1. Part 5: Anti-Reverse-Engineering
Chapter 15: Anti-Disassembly
Chapter 16: Anti-Debugging
Chapter 17: Anti-Virtual Machine Techniques
Chapter 18: Packing and Unpacking

2. Chapter 15: Anti-Disassembly

3. Anti-Disassembly 1. Understanding Anti-Disassembly
2. Defeating Disassembly Algorithms
3. Anti-Disassembly Techniques
4. Obscuring Flow Control
5. Thwarting Stack-Frame Analysis

4. 1. Understanding Anti-Disassembly
Special code to cause disassembly analysis to produce incorrect program listings
Goal is to delay or prevent analysis of malicious code
Thwart automated and manual analysis
Tricking disassembly at an incorrect offset
Examples on p. 328 and 329

5. 2. Defeating Disassembly Algorithms
Two types of algorithms
Linear disassembly
Iterate over a block of code, disassembling one instruction at a time linearly
Decode blindly from start to end, ignores flow-control instructions that cause only a part of the buffer to execute (Examples on p. 331)
Opcode 0xE8 assumed to be “call”, next 4 bytes assumed to be target (can instead contain malicious code)
Flow-oriented disassembly
Builds list of locations to assemble by examining code from entry
p. 332: After unconditional jmp, decoding stops
Arbitrary results based on the order in which conditional branches and calls are followed by disassembler (p. 333, 334)

6. 3. Anti-Disassembly Techniques
Jump instructions with the Same Target
Back-to-back conditional jumps with the same target
jz followed by jnz should be treated as unconditional jmp (Example on p. 335)
Toggle bytes from code to data using “C” and “U”
Jump instruction with a constant condition
XOR reg,reg followed by jz (Example on p. 336)
Note: Both methods use a “rogue” byte (0xE9 or 0xE8)
Impossible disassembly
Using a single byte in two instructions
Disassembler limited to picking one interpretation, but processor can use both
Inward jump (Figure 15-4)
More complex case (Figure 15-5)

7. 4. Obscuring Flow Control
Function pointers
Locations resolved at run-time
Hard to statically reverse engineer
Return pointer abuse
Modify return value on stack at run-time (return-oriented programming)
call $+5 on Example on p. 342
Misusing structured exception handlers
SEH allows program to handle error conditions intelligently
Uses a stack to manage (FS segment register)
Example on p. 346: Push pointer to exception routine onto she stack, then trigger exception (divide-by-zero). Routine is not disassembled

8. 5. Thwarting Stack-Frame Analysis
Stack-frame analysis dependent upon compiler used
Calling conventions vary
Custom management also possible such as management using esp directly
Listing 15-1 does not use ebp, breaking IDA Pro analysis
cmp instruction is more or less predictable, but IDA Pro traces incorrect branch
Misses “add esp, 104h” and shows esp getting into an incorrect range (at -F8)

9. In-class exercise Lab 15-01
In IDA Pro, what anti-disassembly technique is used and how many times is it used? Undo the anti-disassembly in IDA Pro
What order is the input checked?

10. In-class exercise Lab 15-02
Explain the false conditional at 0x A. Patch it. (cfg/idagui.cfg , ENABLE_PATCH_SUBMENU NO)
Explain the false conditional at 0x004011D0. Patch it.
Explain the technique being used at 0x Patch it.
Explain the technique being used at 0x Patch it.
Explain the technique being used at 0x004012E6. Which two methods does it combine? Patch it to reveal Listing 15-7L.
Step through analysis of the malicious code
What do sub_40130F and sub_ do?
Show how the first downloaded file is used to generate the second downloaded file
Show how the second downloaded file is used

11. Chapter 16: Anti-Debugging

12. Anti-Debugging
Anti-analysis technique for malware to recognize when it is under the control of a debugger
Slow down analysis as much as possible to increase window of vulnerability
Hundreds of techniques

13. Anti-Debugging 1. Windows Debugger Detection
2. Identifying Debugger Behavior
3. Interfering with Debugger Functionality
4. Debugger Vulnerabilities

14. 1. Windows Debugger Detection
Using the Windows API
IsDebuggerPresent() returns 0 if no debugger attached by searching the Process Environment Block for field IsDebugged
CheckRemoteDebuggerPresent() allows one to check the IsDebugged flag on other processes
NTQueryInformationProcess using value ProcessDebugPort
OutputDebugString (Listing 16-1)

