This document discusses techniques that malware authors use to frustrate malware analysts, including inserting breakpoints, manipulating timing functions, exploiting Windows internals like debug flags and objects, anti-dumping methods, VM detection, and debugger-specific tricks. The author also announces a public malware repository and API called VXCage for sharing samples.

1. How to drive a malware
analyst crazy
MICHAEL BOMAN, MALWARE RESEARCH INSTITUTE

2. About me
4th year speaking at 44CON
- 2012: Malware as a hobby [P]
- 2013: Controlling a PC using Arduino [WS]
- 2014: Malware analysis as a big data problem [P]
- 2015: Malware anti-reversing [P], Indicators of Compromise [WS]
Malware Researcher, Founder Malware Research Institute
6 kids, one more on the way…

3. Malware Research Lab, 2012

4. Malware Research Lab, 2015

5. Disclaimer
 These are the techniques I’ve come across trying to keep malware
researchers out of the game
 Or just waste a heck of a lot time doing quite silly things…
 Not a complete list of techniques
 The techniques discussed are aimed towards a x86/win32 environment

6. Technique #1: Breakpoints
 INT 3h
 Memory Breakpoints
 Hardware Breakpoints

7. How INT3 breakpoints work
mov eax, fs:[0x30]
mov eax, [eax + 0x0c] // <- Break here
mov eax, [eax + 0x0c]
mov dword ptr [eax + 0x20], NewSize

8. How INT3 breakpoints work
mov eax, fs:[0x30]
int 3h [garbage] // <- EP
mov eax, [eax + 0x0c]
mov dword ptr [eax + 0x20], NewSize

9. How INT3 breakpoints work
mov eax, fs:[0x30]
mov eax, [eax + 0x0c] // <- restored by debugger
mov eax, [eax + 0x0c]
mov dword ptr [eax + 0x20], NewSize

10. Memory Breakpoints
 Allocate memory, mark PAGE_GUARD
 When accessed
STATUS_GUEARD_PAGE_VIOLATION is
raised, handled by program
Allocate memory as buffer
Fill buffer with RET instruction
Mark buffer with PAGE_GUARD
PUSH potential return address to stack
JMP to buffer
If debugger:
RET will jump back to potential return address
else:
STATUS_GUARD_PAGE_VIOLATOIN exception
occur

11. Hardware breakpoints
 Hardware breakpoints are a technology implemented by Intel in their
processor architecture, and are controlled by the use of
 Special registers DR0 - DR7
 DR0 - DR3 - 32 bit registers for the breakpoint address
 DR4, DR5 - obsolete synonyms for DR6 and DR7
 DR6 – Debug status
 DR7 – Debug control

12. Technique #1: Breakpoints
 INT 3h
 Look for code that scans memory for 0xCC [INT3] and/or 0xCD 0x03 [INT
(immediate) 3]
 Memory Breakpoints
 Look for memory allocations with PAGE_GUARD flag set
 Hardware Breakpoints
 Win32 GetThreadContext and SetThreadContext
 Structured Exception Handling

13. Technique #2: Timing
 RDTSC (ReaD TimeStampClock)
 Win32 Timing Functions
 GetTickCount
 timeGetTime
 QueryPerformanceCounter
 […]

14. Technique #2: Timing
 RDTSC (ReaD TimeStampClock)
 Mark RDTSC as a elevated instruction (can then be intercepted and modified)
 Win32 Timing Functions
 Use DLL-injection to overload the function with one that lies nicely in our favour
Please remember to lie consistently to all timing methods.

15. Technique #3: Windows Internals
 ProcessDebugFlags
 Debug Object Handle
 Thread Hiding
 BlockInput
 OutputDebugString

16. ProcessDebugFlags
 Pass undocumented class ProcessDebugFlags (0x1f) to the
NtQueryProcessInformation() function.
 When NtQueryProcessInformation is called with the ProcessDebugFlags
class, returns the inverse of EPROCESS -> NoDebugInherit
 FALSE == Debugger present

17. Debug Object Handle
 Windows XP or later
 When debugged a Debug Object created
 Can be queried using NtQueryInformationProcess
 Originating from kernel -> hard to hide

18. Thread Hiding
 Windows 2000 and later
 HideThreadFromDebugger class, passed into NtSetInformationThread,
 The class prevents debuggers from receiving events from any thread that
has had NtSetInformationThread with the HideThreadFromDebugger class
called on it.
 These events include breakpoints, and the exiting of the program if it is
called on the main thread of an application.

