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A survey of Buffer overflow exploitation on HTC touch mobile phone Advanced Defense Lab CSIE NCU Chih-Wen Ou

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Abstract Buffer overflow issues on ARM based handheld devices (HTC touch mobile phone) Theoretical analysis and practical testing programming

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Acknowledgement Collin Mulliner Exploiting PocketPC, What the Hack! July 2005. Collin Mulliner BlackHat Japen, October 2008 Everybody and

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Outline Introduction Background Programming Evaluation and Discussion Future work Conclusion

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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)

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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. Mulliners work since 2003

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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

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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)

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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)

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Background C programming on WinCE Dangerous string manipulation functions Strcpy, strcat, sscanfetc 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!

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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)

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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

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Programming Code and result

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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); 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

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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

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Programming Code: execution in global data area

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Programming Code: execution in stack

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Evaluation and Discussion Injected instruction in Stack Success(emulator) Failed (device) Injected instruction in global data success

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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

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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

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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

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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

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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