Zero2Auto : CH03 Practical Analysis Challenge - Part II

I encourage you to follow this writeup along with your analysis

Welcome back, in this part-II of analysing the challenge binary from Z2A: Ch03 (Practical Analysis) we gonna take a thorough look at the second payload extracted from .rsrc section and injected into another process of stage01 (main_bin.exe). So, let’s get started from the main of this stage..

Inside main()

• first gonna grab the file path of itself using GetModuleFileNameA(), to get the executable/file name using sub_404A23 aka strtok(identified by flirt in cutter) using a loop

• with filename ie. main_bin.exe (remember base payload spawned a child process of its own) in hand, it will run another loop to get filename’s length ie. 0xC, then call to sub_401660(CRC32 hashing), where filename being the first argument and second being the filename length, then compare it to a hard-coded hash 0xB925C42D

• if not equal then continue to call sub_401210(api_hash_resolving) function which takes a module number among three ie.

  

• and a hard-coded hash, then return the API function name to immediately call it in the next line call eax, in this case it resolves the API hash 8436F795h (IsDebuggerPresent) to check if this process is being debugged or not, if being debugged then exit the process

• Otherwise, go ahead and call another anti-debugging check function sub_401000

Inside sub_401000()

• this at first stores a hard-coded set of hash to xmm0 from xmmword_413CE0 variable
• then move on to resolve some API function using sub_401210 and store them in registers to call’em later, those APIs are
  • 0C1F3B876h ie.CreateToolhelp32Snapshot
  • 8197004Ch ie. Process32FirstW
  • 0BC6B67BFh ie. Process32NextW
• next it calls CreateToolhelp32Snapshot(call esi) to take snapshot of running process, then if successfully retrieved the handle, then call Process32FirstW where the first arg is handle to snapshot and second arg being a PROCESSENTRY32W struct after setting PROCESSENTRY32W->dwSize to 0x22C(556),
• then move on to call Process32FirstW to store the info about first process from snap in PROCESSENTRY32W struct, then lower the string of PROCESSENTRY32W->szExeName (where every character is 2 bytes(aka UTF-16) means if string is x32dbg.exe then its hex will be 78 00 33 00 32 00 64 00 62 00 67 00 2e 00 65 00 78 00 65 00 ) also look at the dump for x32dbg.exe string

• contrary to previous sub_401600 call where string chars are only 1byte long were passed as argument means x32dgb.exe(78 33 32 64 62 67 2e 65 78 65) (pointing this out ‘cause this makes difference in their hash calculated) , then calculate crc32hash to compare the hash the against hardcoded hash set of 4 stored in xmm0 register earlier ie.
  • 7c6ffe70(processhacker.exe)
  • 47742a22(wireshark.exe)
  • d2f05b7d(x32dbg.exe)
  • 659b537e(x64dbg.exe)
• if not matched then call Process32NextW to goto the next process in snapshot, and repeat the comparision until any of’em matches, if none of’em matches then exit the function with return value of 0
• Otherwise, if any of running processes calculated hash matches to any of 4, then exit the function with return value of 1

If last function returned 0(representing not being debugged), then move on to call sub_401D50, which gonna resolve some more API function and store them in for later use, with that done this calls another function sub_401CA0

Inside sub_401CA0()

• first set some memory of 0x40 bytes for struct STARTUPINFOA

• then move on to decrypt some encrypted data stored in xmmword_413C5C and xmmword_413C6C ie. 7C6D1DBD1FEF1D5DDC6CCCBC5FEF891E and 7CAD7CC86D1DDCAC1C4D1DEF0919FC followed by a decryption loop containing shifting and xor to get string C:\Windows\System32\svchost.exe

• then create the process svchost.exe using CreateProcessA (which was resolved in earlier function) in suspended state and returns it PROCESS_INFORMATION struct

• moving ahead in main() (see image below), we see a call to GetModuleHandleW with 0 as argument means the module address of its own(stage02) which will be 0x400000

• then allocate some memory space of size 0x18000(SizeOfImage of stage02) using VirtualAlloc to copy the stage02 to allocated memory using call sub_4037B0

• now this allocates memory of size 0x18000 in suspended svchost.exe using VirtualAllocEx

• then using some loop, gonna perform base relocation(check this out for more on Base Relocation table) for the copied executable based on address returned from VirtualAlloc call, tldr; this performs base relocation which needs to done when PE file is loaded to other address than image base address, in this case this is relocating the copied stage02 binary(present in lpBuffer = lpMem) to adjust the base relocation table with base address equal to address returned from VirtualAlloc

• after relocation, call to a WriteProcessMemory to write the relocated stage02 in the suspended svchost.exe,
• then create a remote thread in svchost.exe with lpStartAddress set to sub_401DC0 relocated to new base address, with dwCreationFlags set to 0 means execute this routine immediately after creating the thread,

TIP

How to break into sub_401DCO ?

• Now, in order to analyse this function in x32dbg , again attach the svchost.exe process before calling CreateRemoteThread
• follow the subroutine address(at offset 0x1DC0) in disassembler and place a breakpoint there
• then after calling CreateRemoteThread in parent process, jump onto the svchost.exe process and resume the debugger and you’ll break on this function, if all goes good.