15. 1. Windows Debugger Detection
Manual checks
Bypass Windows API to check memory locations directly
Preferred by malware since calls can be hooked by anti-virus
BeingDebugged flag
Loading of PEB structure address fs:[30h] (Listing 16- 2)
Followed by access of BeingDebugged flag at offset 0x2 (Table 16-1)
Debugger heap check
Get address of first ProcessHeap by loading value at 0x18 into PEB structure, then access flag field at 0x10 (XP) or 0x44 (Win7) (Listing 16-3)

16. 1. Windows Debugger Detection
Manual checks
NtGlobalFlag check
Heap management different for debugged programs
Specified at 0x68 offset in PEB. Set to 0x70 if debugged (Listing 16-4)
Registry values used by debuggers (HKLM\....\AeDebug)
Window names (e.g. OLLYDBG)
File system
Debugging services, API hooks (OllyDbg detour of OpenProcess), well-known fixed values in memory (e.g. OllyDbg stores some strings at 0x004B064B)

17. 2. Identifying Debugger Behavior
INT scanning
INT 3 inserted by debugger to temporarily replace an instruction so that debug exception handler can run when software breakpoints are hit (Opcode 0xCC)
Search for 0xCC in code (Listing 16-6)
Performing code checksums
Malware performs checksum on its code pages and exits if tampering detected
Timing checks
Malware take timestamps and exits if there is a lag
Especially effective when taken before and after an exception
Implemented via rdtsc instruction (Listing 16-7), QueryPerformanceCounter, or GetTickCount (Listing 16-8)

18. 2. Identifying Debugger Behavior
Debugger artifacts
INT 1 overwrites 6 bytes below current ESP with return values for IP, CS, and Flags
PUSH AX
POP AX
DEC SP
POP BX
CMP AX,BX
JNE CODE_IS_TRACED
Force INT 1/INT 3 tracing to disable essential
Use a canary similar to StackGuard
Hide critical value (e.g. decryption key) on stack directly without modifying stack pointer
Debugger overwrites value if it runs

19. 2. Identifying Debugger Behavior
Debugger artifacts
Check registers/flags saved by debugger such as DR0-DR7
Set handler and force exception (divide by zero)
Debug registers saved on stack on context switch
Read and write values directly
Execute exception with Trap flag set
No debugger = SEH occurs
Debugger attached = SEH will not occur

20. 3. Interfering with Debugger Functionality
Using TLS (thread local storage) callbacks
Debuggers pause at program entry point defined by the PE header
TLS implemented in an executable contains a .tls section that is initialized before program entry point
Most debuggers can be configured to pause before TLS callback code if a .tls section is present in malware
Using exceptions
Debuggers can be configured to either trap exceptions or pass them through automatically to application
Malware probes to ensure exceptions are passed through quickly

21. 3. Interfering with Debugger Functionality
Inserting interrupts
Inserting a long loop of INT 3 instructions
Inserting 0xCD03 (STATUS_BREAKPOINT) to generate an INT 3.
Inserting INT 2D (kernel debugger breakpoint)
Running line of code
Hook INT 1
Decrypt next instruction, encrypt previous one
Only one instruction decrypted in memory at a time
Hard to analyze
Side-effects of having a debugger attached result in malware changing how it executes
Have malicious code be a part of an SEH handler
INT 3 without debugger returns exception directly back into program to handle
INT 3 with debugger goes elsewhere (Listing 16-9)

22. 3. Interfering with Debugger Functionality
Modifying expected interrupt behavior
Continually overwrite Interrupt Vector of INT 1/3 instructions to point to garbage code to crash debugger
Turning off keyboard interrupts
IN AL, 20h
OR AL, 02
OUT AL, 20
<virus code>
IN AL, 20
AND AL, NOT 2
OUT AL,20

23. 4. Debugger Vulnerabilities
PE header vulnerabilities
OllyDbg follows specifications of PE headers more strictly than Windows. Crashes on malformed headers that will run without debugger
Code vulnerabilities
OutputDebugString vulnerable to format string vulnerability in OllyDbg v Pass malformed string to crash debugger
Exploit instructions that OllyDbg handles differently than CPU to crash debugger
Exploit exceptions that OllyDbg handles differently than CPU to crash debugger (memory handling)

24. In-class exercise Lab 16-01
Load the binary in IDA Pro. Bring up Figure 16-1L and explain what the three jz checks are doing
Bring up sub_ and Listing 16-1L. What does this code do?
Load the binary in OllyDbg
Set a breakpoint at 0x What is the value of eax? Step over several instructions. What happens?
Bring up Figure 16-2L or Figure 16-3L (via the Command Line plug-in or Phant0m plug-in) to reset the flag
Re-run the first OllyDbg step. What happens?
Explain the second anti-debugging check at 0x and how to bypass it
Explain the third anti-debugging check at 0x and how to bypass it
Set the argument to “-in” and single-step to reach 0x004035D5.

