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Part 6: Special Topics

This chapter explores shellcode analysis, covering topics such as position-independent code, execution location identification, manual symbol resolution, shellcode encodings, and NOP sleds.

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Part 6: Special Topics

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  1. Chapter 19: Shellcode Analysis Chapter 20: C++ Analysis Chapter 21: 64-bit Malware Part 6: Special Topics

  2. Chapter 19: ShellcodeAnalysis

  3. Shellcode Analysis Shellcode • Payload of raw executable code that allows adversary to obtain interactive shell access on compromised system • Often used alongside exploit to subvert running program Issues to overcome • Getting placed in preferred memory location • Applying address locations if it cannot be placed properly • Loading required libraries and resolving external dependencies

  4. Shellcode Analysis 1. Position-Independent Code 2. Identifying Execution Location 3. Manual Symbol Resolution 4. Shellcode Encodings 5. NOP Sleds 6. Finding Shellcode

  5. 1. Position-Independent Code No hard-coded addresses • All branches and jumps must be relative so that code can be placed anywhere in memory and still function as intended • Essential in exploit code and shellcode being injected from a remote location since addresses are not necessarily known • Table 19-1, p. 408

  6. 2. Identifying Execution Location Shellcode may need to find out its execution location • 32-bit x86 does not provide EIP-relative access to embedded data as it does for control-flow instructions • Must load EIP into general purpose register • Problem: "mov %eip, %eax" not allowed Two methods • call/pop • fnstenv

  7. 2. Identifying Execution Location • call/pop • call pushes EIP of next instruction onto stack, pop retrieves it (Listing 19-1, p. 410)

  8. 2. Identifying Execution Location • fnstenv • stores 28-byte structure of FPU state to memory • FPU runs parallel to CPU, so must be able to accurately identify faulting instruction • fpu_instruction_pointer = EIP of last instruction that used FPU (Listing 19-2, 19-3, p. 412)

  9. 2. Identifying Execution Location • fnstenv

  10. 3. Manual Symbol Resolution Shellcode can not use Windows loader to find where libraries are in process memory • Must dynamically locate functions such as LoadLibraryA and GetProcAddress (both located in kernel32.dll) Finding kernel32.dll in memory • Undocumented structure traversal (Figure 19-1, Listing 19-4, p. 414, 415)

  11. Manual Symbol Resolution

  12. Manual Symbol Resolution Once found, then parse PE Export Data in kernel32.dll • Addresses of exported calls in header (relative virtual addresses in IMAGE_EXPORT_DIRECTORY ) • AddressOfFunctions, AddressOfNames, AddressOfNameOrdinals arrays (Figure 19-2, p. 417)

  13. Manual Symbol Resolution • Algorithm for resolution • Iterate over the AddressOfNames array looking at each char* entry, and perform a string comparison against the desired symbol until a match is found. Call this index into AddressOfNamesiName. • Index into the AddressOfNameOrdinals array using iName. The value retrieved is the value iOrdinal. • Use iOrdinal to index into the AddressOfFunctions array. The value retrieved is the RVA of the exported symbol. Return this value to the requester.

  14. Manual Symbol Resolution Using hashes to parse PE Export Data in kernel32.dll • To make shellcode compact and to avoid signature detection on strings, hashes of function names can be used to compare (Listing 19-5, 19-6, p. 418-419)

  15. Manual Symbol Resolution

  16. Manual Symbol Resolution

  17. 4. Shellcode Encodings Many exploits target unsafe string functions • strcpy, strcat • Injection of malicious code must avoid NULL bytes which will terminate buffer overflow pre-maturely (Listing 19-8, p. 421) strcpy( buffer, argv[1] ); moveax, 0 ; 4 bytes of NULL xoreax, eax ; No NULL bytes

  18. Shellcode Encodings ASCII armor • Hardens against buffer overflow • If exploit requires injection of addresses, then force addresses to include NULL by putting all code/data within ASCII armor at beginning of address space

  19. 5. NOP Sleds Sequence of NOPs preceing shell code • Address targeting inexact • Allows exploit to increase likelihood of hits by giving a range of addresses that will result in shellcode executing Buffer[0..256] [stuff] Return addr [stuff] New Addr New Addr New Addr Shell Code New Addr

