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SEH overwrite and its exploitability
The document discusses SEH (structured exception handling) overwrites and techniques to bypass protections against them. It explains that SEH overwrites can be used to exploit Windows software by overwriting exception handlers. Common protections like SafeSEH, software DEP, hardware DEP, and SEHOP aim to prevent this. However, the document demonstrates that under certain conditions, it is possible to bypass all of these protections simultaneously. Specifically, if an attacker knows addresses needed to recreate the SEH chain and has a non-SafeSEH module containing suitable "add esp, ret" instructions, they can exploit a buffer overflow to execute arbitrary code despite all protections. The document provides a detailed example of this technique on Windows 7.
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SEH overwrite and its exploitability
- 1. SEH overwrite andits exploitability Shuichiro Suzuki Fourteenforty Research Institute Inc. Research Engineer
- 2. Agenda • Theme andGoal • Review of SEH overwrites • Protection mechanisms for SEH overwrites • Bypassing protection mechanisms. • Demonstration • Conclusion
- 3. Theme and goal Theme •SEH overwriting is one of the major methods for exploiting Windows software. • We already have several protection mechanisms for SEH overwrites. • Is the protection provided by DEP and SEHOP enough? • How about the protection from SafeSEH and SEHOP? Goal • To reveal which combinations of the known protection mechanisms for SEH overwrites are really effective. • On the way to the goal, I will show you that we can bypass SafeSEH and Software DEP and Hardware DEP and SEHOP, all at the same time, under certain conditions.
- 4. Target environment • WindowsXP SP3 • Windows Vista SP1 • Windows 7 • 32bit processes on x86 • 32bit processes on x64 (WOW64) • Visual Studio 2008 was used to compile all the programs in this presentation.
- 5. Review of SEHOverwrite
- 6. About SEH • SEHis “Structured Exception Handling” • Exception handling system provided by Windows int test(void){ __try{ // Exception may occur here } __except( EXCEPTION_EXECUTE_HANDLER ){ // This handles the exception } return 0; }
- 7. Inside Windows ExceptionDispatching 0x0012ff50 0x0012ff7C ExceptionList FS Register NT_TIB 0x0012ff90 0x0401790 0x0408800 Stack EXCEPTION_RECORD typedef struct _EXCEPTION_RECORD{ struct _EXCEPTION_RECORD *_next; PEXCEPTION_ROUTINE _handler; } EXCEPTION_RECORD; SEH chain 0xffffffff 0x070BBCC Thread Information Block _next _handler
- 8. SEH Overwrite Attack 0x0012ff50 0x0401790 StackProcess Memory Space Module(EXE or DLL) Handler1 Buffer Overflow! 0x0505690
- 9. Stack after anexception handler is called 0x0401790 Stack 0x909006eb EXCEPTION_DISPOSITION __cdecl _except_handler ( struct _EXCEPTION_RECORD *_ExceptionRecord, void * _EstablisherFrame, struct _CONTEXT *_ContextRecord, void * _DispatcherContext ); ESP _DispatcherContext _ContextRecord _EstablisherFrame _ExceptionRecord Address to return _handler ESP+8 pop eax pop eax retn Attackers Arbitrary Code
- 10.
- 11. Protection mechanisms • /GS •SafeSEH • SoftwareDEP • SEHOP • Hardware DEP • ASLR
- 12. /GS • Stack Guardby Visual Studio compiler • This checks if a buffer overflow occurs before returning from a function. • This is irrelevant to SEH overwrites because an exception can be generated before the check.
- 13.