25. Chapter 17: Anti-Virtual Machine Techniques

26. Anti-Virtual Machine Techniques
Virtual machines initially used only by malware analysts
Malware benefited from detecting VM (especially VMware) and shutting down to escape analysis
Method is increasingly uncommon as a result of the prevalent use of VMs by normal users
Rollback recovery easy
Portability

27. Anti-Virtual Machine Techniques
1. VMware Artifacts
2. Vulnerable Instructions
3. Tweaking Settings
4. Escaping the Virtual Machine

28. 1. VMware Artifacts
Filesystem (e.g. C:\Program Files\VMware\VMware Tools)
Registry
p. 371
Process listing
Figure 17-1
Memory (invariant strings in VMware virtual machine)
Networking
MAC addresses assigned for use by IEEE for VMware NICs begin with 00:0C:29

29. 1. VMware Artifacts Example code to check Circumventing checks
Listing 17-1
Circumventing checks
Patch condition on branch to bypass in debugger
Use hex editor to modify VMware string
Uninstall VMware tool being checked

30. 2. Vulnerable Instructions
Instructions exposing host machine
For performance, virtual machine monitors allow certain instructions to execute directly
Instructions such as sidt, sgdt, sldt access hardware registers directly without generating an interrupt
Can expose inconsistency within guest VM
Must NOP out the check
Querying the I/O communication port (Phatbot, Storm)
VMware virtualizes I/O ports
Port can be queried to detect presence of VMware
Obtaining VMware version via IO port (Listing 17-3)
Common Anti-VM instructions
sidt, sgdt, sldt, smsw, str, in, cpuid
20 instructions designated by VMware as “not virtualizable”

31. 3. Tweaking Settings
VMware provides options to hide itself from malware
Listing 17-5
Protects against all checks implemented by ScoopyNG, a free VMware detection tool
Last-resort since performance will crater if used

32. 4. Escaping the Virtual Machine
Exploiting VMware bugs to crash host or run code in it
Shared folder feature
Drag-and-drop functionality in VMware Tools
VM display function

33. In-class exercise Lab 17-01
In IDA Pro, show and explain the three anti-VM checks being performed
Run the code
Break before the first anti-VM check. Does this check succeed? If so, NOP or skip the check and run again.
Break before the second anti-VM check. Does this check succeed? If so, NOP or skip the check and run again.
Break before the third anti-VM check by setting a breakpoint at 0x004012CB and stepping into sub_ Does this check succeed? If so, NOP or skip the check and run again.
Reach the beginning of malware code at 0x004012DF and generate Listing 17-5L

34. Chapter 18: Packers and Unpacking

35. Packers Used to shrink malware and thwart detection by antivirus
Thwarts static analysis since malware must be unpacked before it can be analyzed
Original executable transformed to a new self-extracting one via compression, encryption, or obfuscation making it harder to recognize and reverse-engineer
Typically employs anti-disassembly, anti-debugging, and anti-VM techniques to prevent unpacking on an analyst machine

36. Packers and Unpacking 1. Packer Anatomy 2. Identifying Packed Programs
3. Unpacking Options
4. Tips and Tricks for Common Packers
5. Packed DLLs

37. 1. Packer Anatomy Unpacking Stub
Small piece of code loaded by the operating system just as a normal program
Unpacking stub then loads original program
Step #1: Unpacking original executable into memory
Loader reads PE header and copies sections into allocated memory normally
Unpacking code does the same for packed code
Step #2: Resolve imports of original executable
Loader reads PE header to find library functions to import and their addresses
Unless packed code's imports included in unpacking code's import section, unpacker must resolve imports manually using LoadLibrary and GetProcAddress
Step #3: Transfers execution to original execution point
Tail jump to entry point