  20. IP? IP? IP? IP? 5. NOP Sleds NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP NOP xor eax, eax mov al, 70 xor ebx, ebx xor ecx, ecx int 0x80 jmp short two one: pop ebx xor eax, eax mov [ebx+7], al mov [ebx+8], ebx mov [ebx+12], eax mov al, 11 lea ecx, [ebx+8] lea edx, [ebx+12] int 0x80 two: call one db '/bin/shXAAAABBBB' xor eax, eax mov al, 70 xor ebx, ebx xor ecx, ecx int 0x80 jmp short two one: pop ebx xor eax, eax mov [ebx+7], al mov [ebx+8], ebx mov [ebx+12], eax mov al, 11 lea ecx, [ebx+8] lea edx, [ebx+12] int 0x80 two: call one db '/bin/shXAAAABBBB' Shell Code Shell Code

  21. 6. Finding Shellcode In Javascript • Binary instructions encoded via escaped bytes • %u1122%u3344%u5566%u7788 = 22 11 44 33 55 66 88 77 • Example: FBI Playpen exploit

  22. 6. Finding Shellcode In payloads • Common API calls (VirtualAllocEx, WriteProcessMemory, CreateRemoteThread) • Common opcodes (Call, Unconditional jumps, Loops, Short conditional jumps)

  23. Chapter 20: C++ Analysis

  24. C++ Analysis 1. Object-Oriented Programming 2. Virtual vs. Nonvirtual Functions 3. Creating and Destroying Objects

  25. 1. Object-Oriented Programming Functions (i.e. methods) in C++ associated with particular classes of objects • Classes used to define objects • Similar to struct, but also include functions “this” pointer • Implicit pointer to object that holds the variable being accessed • Implied in every variable access within a function that does not specify an object • Passed as a compiler-generated parameter to a function (typically the ECX register, sometimes ESI) • “thiscall” calling convention (compared to stdcall, cdecl, and fastcall calling conventions) • Listing 20-2, Listing 20-3, p. 429, 430 • 3) loads this pointer into ecx • 4) puts ecx into eax, then access x to compare it to 10

  26. Object-Oriented Programming Overloading and Mangling • Method overloading allows multiple functions to have same name, but accept different parameters • When function called, compiler determines which version to use (Listing 20-4, p. 431) • C++ uses name mangling to support this construct in the PE file • Algorithm for mangling is compiler-specific • IDA Pro demangles based on what it knows about specific compilers (Figure 20-1, p. 431) ?TestFunction@SimpleClass@@QAEXHH@Z public: void __thiscallSimpleClass::TestFunction(int,int) • Can use c++filt on linux (CTF level?) mashimaro <~> 9:19AM % c++filt -n _Z1fv f()

  27. 2. Virtual vs. Nonvirtual Functions Virtual functions • Can be overridden by a subclass (polymorphism) • Execution is determined at runtime with the child subclass overriding the parent • Can keep parent functionality by changing the type of the object to be an instance of the parent class Nonvirtual functions • Execution is determined at compile time • If object is an instance of the parent, the parent class's function will be called, even if the object at run-time belongs to the child class • Table 20-1, p. 433 • On left g() compiled to call A’s foo() • On right g() compiled to check at run-time which foo()to call

  28. Virtual vs. Nonvirtual Functions Implementation • Nonvirtual functions the same • Polymorphism via virtual function tables (vtables), effectively an array of pointers to code (Table 20-2, p. 434) • On left, nonvirtual call as before • On right, vtable traversed in 1) and 2), then function pointer loaded into eax and called • Recognizing vtables • Should point to legitimate subroutines in IDA Pro (sub_####) (Listing 20-6, p. 435) • Switch tables similar but point to legitimate code locations within code in IDA Pro without function prologue (loc_#####) • Virtual functions never directly called • Code xrefs to functions within IDA Pro yield nothing due to use of registers in call • vtable similarity can be used to associate objects • Listing 20-7, p. 436

  29. 3. Creating and Destroying Objects Constructor and Destructor functions • Object either stored on stack for local variables • Object stored in heap if “new” is used • Note: Implicit “deletes” for objects that go out of scope may be handled as exceptions • Listing 20-8, p. 437 • Initializes vtable for object stored on stack • Does multiple loads of vtable (parent, then child) • Creates another object via “new” call (note the name mangling) ??2@YAPAXI@Z void * __cdecl operator new(unsigned int)