19. BlockInput
 BlockInput() blocks mouse and keyboard messages from reaching the
desired application
 Only the thread that called BlockInput can call it to remove the block
 Not really Anti-RE, but can mess with you

20. OutputDebugString
 Call OutputDebugString()
 GetLastError()
 No error == debugger present

21. Technique #3: Windows Internals
 ProcessDebugFlags
 Check NtQueryProcessInformation() calls for [undocumented] ProcessDebugFlags (0x1f) object
 Hook NtQueryProcessInformation(), lie about the ProcessDebugFlags value
 Debug Object Handle
 Hook NtQueryInformationProcess(), remove any links to debug objects
 Thread Hiding
 Remove any HideThreadFromDebugger class passed into NtSetInformationThread
 BlockInput
 Hook it to a NO-OP
 OutputDebugString
 Hook it to always return error

22. Technique #4: Process Exploitation
 Open Process
 Parent Process
 Self-Debugging
 UnhandledExceptionFilter
 NtQueryObject

23. Open Process
 Debugger not properly resets process privileges
 Open privileged process like csrss.exe
 If succeed we are running under a debugger

24. Parent Process
 Check if GetParentProcessId() and GetExplorerPIDbyShellWindow()) is the
same
 Or however you are expecting your malware to be executed

25. Self-Debugging
 Parent spawns child who debugs the
parent
 Prevents debugger to attach to
parent
Child
Parent

26. UnhandledExceptionFilter
 UnhandledExceptionFilter is the
exception handler that is called when
there are no other handlers to handle
the exception.
 When utilizing this technique, the
process will exit instead of resuming
execution which is fine for Anti-RE
purposes.
UnhandledExceptionFilter
SEH Chain
Vectored Exception Handlers

27. NtQueryObject
 NtQueryObject() called with ObjectAllTypesInformation class, returns
information about the host system and the current process including
DebugObjects in the environment.
 ObjectAllTypesInformation can be traversed to locate DebugObjects

28. Technique #4: Process Exploitation
 Open Process – Make sure debugger drops SeDebugPrivilege
 Parent Process – Fake GetParentProcessId()
 Self-Debugging - Set PsGetProcessId()->EPROCESS->DebugPort to 0
 UnhandledExceptionFilter – Make sure the debugger do “the right thing”
 NtQueryObject – Intercept and filter

29. Technique #5: Anti-dumping
 Nanomites
 Stolen Bytes (Stolen Code)
 SizeOfImage
 Virtual Machines
 Guard Pages
 Removing the PE Header

30. Nanomites
 Replace JUMP (Jxx) instructions with INT 3h breakpoints
 Store original JUMP (Jxx) instruction in an encrypted table
 Use self-debugging, debugger process will substitute the INT 3h code with
the correct JUMP instruction depending on encryption algorithm.
 Put some stray INT 3h in the execution flow and you have made a real
mess

31. Stolen Bytes (Stolen Code)
 Code or bytes from the original process protected by the packer are
copied and encrypted somewhere inside the packing code
 The original (copied) code is replaced with jumps to a dynamic allocated
buffer for the decrypted bytes and then jumps back to the original flow

32. SizeOfImage
 Modifying PE -> IMAGE_OPTION_HEADER -> SizeOfImage can cause
problems for tools that weren't developed to handle this problem.

33. Virtual Machines (think JVM, not VBox)
 Protectors like Themida and VMProtect already use virtual machines in
their protection schemes.
 Themida uses a technology that creates a unique virtual machine for
every protected executable
 Prevents the use of a generic attack against its virtualization protection
 Many protection schemes implement junk code instructions

34. Guard Pages
 Discussed earlier
 Can be used for an on-demand decryption/decompression system
 Mark all pages that were not immediately needed as guard pages
 When accessed, an EXCEPTION_GUARD_PAGE exception will be raised
 Additional data can be decrypted or decompressed either from file or
memory.