38. 1. Packer Anatomy Unpacking samples
Save flags and all registers (PUSHFD, PUSHAD), call unpacking routine
Within unpacking routine
Jump to OEP (original execution point)
Tail jump (POPAD/POPFD restoration, PUSH followed by a RET!)
Set breakpoint at 0x0040CA96 and dump memory image
0040AC44 FFFF INVALID
0040AC4C 9C PUSHFD
0040AC4D PUSHAD
0040AC4E E CALL AC55
**If you step over this CALL using F10, the program will run.
Thus, reload the program and step into this CALL using F8 next time.
aaaaaaaa
...
wwwwwwww
xxxxxxxx JNZ zzzzzzzz <-- Loop back to aaaaaaaa
yyyyyyyy JMP aaaaaaaa
zzzzzzzz New Instructions
0040CA83 8BBD2E MOV EDI,[EBP E]
0040CA89 E85E CALL CEEC
0040CA8E POPAD
0040CA8F 9D POPFD
0040CA PUSH EAX
0040CA91 68CC PUSH CC
0040CA96 C RET

39. 1. Packer Anatomy Unpacking samples Alternate jump to OEP
Can also use a jmp (Listing 18-1)
Note: empty bytes after JMP and huge offset
IDA Pro can identify JMP goes to garbage and flags it red (Figure 18-5)
015F: MOV [ESP+1C],EAX
015F: POPAD
015F: JNZ (JUMP )‏
015F: PUSH EAX *** Take note of the value
of EAX!
015F: RET *** Stop here!!!

40. 2. Identifying Packed Programs
Simple indicators
Program with few imports and imports are LoadLibrary and GetProcAddress
IDA Pro recognizes a small amount of code
Presence of UPX0 section (a specific packer)
Abnormal section sizes
Used by tools such as PEiD to determine if code is packed
Entropy calculation
Disorder in a program much larger in encrypted and compressed payloads

41. 3. Unpacking Options Automated static unpacking
Decompress and decrypt executable to restore original code
Specific to a packer (i.e. you must know which packer was used)
PE Explorer
Supports NSPack, UPack, and UPX
Automated dynamic unpacking
Program is run and unpacker stub is allowed to unpack original executable
Once tail jump is reached, memory is dumped and original program written to disk
Fails if the end of unpacking stub is not identified properly
Not many publicly available tools for this

42. 3. Unpacking Options Manual dynamic unpacking
Option #1: Discover packing algorithm and write a program to run it in reverse
Option #2: Run packed program so unpacking stub does the work
Break and dump the process out of memory (Listing 18-2 and 18-3)
Manually fix up PE header so program is complete
Helpful tools
OllyDump plug-in for OllyDbg (performs OEP identification, import table reconstruction, entry point patching)
ImpRec (Import Reconstructor) when OllyDump fails to build a proper import table

43. 3. Unpacking Options Manual dynamic unpacking
Finding OEP via stack trace
Upon entry into unpacking stub, registers often pushed
Set a breakpoint for esp accessing those stack locations again
Indicates unpacking code is finished and a jump to original entry point forthcoming
Finding OEP via iteration
Break at the end of each loop and iterate until tail jump identified
Manual import table patching
Two tables: table of function names, table of addresses
Listing 18-4 when import table broken
Cross-reference between OllyDbg and IDA Pro to patch import table with function name

44. 4. Tips and Tricks for Common Packers
UPX (Ultimate Packer for eXecutables)
Open-source
Designed for compression not for security
OllyDump finds easily using heuristics previously described
PECompact
Similar to UPX, uses a tail jump of jmp *eax
ASPack
Uses self-modifying code to thwart analysis

45. 4. Tips and Tricks for Common Packers
Petite
Uses single-step exceptions to break into debugger
Must pass single-step exceptions back to Petite or employ hardware breakpoints to find OEP
WinUpack
Uses PUSH followed by RET for tail jump
Placed in the middle of stub (Listing 18-5)
Themida
Secure packer employing anti-debugging, anti-analysis, and anti-VM techniques
Contains a kernel component making it difficult to follow
Runs code continuously
Use ProcDump to dump memory without attaching debugger

46. 5. Packed DLLs Similar to executables
Unpacking stub contained in DllMain
DllMain unpacks original DLL
Some debuggers execute DllMain before breaking
Can set IMAGE_FILE_HEADER values to cause DLL to be interpreted as executable

47. In-class exercise Lab 18-1
Load executable in IDA Pro to identify packed code
Run PEiD on binary and find section UPX2. Perform a “deep scan”. What does PEiD return?
In OllyDbg, locate the jump to the unpacking stub by finding the register save instruction
Set a breakpoint at this location and execute unpacking code. Single-step to the OEP.
Use OllyDump to dump the program into a new executable and load the new executable in IDA Pro