- 14. SafeSEH Handler1 0x00002550 Handler2 0x00049080 Handler3… … … A module protected by SafeSEH Handler1 Handler2 0x00002550 0x00049080 Header Code “pop,pop,ret” sequence ×
- 15. Weakness of SafeSEH Module SafeSEHProtected Module Not SafeSEH Protected pop pop ret× pop pop ret pop pop ret pop pop ret Memory Space Data area without executable attribute Data area with executable attribute Data area in a module
- 16. Modules not protectedby SafeSEH From Thunderbird 2.0.0.23 * All modules in Thunderbird 3.0.3 are compiled with /SafeSEH
- 17. pop pop retin the wild From thunderbird.exe
- 18. Additional check bySoftware DEP Module SafeSEH Protected Module Not SafeSEH Protected pop pop ret× pop pop ret pop pop ret pop pop ret Memory Space Data area without executable attribute Data area with executable attribute Data area in a module Weekness
- 19. Software DEP 0: kd>dt nt!_KEXECUTE_OPTIONS +0x000 ExecuteDisable : Pos 0, 1 Bit +0x000 ExecuteEnable : Pos 1, 1 Bit +0x000 DisableThunkEmulation : Pos 2, 1 Bit +0x000 Permanent : Pos 3, 1 Bit +0x000 ExecuteDispatchEnable : Pos 4, 1 Bit +0x000 ImageDispatchEnable : Pos 5, 1 Bit +0x000 DisableExceptionChainValidation : Pos 6, 1 Bit +0x000 Spare : Pos 7, 1 Bit KPROCESS structure _KEXECUTE_OPTIONS
- 20. Software DEP pop popret× pop pop ret pop pop ret pop pop ret Memory Space × × Data area in a module× Module SafeSEH Protected Module Not SafeSEH Protected Data area without executable attribute Data area with executable attribute
- 21. Memory Space 0x00405600 SEHOP 0x12ff50 0x0012ff90 0x04224550 0x0701790 0x12ffe4 0x0701790 0xffffffff FinalExceptionHandler 0x909006eb × Overwrite! NTDLL.DLL Module 2 poppop ret Module 1 Proper Handler SEH Chain Stack SEH overwrites protection
- 22.
- 23. Hardware DEP andASLR • Hardware DEP prevents code without executable attribute from being executed. • ASLR has several impacts on the SEH overwrites.( Explain later )
- 24.
- 25. SafeSEH and SoftwareDEP pop pop ret pop pop ret pop pop ret pop pop ret Memory Space Data area in a module pop pop ret× pop pop ret pop pop ret pop pop ret Memory Space Data area in a module × × × × × × × × Module SafeSEH Protected Module Not SafeSEH Protected Non module area (Data area) Non module area with executable attribute
- 26. Bypassing SEHOP • Itis weak if the address of a buffer overflow and FinalExceptionHandler is known. • Attackers can recreate a proper SEH chain. 0x00405600 0x12ff50 0x0012ff90 0x04224550 0x0701790 0x12ffe4 0x0701790 0xffffffff FinalExceptionHandler 0x12ff50 Overwrite!
- 27. Bypassing Hardware DEP •Return-into-libc • Return-oriented programming VirutalAlloc Stack Args for VirtualAlloc memcpy Args for memcpy CreateFile Args for CreateFile ... ESP Buffer overflow
- 28. Bypass /GS, SafeSEH,Software DEP, Hardware DEP, SEHOP • Recreate SEH Chain (bypassing SEHOP) • Overwritten exception handler address must be in a module which is not SafeSEH enabled ( bypassing SafeSEH ) • Create a stack to execute desired code ( bypassing Hardware DEP ) • To execute the code using the stack above, we have to set an exception handler to some stack rewind and return instructions ( bypassing Hardware DEP ) • Trigger an exception
- 29. Recreating the SEHChain and Setting the Exception Handler Address 0x0012ff50 0x0401790 Stack Process Memory Space Module(EXE or DLL) No SafeSEH Buffer 0xffffffff FinalExceptionHandler Module(EXE or DLL) Handler1 SEH Chain Exception! 0xffffffff FinalExceptionHandler 0x0505690 0x0012ff4C Attackers Arbitrary Code and Data
- 30. Good instruction tobypass Hardware DEP 0x0012ff50 Module(EXE or DLL) No SafeSEH Buffer 0x0505690 0xffffffff FinalExceptionHandler Module(EXE or DLL) Handler1 add esp, 0x0C24 retn Exception Information 0x0c24 Attackers Arbitrary Code Stack Process Memory Space
