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Part 5: Anti-Reverse-Engineering

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

  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 • Naïve disassembly p. 328, Loc. 8288 • Corrected disassembly p. 329, Loc. 8300

  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 • Example previously on p. 328-329 • Bogus, injected opcode 0xE8 assumed to be “call” • Next 4 bytes assumed to be target (can instead contain malicious code) • Example p. 330, Loc. 8344 • Jump table correctly disassembled • Turns to code in a linear disassembler (p. 331, Loc. 8357)

  6. Defeating Disassembly Algorithms • Two types of algorithms • Flow-oriented disassembly • Builds list of locations to assemble by examining code from entry • Example • p. 332, Loc. 8368: Decoding correctly stops after unconditional jmp • p. 332, Loc. 8388: Linear disassembly parses ‘Failed’ as code • Example • p. 333, Loc. 8401: Call-pop to get pointer to "hello" into a register • p. 334, Loc. 8415: Instructions after call disassembled first parse string as code • p. 334, Loc 8428: Manual cleanup • Issues with flow-oriented disassembly • On conditional branch, which branch should be followed first? • On a call, do you disassemble the call or instructions after first? • Arbitrary results based on your choice!

  7. 3. Anti-Disassembly Techniques • Fake conditionals • Take advantage of choice in disassembly • Jump instructions with the Same Target • Back-to-back conditional jumps with the same target • Consider Figure 15-2, p. 335, Loc. 8477 • Unconditional jump that *always* skips bogus 0xE8 • Confused as code if fall-through followed firstin flow-disassembler • Incorrect disassembly at p. 335, Loc. 8448 • Correct disassembly at p. 335, Loc. 8462 • Toggle bytes from code to data using “C” and “U” • Jump instruction with a constant condition • Figure 15-3, p. 336, Loc. 8505 • XOR reg,reg followed by jz • Incorrect disassembly: p. 335, Loc. 8477 • Correct disassembly: p. 336, Loc. 8491 • Note: Methods use a “rogue” call/jmp byte (0xE9 or 0xE8)

  8. Anti-Disassembly Techniques • 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, p. 337, Loc. 8517) • More complex case (Figure 15-5, p. 338, Loc. 8533) • Incorrect disassembly (p. 338, Loc. 8541) • Correct disassembly (p. 339, Loc. 8555) • Must manually patch code or NOP out bogus instructions • IDAPython script on p. 339, Loc. 8574

  9. 4. Obscuring Flow Control Function pointers • Locations resolved at run-time • Hard to statically reverse engineer • p. 341, 2 calls to sub_4011C0 via function pointers not labeled Return pointer abuse • Modify return value on stack at run-time (return-oriented programming) • call $+5 coupled with rogue ret instruction on p. 342 • retn makes it appear sub_4011C0 is finished, but control goes just beyond retn instruction

  10. Obscuring Flow Control Misusing structured exception handlers • SEH allows program to handle error conditions intelligently • Uses a stack to manage (FS segment register) • Example on p. 346 • Craft pointer to exception routine (0x401080) via top two instructions • Push address onto stack as exception handler in third instruction • Trigger exception (divide-by-zero). • Routine at 0x401080 is not disassembled • Use IDA to manually disassemble

  11. 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, p. 347) does not use ebp, breaking IDA Pro analysis • cmp instruction sets up a fake conditional, but IDA Pro traces incorrect branch • Assumes “add esp, 104h” is actually executed and shows esp getting into an incorrect range (at -F8) • cmp should never trigger

  12. Talk • https://www.youtube.com/watch?v=wdFLK_eX0QY

  13. In-class exercise Lab 15-01, 15-02

  14. Chapter 16: Anti-Debugging

  15. 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

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

  17. 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

  18. Windows Debugger Detection Using the Windows API • OutputDebugString() (Listing 16-1, p. 353) • Get errorValue initially • Call function • If no debugger attached when attempting to output debug string, errorValue changed to indicate • Do the malicious thing

  19. Windows Debugger Detection Direct checks of structures in memory • Bypass Windows API • Preferred by malware since API calls can be hooked by anti-virus

  20. Windows Debugger Detection Direct checks of structures in memory • BeingDebugged flag • Load PEB structure address fs:[30h] • Access BeingDebugged at offset 0x2 • (Listing 16-2, Table 16-1, p. 354)

  21. Windows Debugger Detection Direct checks of structures in memory • Flags indicate if process heap was created by the debugger • Debugger heap check • Get address of process’s first heap (ProcessHeap) by loading value at 0x18 into PEB structure, then accessing flag field at 0x10 (XP) or 0x44 (Win7) (Listing 16-3, p. 355)

  22. Windows Debugger Detection Direct checks of structures in memory • Heap management different for debugged programs • NtGlobalFlag check • Specified at 0x68 offset in PEB. Set to 0x70 if debugged (Listing 16-4, p. 355)

  23. Windows Debugger Detection System artifacts • API hooks (OllyDbg detour of OpenProcess) • Registry values used by debuggers (HKLM\....\AeDebug) • Window names (e.g. OLLYDBG) • Debugging services running on system • File system artifacts • Memory artifacts (e.g. OllyDbg stores some strings at 0x004B064B)

  24. 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, p. 357) Performing code checksums • Malware performs checksum on its code pages and exits if tampering detected Get address Set address to start of code Check 4096 bytesfor 0xCC 0xCC found!

