part 5: anti-reverse-engineering chapter 15: anti-disassembly chapter 16: anti-debugging chapter 17:...

Part 5: Anti-Reverse-Engineering

Chapter 15: Anti-DisassemblyChapter 16: Anti-DebuggingChapter 17: Anti-Virtual Machine

TechniquesChapter 18: Packing and Unpacking

Chapter 15: Anti-Disassembly

Anti-Disassembly1. Understanding Anti-Disassembly

2. Defeating Disassembly Algorithms

3. Anti-Disassembly Techniques

4. Obscuring Flow Control

5. Thwarting Stack-Frame Analysis

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

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)

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)

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

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)

In-class exercise

Lab 15-01, 15-02

Chapter 16: Anti-Debugging

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

Anti-Debugging1. Windows Debugger Detection

2. Identifying Debugger Behavior

3. Interfering with Debugger Functionality

4. Debugger Vulnerabilities

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)


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)


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)


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)


Debugger artifacts INT 1 overwrites 6 bytes below current ESP with

return values for IP, CS, and FlagsPUSH AXPOP AXDEC SPDEC SPPOP BXCMP AX,BX JNE CODE_IS_TRACED

Force INT 1/INT 3 tracing to disable essential functions

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

Re-pop value from stack. If not the same,then single stepping has clobbered it


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

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

Malware can hide functionality in TLS 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

3. Interfering with Debugger FunctionalityInserting 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)

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 interruptsIN AL, 20hOR AL, 02OUT AL, 20<virus code>IN AL, 20AND AL, NOT 2OUT AL,20

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)

In-class exercise

Lab 16-01

Chapter 17: Anti-Virtual Machine Techniques

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

Anti-Virtual Machine Techniques1. VMware Artifacts

2. Vulnerable Instructions

3. Tweaking Settings

4. Escaping the Virtual Machine

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

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

2. Vulnerable InstructionsDescriptor 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 so VM software creates separate locations But, Guest VM can directly execute x86 instructions that

directly access underlying registers to check for inconsistency Must NOP out these checks

Red Pill x86 instruction sidt loads value of IDTR (Listing 17-2)

No Pill x86 instruction sldt loads value of LDTR x86 instruction sgdt loads value of GDTR

2. Vulnerable InstructionsQuerying 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”

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

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, VM display function

In-class exercise

Lab 17-01

Chapter 18: Packers and Unpacking

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

Packers and Unpacking

1. Packer Anatomy2. Identifying Packed Programs3. Unpacking Options4. Tips and Tricks for Common Packers5. Packed DLLs

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

1. Packer AnatomyUnpacking 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 INVALID0040AC4C 9C PUSHFD0040AC4D 60 PUSHAD0040AC4E 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.

aaaaaaaa...wwwwwwwwxxxxxxxx JNZ zzzzzzzz <-- Loop back to aaaaaaaayyyyyyyy JMP aaaaaaaazzzzzzzz New Instructions

0040CA83 8BBD2E744000 MOV EDI,[EBP+0040742E]0040CA89 E85E040000 CALL 0040CEEC0040CA8E 61 POPAD0040CA8F 9D POPFD0040CA90 50 PUSH EAX0040CA91 68CC104000 PUSH 004010CC0040CA96 C20400 RET 0004

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:01017554 MOV [ESP+1C],EAX015F:01017558 POPAD015F:01017559 JNZ 01017563 (JUMP )015F:01017563 PUSH EAX *** Take note of the valueof EAX!015F:01017564 RET *** Stop here!!!

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

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


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


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

4. Tips and Tricks for Common PackersUPX (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

4. Tips and Tricks for Common PackersPetite

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

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

In-class exercise

Lab 18-1

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