- 31. Create proper stackfor return-into-libc 0x0012ff50 Module(EXE or DLL) No SafeSEH Buffer 0x0505690 0xffffffff FinalExceptionHandler Module(EXE or DLL) Handler1 add esp, 0x0C24 retn Exception Information 0x0c24 Attackers Arbitrary Code Stack Process Memory Space 0x00401110 0x30000000 0x00000800 0x00003000 0x00000040 0x30000000 0x30000000 0x0012F758 0x00000800 Address of VirtualAlloc Address of memcpy Address to be allocated The size to be allocated MEM_COMMIT|MEM_RESERVE PAGE_EXECUTE_READWRITE Address to return after memcpy Dest to copy Src of copy The size to be copied 0x7c8622A4
- 32. Exception handlers whichcan be used 0x0012ff50 Module(EXE or DLL) No SafeSEH Buffer 0x0505690 0xffffffff FinalExceptionHandler Module(EXE or DLL) Handler1 add esp, 0x0C24 retn Exception Information 0x0CF8 Attackers Arbitrary Code Stack Process Memory Space 0x00401110 0x30000000 0x00000800 0x00003000 0x00000040 0x30000000 0x30000000 0x0012F758 0x00000800 Address of VirtualAlloc Address of memcpy Address to be allocated The size to be allocated MEM_COMMIT|MEM_RESERVE PAGE_EXECUTE_READWRITE Address to return after memcpy Dest to copy Src of copy The size to be copied 0x7c8622A4 add esp, 0x0024 retn Doesn’t work Too small rewind add esp, 0xFF24 retn May be too big. Depends on the stack size. add esp, 0x0CF8 retn
- 33. Example of “addesp + retn” From Thunderbird 2.0.0.23
- 34. Summarize the condition •The address of the stack where buffer overflow occurs is known. • The address of the FinalExceptionHandler in ntdll.dll is known. • A process has a module not protected by /SafeSEH • “add esp + retn” instructions which matches followings can be found in the module. – The amount of “add esp” rewind is larger than the stack made by windows exception dispatcher. – The amount of “add esp” rewind is smaller than the stack size when the exception handler is called. • The address of VirtualAlloc and memcpy is known ( or some alternatives can be used). • Attacker can write 0x00 on the stack by using buffer overflow, which means string functions can’t be used to exploit. ( At least the method I showed here can not be used if this doesn’t match)
- 35. Demonstration • Demonstration onWindows 7 • I assumed that “add esp,0x0c24 – retn” can be found in a non /SafeSEH protected module. • /GS, Software DEP , Hardware DEP, SEHOP are all enabled. • ASLR is disabled.
- 36. Importance of ASLR •Makes it difficult to recreate proper SEH chain • Makes it difficult to find “add esp + retn” instructions at a fixed address. • Makes it difficult to set FinalExceptionHandler address in the last element of SEH chain.
- 37. Conclusion Protections Windows XPSP3 Windows Vista SP1 Windows 7 /GS + SafeSEH Exploitable by using data area as an exception handler Exploitable by using data area as an exception handler Exploitable by using data area as an exception handler /GS + SafeSEH + Software DEP If all modules are SafeSEH protected its difficult to exploit If all modules are SafeSEH protected its difficult to exploit If all modules are SafeSEH protected its difficult to exploit /GS + Software DEP + Hardware DEP Exploitable by Return- into-libc or Return- oriented programming Exploitable by Return-into- libc or Return-oriented programming Exploitable by Return-into- libc or Return-oriented programming /GS + Software DEP + SEHOP - Exploitable by recreating proper SEH chain Exploitable by recreating proper SEH chain /GS + SafeSEH + SEHOP - Exploitable by recreating proper SEH chain and using data area as an exception handler Exploitable by recreating proper SEH chain and using data area as an exception handler /GS + Software DEP + SEHOP + Hardware DEP - Exploitable by recreating proper SEH Chain and using data area and return-oriented programming Exploitable by recreating proper SEH Chain and using data area and return- oriented programming /GS + SEHOP + ASLR - Difficult to exploit Difficult to exploit /GS + Software DEP + SEHOP + Hardware DEP + ASLR - Difficult to exploit Difficult to exploit
- 38. ASLR + DEP •“Bypassing Browser Memory Protections” ( Alexander Sotirov, Mark Dowd ) shows how to bypass them. • But it was without SEHOP
- 39.