A survey of Buffer overflow exploitation on

HTC touch mobile phone

Advanced Defense Lab

CSIE NCU

Chih-Wen Ou

Abstract

Buffer overflow issues on ARM based handheld devices (HTC touch mobile phone)

Theoretical analysis and practical testing programming

Acknowledgement

怡群 Collin Mulliner

– Exploiting PocketPC, What the Hack! July 2005.

Collin Mulliner – BlackHat Japen, October 2008

Everybody and 老師

Outline

Introduction Background Programming Evaluation and Discussion Future work Conclusion

Introduction

ARM based handheld devices– Most widely used processor

PDAs GPS devices

– Mobile phone devices Symbian WinCE iPhone

Our test platform: HTC touch ARM processor Windows mobile 6.0 (WinCE 5)

Introduction

Buffer overflow issues– Without social engineering– Computer compromised– String data manipulation without proper bound check– Memory corruption and possible malicious execution flow

redirection

shellcode programming on ARM based WinCE– ARM assembler and disassembler

Visual studio with windows mobile 6.0 sdk– ARM instruction set reference– C. Mulliner’s work since 2003

Background

ARM– RISC Instruction set architecture

32 bit word (4 bytes long) Separated Instruction and data cache

– Register organization (user32/system mode) r0-r12 are general purpose register r13 is stack pointer SP r14 is subroutine link register LR r15 is program counter PC

Background

WinCE– Slot virtual memory designed

32-bit addressing, 4G address space Divided into 32 MB sized slots Slot 0 mapped for the “current execution” process 33 slots used for user processes (including slot 0) 1 slot for DLLs Others slots used for kernel Memory protection exists ( claimed by C. Mulliner)

Background

WinCE– Processes

Basically no thread limit (by C. Mulliner)

All processes share the same 4G virtual address space

Only few slots can be accessed by a certain process

– XIP DLLs eXecutin In Place DLLs ROM Function addresses are always

the same (By C. Mulliner)

Background

C programming on WinCE– Dangerous string manipulation functions

Strcpy, strcat, sscanf…etc

– Excution flow control variable in stack LR is designed for resuming the execution address when subroutine call is

finished – (mov pc,lr)– Hard to change the execution flow

Actually in our test program, saving return address in stack is still used on WinCE, when issuing a further subroutine call and current LR needs to be save in stack

The saved return address is always directly loaded to PC – (ldr pc, [sp],#4)

– Buffer overflow vulnerabilities may exist!

Programming

Memory analysis program– Show the address of global variables

0x000140dc (slot 0)– Show the address of local variables

in stack 0x??07fe7c (device) 0x1807fe7c (emulator) Different slot

– Show the start address of function exectest()

0x00011050 (slot 0)– Show the address of function

MessageBoxW 0x03f7f720 (fixed in slot 1)

Programming

Execution flow redirection testing– By directly rewriting the guessed memory address

of first local variable plus offsets– The new redirected address point to a static link

compiled target procedure in code segment because of leak information of :

Execution in stack Execution in global data Execution among unknown memory layout

Programming

Code and result

Programming

Simple MessageBox pop up Shellcode– Call MessageBoxW(0,0,0,0) by directly issuing a function pointer

call from 0x03f7f720 ((int(*)(DOWRD, DOWRD, DOWRD, DOWRD))(0x3f7f720))(0,0,0,0);感謝怡群

– 32 bytes of 8 instructions “\x00\x30\xa0\xe3” mov r3, #0 “\x00\x20\xa0\xe3” mov r2, #0 “\x00\x10\xa0\xe3” mov r1, #0 “\x00\x00\xa0\xe3” mov r0, #0 “\xfe\x47\xa0\xe3” mov r4, #0xEF, 14 “\x8e\x4e\x44\xe2” sub r4, r4, #0x8E, 28 “\x0f\xe0\xa0\xe1” mov lr, pc “\x04\xf0\xa0\xe1” mov pc, r4

Programming

According to result of analysis so far and the finished shellcode, we can write a test program on our HTC touch phone.

To test executing shellcode in global data area

To test executing shellcode in stack Both above execution are launched by

rewriting the return address in stack

Programming

Code: execution in global data area

Programming

Code: execution in stack

Evaluation and Discussion

Injected instruction in Stack– Success(emulator)– Failed (device)

Injected instruction in global data– success

Evaluation and Discussion

Executing in stack failed– Instruction cache?

Global data is much closer to code segment composed of instructions compared to local variable, which is in stack

Therefore, global data may be cached into instruction cache with other instructions (just guessing…)

– Address range? Any execution limitation of program counter? Other possible execution limitations cause such failure

Found GS function on WinCE– __security_check_cookie– I will test it in the future

Evaluation and Discussion

Programs on ARM based WinCE platform– Extremely similar layout between emulator and HTC device.– No variation of layout when re-executing the program– Easy to decide addresses of functions within XIP DLLs

without changed (ROM)– By default, GS function always protects our execution flow

from control variable in stack being changed by malicious craft attacking string

Good for security

Evaluation and Discussion

Programming on ARM based WinCE platform– Once program are compiled without GS on.– Once execution control variables can be changed through

buffer overflow vulnerabilities– Once there is at least one enough writable global data

space, especially string( because of XIP DLLs, may not be necessary)

– We induce that such kind of program on a device is dangerous for compromising

Future work

Vulnerable program threat analysis– How much possibility for attacker changing the value of control variable in

stack– GS function

Well attacking execution– Execution in global data– Execution by repeated calls within XIP DLLs

Completely proof of concept– A vulnerable program

buffer overflow vulnerable program on HTC touch phone– A classic attacking string

Malicious craft attacking string– A practical compromising

Download and execution … etc

Conclusion

Introduction of ARM register usage and its operation during subroutine call

WinCE memory layout analysis on emulator and HTC touch

Practical shellcode programming on ARM Practical shellcode execution on HTC touch GS function found