMMIT– And flProtect to PAGE_EXECUTE_READWRITE

The Hard way

LPVOID WINAPI VirtualAlloc( __in LPVOID lpAddress, __in SIZE_T dwSize, __in DWORD flAllocationType, __in DWORD flProtect );

– After VirtualAlloc returned , we call memcpy and copy shellcode into allocated RWX memory region

– set dest to Allocated unit by VirtualAlloc– And scr to shellcode address in buffer– Trick : You can also call VirtualProtect to make

shellcode location executable ( no memcpy needed )

The Hard way ( cont. )

void *memcpy( void *dest, const void *src, size_t count );

– VirtualAlloc function has __cdecl calling convention

– So ,Calling function pops it’s the arguments from the stack

– What we have to do if we want to call another function which has calling convention that not clear stack after call ?

– We can use pop,ret instruction

The Hard way ( cont. )

Pop regPop regPop regret

N time

N = function arguments count

Finall attack scenarioThe hard wayTarget : XP SP3 EN

Shellcode

VirtualAlloc

Memcpy

Mem Addr

Shellcode Size

MEM_COMMIT

PAGE_EXECUTE_READWRITE

Mem Addr

Shellcode Addr

Mem Addr

Shellcode Size

Saved Return Address

Bypassing DEP Protection ( hard way ) inAction

Just trigger debuuger

– In this case we can use 0x00 in data– If can’t ?• Use swquence of functions which dosent need 0x00 in

arguments• Use NtSetInformationProcess method which dosnt

need any null byte

DEP Null Byte problem

– In Windows Vista, NX cannot be disabled once turned on for a process

– now?• If can use null byte , VirtualAlloc+memcpy or

VritualProtect functions are handy ! ( without ASLR , in browser exploitation also with ASLR )• If cant , i dont know the asnwer ...

DEP Vista problem

– ASLR randomizes the location of images (PE files mapped into memory), heaps, stacks,the PEB and TEBs

– When a new address is being selected as an image base for an executable, a random delta value

– is added to or subtracted from the ImageBase value in the executable's PE header

– This delta value is calculated by taking a random 8-bit value from the RDTSC counter and multiplying it by 64KB

Address Space Layout Randomazation

– Heap Randmoazation• 5-bit random value is generated• multiplied by 64K• This value used as an offset from the base address

returned by the NtAllocateVirtualMemory

Address Space Layout Randomazation ( cont. )

– Stack Randmoazation• base of the stack is chosen randomly• offset into the initial page where the stack starts getting

used is also chosen at random

Address Space Layout Randomazation ( cont. )

– Heap Spray• Allocating larg amount of heap blocks• Filling Heap Blocks withNop and Shellcode• Use randomly address base on allocated units base

address and 64kb alignment• Filling buffer with return address • JavaScript !!!• Browser dependent limitation ( or every progarm which

allow us to make big block of heaps)

Bypassing ASLR Scenario

Memory Protections

DemoHeapSpray Technique

– IE disallow address which contain non-ascii bytes– Address most be taken base on heap blocks count– Using ascii sequnce address are highly recommend– Address such as : 0x05050505 , 0x0d0d0d0d ,

0x0a0a0a0a– You can also use Heap-Feng-Shui method instead

of HeapSpray

Heap Spray notes

Bypassing ASLR Protection inAction


– FinalExceptionHandler function in NTDLL.DLL is registered as the first exception handler in all theads

– As additional exception handlers are registered, they form a linked list with the last record always pointing to FinalExceptionHandler

– The exception dispatcher walks this linked list and verifies that the last record still points to that Function

– If an attacker overwrites the Next field of an exception handler record, the validation loop will not reach the last record and the SEH chain corruption will be detected

SEH Overwrite Protection ( SEHOP )

– Skape offer ( current implementation is abit diff )

SEHOP ( Implementation )

CurrentRecord = fs:[0];ChainCorrupt = TRUE;while (CurrentRecord != 0xffffffff) {

if (IsInvalidAddress(CurrentRecord->Next))break;

if (CurrentRecord->Next == ValidationFrame) {ChainCorrupt = FALSE;break;

}CurrentRecord = CurrentRecord->Next;

}if (ChainCorrupt == TRUE)

ReportExploitationAttempt();

– Exist theory• One potential way to bypass this protection is to point

the overwritten Next pointer to a fake SEH record that points to the FinalExceptionHandler function. However, the ASLR implementation inVista randomizes the address of the function and makes it impossible to for an attacker to terminate the SEH chain unless they have a way to bypass ASLR

– My theory

SEHOP bypassing theories

– /Gs , /SafeSEH– /Gs , /SafeSEH , DEP– ASLR , DEP– /Gs , /SafeSEH , ASLR , DEP

Combined Protections Bypassing Scenarios

Bypassing Combined Protection inAction


Question ?

Thank you for attending

bypassing ms windows memory protections shahriyar jalayeri [email protected] 1 st iranian...

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