35. Removing the PE Header
 Removes an executable's portable executable from memory at runtime
 A dumped image would be missing important information such as the RVA
(Relative Virtual Address) of important tables (Reloc, Import, Export etc..),
the entry point, and other information that the Windows loader needs to
utilize when loading an image

36. Technique #5: Anti-dumping
 Nanomites
 Stolen Bytes (Stolen Code)
 SizeOfImage
 Virtual Machines
 Guard Pages
 Removing the PE Header

37. Technique #6: Exploiting IA-32 Instructions
 Interrupt 2D
 Stack Segment
 Instruction Prefixes

38. Interrupt 2D
 INT 2D instruction can be used as a debugger detection method
 When executed
 No Debugger Present -> Exception
 Debugger Present -> No Exception
 Debugger specific

39. Stack Segment
 Manipulate stack segment using push
ss and pop ss cause the debugger to
execute instructions unwillingly
 In the following code, when stepping
over the code with any debugger, the
mov eax, 9 line will execute, but will
not be stepped on by the debugger.
push ss
pop ss
mov eax, 9 // This line executes
but is stepped over
xor edx, edx // This is where the
debugger will step to

40. Instruction Prefixes
 Takes advantage of the way debuggers
handle instruction prefixes.
 When stepping over this code in OllyDBG
or in Visual Studio 2008, we will reach the
first emit and immediately be taken to
the end of the __try block. What
happens is that the debugger essentially
skips over the prefix and handles the INT
1.
 When running this code without a
debugger, there will be an exception
that SEH will catch and the program will
continue along.
inline bool IsDbgPresentPrefixCheck()
{
__try
{
__asm __emit 0xF3 // 0xF3 0x64 disassembles as PREFIX REP:
__asm __emit 0x64
__asm __emit 0xF1 // One byte INT 1
}
__except(EXCEPTION_EXECUTE_HANDLER)
{
return false;
}
return true;
}

41. Technique #6: Exploiting IA-32
Instructions
 Interrupt 2D
 Stack Segment
 Instruction Prefixes

42. Technique #7: VM Detection
 VM Artefacts
 Hardware
 Drivers
 OS version / serial number
 Add-ons
 WMI calls
 Interactivity
 Is the computer being used?
 Click on invisible or very small buttons no human could see

43. Technique #7: VM Detection
 VM Artefacts
 Hardware – Clone real system configuration
 Drivers – Don’t use VM-specific drivers
 OS version / serial number – Use ”real” serial numbers
 Add-ons – Never install VM Guest tools
 WMI calls – Patch hypervisor, use real hardware
 Interactivity
 Is the computer being used? – Fake interactivity
 Click on invisible or very small buttons no human could see – Make sure your
fake interactivity is plausible

44. Debugger specific techniques
 OllyDBG
 FindWindow
 OutputDebugString Exploit
 WinDBG
 FindWindow
 Cuckoo Sandbox
 Check if hooked

45. Debugger specific techniques
 OllyDBG
 FindWindow – Hijack function call or modify OllyDBG binary
 OutputDebugString Exploit – Run patched version
 WinDBG
 FindWindow – Hijack function call or modify WinDBG binary
 Cuckoo Sandbox
 Check if hooked – Run unhooked, patch the hook-check function

46. Other Techniques
 Junk Code
 Native Code Permutations

47. Other Techniques
 Junk Code
 Native Code Permutations
Unfortunately there are no quick-fixes for these techniques

48. Announcement
Riddle

49. Announcement
 Public VXCage-server
 Available at vxcage.malwareresearch.institute (http, soon https)
 Feel free to apply for a personal account, free of charge:
 TO: michael@michaelboman.org
 SUBJECT: VXCage Access
 BODY:
 Who you are: name, twitter handle (if any, for cyberstalking), other contact info
 Why you want access
 Proposed username for the system (the password will be generated for you)
 Please contact me at the above address for raw access to the archive

50. VXCage API: Quick intro
 REST with JSON output
 /malware/add – upload sample
 /malware/get/<sha256> - download sample
 /malware/find – search sample based on hash, date, tag
 /tags/list – list tags
 Docs & Source code at https://github.com/mboman/vxcage

51. Thank you
 Michael Boman (@mboman)
 michael@michaelboman.org (soon also
michael.boman@malwareresearch.institute)
 Malware repository: vxcage.malwareresearch.institute
 Malware blog: blog.malwareresearch.institute