  25. Identifying Debugger Behavior Timing checks • Malware takes timestamps and exits if there is a lag • Especially effective when taken before & after an exception • Implemented via rdtscinstruction (Listing 16-7, p. 358), QueryPerformanceCounter, or GetTickCount (Listing 16-8, p. 359)

  26. Identifying Debugger Behavior Debugger artifacts • INT 1osed to single-step program in debugger • Overwrites 6 bytes beyond current sp with return values for IP, CS, and Flags • Put canary just beyond stack and check that it survives PUSH AX POP AX DEC SP DEC SP POP BX CMP AX,BX JNE CODE_IS_TRACED Put value on stack Get stack pointer to point back to it Read it again If not the same, then single stepping has clobbered it

  27. Identifying Debugger Behavior Debugger artifacts • Force INT 1/INT 3 tracing to disable essential functions • Hide critical value (e.g. decryption key) on stack directly without modifying stack pointer • Debugger overwrites value if it runs • Registers/flags saved by debugger on context switches • Debug registers (DR0-DR7) saved on stack on context switch • Set handler and force exception (divide by zero) • Read and write values directly • Execute exception with Trap flag set • No debugger = SEH occurs since no one handles trap • Debugger attached = SEH will not occur (trap sent to debugger)

  28. 3. Interfering with Debugger Functionality Using TLS (thread local storage) callbacks • Debuggers typically execute until program entry point defined by the PE header • TLS implemented in an executable contains a .tls section that can be executed before program entry point • Malware can hide functionality in TLS • Must configure debugger to pause before TLS callback code

  29. Interfering with Debugger Functionality Using exceptions • Debuggers typically configured to trap exceptions and not pass them through to program • Malware probes to ensure exceptions are passed through quickly • Must configure debugger to pass them through automatically

  30. Interfering with Debugger Functionality Inserting interrupts • Inserting a long loop of INT 3 instructions or 0xCD03 (STATUS_BREAKPOINT) to generate an INT 3. • Trolls the reverse-engineer • Doing the same with INT 2D (kernel debugger breakpoint)

  31. Interfering with Debugger Functionality Putting payload in interrupt handler • Having a debugger attached results in disabling malware • Make malicious code be a part of an SEH handler • INT 3 without debugger invokes SEH handler to execute malicous code • INT 3 with debugger goes to next instruction unless debugger configured appropriately (Listing 16-9, p. 363) Place SEH on stack (continue) Trigger exception Code that does nothing and exits Malicious code

  32. Interfering with Debugger Functionality Use interrupt handling to obfuscate execution • Example: 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 • Themida, last level of Microcorruption

  33. 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 • Turn off keyboard interrupts IN AL, 20h OR AL, 02 OUT AL, 20 <malicious code> IN AL, 20 AND AL, NOT 2 OUT AL,20

  34. 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 • Pass malformed string to crash debugger • OutputDebugString vulnerable to format string vulnerability in OllyDbg v. 1.1. • See fuzzing lecture • Exploit instructions that OllyDbg handles differently than CPU to crash debugger • Exploit exceptions that OllyDbg handles differently than CPU to crash debugger (memory handling)

  35. In-class exercise Lab 16-01

  36. Chapter 17: Anti-Virtual Machine Techniques

  37. 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 • Rollback recovery easy • Portability

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

  39. 1. VMware Artifacts Process listing (Figure 17-1, p. 370)

  40. VMware Artifacts Filesystem • (e.g. C:\Program Files\VMware\VMware Tools) Registry keys (p. 371)

  41. VMware Artifacts Networking • MAC addresses assigned for use by IEEE for VMware NICs begin with 00:0C:29 Memory (invariant strings in VMware virtual machine)

  42. VMware Artifacts Example code to check • Listing 17-1, p. 372 Must disable checks • Patch condition on branch to bypass in debugger • Use hex editor to modify VMware string • Uninstall VMware tool being checked

  43. 2. Vulnerable Instructions Descriptor Table Instructions • 3 special x86 registers for pointing to machine-wide data structures • IDTR: points to Interrupt Descriptor Table Register • GDTR: points to Global Descriptor Table Register (memory lookups) • LDTR: points to Local Descriptor Table Register (unused in Windows) • Guest VM must have a different location for these tables than Host VM • VM software creates separate locations • But, Guest VM can directly execute x86 instructions that directly access underlying registers to check for inconsistency (user-mode instructions not caught by hypervisor) • Must NOP out these checks

  44. 2. Vulnerable Instructions Examples of Descriptor Table checks • Red Pill • x86 instruction sidt loads value of IDTR (Listing 17-2, p. 374) • VMware sets 5th byte of IDTR to 0xFF • No Pill • x86 instruction sldt loads value of LDTR • Zero on non-virtualized, Non-zero on virtualized • x86 instruction sgdt loads value of GDTR

  45. Vulnerable Instructions 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, p. 376) • Must NOP out the check

  46. Vulnerable Instructions Common Anti-VM instructions • sidt, sgdt, sldt, smsw, str, in, cpuid • 20 instructions designated by VMware as “not virtualizable” • Malware will only use them if checking for VMs

  47. 3. Tweaking Settings VMware provides options to hide itself from malware • Listing 17-5, p. 379 • Protects against all checks implemented by ScoopyNG, a free VMware detection tool • Last-resort since performance will crater if used

  48. 4. Escaping the Virtual Machine Exploiting VMware bugs to crash host or run code in it • Prior exploits (now patched) include shared folder feature, drag-and-drop functionality in VMware Tools, shared clipboard, VM display function

  49. Talk • Blue Pill, Don't Tell Joanna

  50. In-class exercise Lab 17-01

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