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Part 5: Anti-Reverse-Engineering

Part 5: Anti-Reverse-Engineering. Chapter 15: Anti-Disassembly Chapter 16: Anti-Debugging Chapter 17: Anti-Virtual Machine Techniques Chapter 18: Packing and Unpacking. Chapter 15: Anti-Disassembly. Anti-Disassembly. 1. Understanding Anti-Disassembly 2. Defeating Disassembly Algorithms

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Part 5: Anti-Reverse-Engineering

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  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 0x0040115A. Patch it. (cfg/idagui.cfg , ENABLE_PATCH_SUBMENU NO) • Explain the false conditional at 0x004011D0. Patch it. • Explain the technique being used at 0x00401215. Patch it. • Explain the technique being used at 0x00401269. 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_401386 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 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. 1.1. 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_401000 and Listing 16-1L. What does this code do? • Load the binary in OllyDbg • Set a breakpoint at 0x00403554. 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 0x00403573 and how to bypass it • Explain the third anti-debugging check at 0x00403594 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 • 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) • Must NOP out the check 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_401100. 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 0040AC44 FFFF INVALID 0040AC4C 9C PUSHFD 0040AC4D 60 PUSHAD 0040AC4E E802000000 CALL 0040AC55 **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. 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 aaaaaaaa ... wwwwwwww xxxxxxxx JNZ zzzzzzzz <-- Loop back to aaaaaaaa yyyyyyyy JMP aaaaaaaa zzzzzzzz New Instructions 0040CA83 8BBD2E744000 MOV EDI,[EBP+0040742E] 0040CA89 E85E040000 CALL 0040CEEC 0040CA8E 61 POPAD 0040CA8F 9D POPFD 0040CA90 50 PUSH EAX 0040CA91 68CC104000 PUSH 004010CC 0040CA96 C20400 RET 0004

  39. 1. Packer Anatomy 015F:01017554 MOV [ESP+1C],EAX 015F:01017558 POPAD 015F:01017559 JNZ 01017563 (JUMP )‏ 015F:01017563 PUSH EAX *** Take note of the value of EAX! 015F:01017564 RET *** Stop here!!! 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)

  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

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