  30. In-class exercise Lab 20-1

  31. Chapter 21: 64-bit Malware

  32. 64-bit Malware 1. Why 64-bit Malware? 2. Differences in x64 architectures 3. Windows 32-Bit on Windows 64-Bit 4. 64-Bit Hints at Malware Functionality

  33. 1. Why 64-bit Malware? AMD64 now known as x64 or x86-64 • Similar to 32-bit x86 with quad extensions • Not all tools support it • Backwards compatibility support for 32-bit ensures that 32-bit malware can run on both 32-bit and 64-bit machines Reasons for 64-bit malware • Kernel rootkits must be 64-bit for machines to run on a 64-bit OS • Malware and shellcode being injected into a 64-bit process must be 64-bit

  34. 2. Differences in x64 Architecture 64-bit vs. 32-bit x86 • All addresses and pointers 64-bit • All general purpose registers 64-bit (%rax vs. %eax) • Special purpose registers also 64-bit (%rip vs. %eip) • Double the general purpose registers (%r8-%r15) • %r8 = 64-bits • %r8d = 32-bit DWORD • %r8w = 16-bit WORD • %r8l = 8-bit LOW byte

  35. 2. Differences in x64 Architecture 64-bit vs. 32-bit x86 • RIP-relative data addressing (Listing 21-2 vs. Listing 21-3, p. 443, 444) • Recall “call/pop” needed previously to get location of data attached to exploit code • Allows compiler to easily generate position-independent code • Decreases the amount of relocation needed when code/DLLs are loaded • Without call/pop, it is much harder to identify exploit code offset 0xD3A2

  36. Differences in x64 Architecture 64-bit vs. 32-bit x86 calling convention and stack usage • Similar to “fastcall” • First six parameters passed in %rdi,%rsi,%rdx,%rcx,%r8,%r9 • Additional parameters passed on stack • Hand-coded malware may completely deviate from this convention to confuse analysis • Stack space is allocated at beginning of function call for the duration of the call (i.e. no push/pop within function) (Figure 21-1, p. 445) • Microsoft's 64-bit exception handling model assumes a static stack

  37. Differences in x64 Architecture 64-bit vs. 32-bit x86 calling convention and stack usage • Reverse engineering complicated since local variables and function parameters co-mingled (Listing 21-4 vs 21-5, p. 445, 446)

  38. Differences in x64 Architecture 64-bit vs. 32-bit x86 • Leaf and Nonleaf Functions • Functions that call other functions are non-leaf or frame functions as they require a frame to be allocated • Nonleaf functions required to allocate at least 0x20 bytes of stack space when calling another function to save register parameters (local variables bump allocation beyond 0x20) • 64-Bit Exception Handling • For 32-bit, SEH handling uses stack • For 64-bit, static exception information table stored in PE file and stored in .pdata section per function. (Does not use stack)

  39. 3. Windows 32-Bit on Windows 64-Bit WOW64 subsystem allows 32-bit Windows to run on 64-bit Windows • Redirects accesses to C:\Windows\System32 to C:\Windows\WOW64 • So if 32-bit malware running on 64-bit Windows writes a file to C:\Windows\System32, it shows up in C:\WINDOWS\WOW64 • Similar redirection for registry keys • 32-bit malware wishing to “break-out” of WOW64 to infect 64-bit system can disable redirection (IsWow64Process, C:\Windows\Sysnative, Wow64DisableWow64FsRedirection to disable for a thread)

  40. 4. 64-Bit Hints at Malware Functionality 64-bit shortcuts • Integers often stored in 32-bit values • Pointers are always 64-bit • Can differentiate non-pointers from pointers via size of register used (Table 21-1, p. 448)

  41. Extra

  42. Machine learning in malware • Polymorphic malware difficult to catch with signatures • After unpacking, shares common functions • Can use RCE to identify features to use in an ML model • Humans (you) decide labels of good and bad • Humans (you) decide features that are helpful or not • Example: Cylance (End Point See) • Issue • Difficult to get explainability out of results • What if polymorphic game code gets flagged? • Can one reason about what ML algorithm is doing?

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