We’ve covered enough for a quick break, see you back in a bit

---

Inside sub_401DC0()

Welcome back, i saw there you didn’t had a break, nvm:) let’s carry on..

Now, let’s examine this function, as this function is executed as a new thread within svchost.exe process

• first gonna resolve 4 API functions related to Internet, then decrypt some encrypted data at xmmword_41C7C , xmmword_413C8C, 0xEA using rotate left by 4, then xor by 0xC5 to get this URL string https://pastebin.com/raw/mLem9DGk:

• now pass the decrypted URL as only argument to sub_401290() ie. getDataFromUrl which gonna reach out to URL and return the data received ie. another URL https://i.ibb.co/KsfqHym/PNG-02-Copy.png of an image
• again pass the data received (another URL) to sub_4013A0() as only argument

---

Inside sub_4013A0()

• get data ie. a PNG file from URL passed to this function using sub_401290() ie. getDataFromUrl,
• then decrypt the data at qword_413CA4, qword_413CAC by rotating left by 4 then xor by 1F to get string \output.jpg (keep endians in mind) and convert this string to UTF-16(wide character) by calling MultiByteToWideChar
• resolve 4 more API function using api_hash_resolving_401210 ie. GetTempPathW, CreateDirectoryW, CreateFileW, WriteFile
• grab temporary directory path using GetTempPathW, to create a folder named cruloader using CreateDirectoryW, within this folder create a file named output.jpg and write the PNG file data received earlier using WriteFile,
• again decrypt data stored at word_413CCC, qword_413CC4 ie. 8E FF EF BF 5F 6F FE 8E 9F by rotating them left by 4 to xor them by 0x9A to get string redaolurc
• Now using loop it finds the offset of data after string redaolurc (at offset 0x41107) in PNG file ie. 0x41110, then get the size of this data ie. 9384bytes(0x24A8) followed by another loop to decrypt the data ie. xor by 0x61 which reveals another executable say stage03
• again call sub_401D50 to resolve some api function related to process creation and injection, then spawn another svchost.exe process using same call to sub_401CA0 as earlier, then gonna call sub_401750

---

Inside sub_401750()

• this functions starts off by capturing the thread Context of newly spawned child svchost.exe process using GetThreadContext(), if function returns success then proceed otherwise return 1

• on success, read the image base address of suspended svchost.exe from PEB->ImageBaseAddress using ReadProcessMemory()

• on success, will compare the image base address of stage03 and suspended svchost.exe, if equal then NtUnmapViewOfSection() is called to unmap view at child svchost image base address, otherwise if base addresses are not equal

• then move on to allocate memory of size 0x6000(size of image of stage03) in child svchos.exe using VitualAllocEx where lpBaseAddress set to 0x400000(image base of stage03), if allocation not successfull, then retry memory allocation at any place (lpBaseAddress = 0), if this fails again then exit

• elsewsie, move on to replace PEB->ImageBaseAddress of child svchost.exe with allocated memory in svchost.exe using WriteProcessMemory(),

• then modify the stage-03 payload by changing value of windows subsystem(IMAGE_NT_HEADERS->OptionalHeader.Subsytem) from 3(IMAGE_SUBSYSTEM_WINDOWS_CUI) to 2(IMAGE_SUBSYSTEM_WINDOWS_GUI)

• then check if previously allocated memory in child svchost.exe is equal to image base address of stage03 payload, if not equal then relocation is done resp. to address allocated, otherwise if they’re equal

• then make a jump to set the thread Context->eax (of child svchost) to the entry point(at offset 0x126F of stage-03) then call SetThreadContext to set modified thread context in child svchost

• now using WriteProcessMemory, VirtualProtectEx gonna map the headers, sections with their protections of stage-03 payload to the allocated memory in child svchost.exe, like this

• then finally call ResumeThread() to continue execution of child svchost.exe from stage03’s entry point

---

TL;DR

Now that we know that this stage injects itself in a created svchost process to execute sub_401DC0, which reach out to internet to get an url to get a png file which contains an executable(stage03) file which is then again injected to a created svchost process, next we’ll see what stage03 is capable of.

---

Stage03 : Inside main()

All this stage does is display a message box and exit.

Hope you had fun reading this walkthrough and I’ll see you in another post.

Till then have a nice time :)

---

API hash resolve

# api_name_resolve.py
# Author: ME :D
 
import pefile
import zlib
 
def api_hash_resolve(dllToLoad, hashToResolve):
    pe = pefile.PE(f'C:\Windows\SysWOW64\{dllToLoad}')
 
    for exp in pe.DIRECTORY_ENTRY_EXPORT.symbols:
        if exp.name is not None:
            if zlib.crc32(exp.name) == int(hashToResolve, 16):
                print(exp.name.decode())
 
def main():
    dllList = ["kernel32.dll", "ntdll.dll", "wininet.dll"]
    
    print(f"DLLs Available: \n0: {dllList[0]}\n1: {dllList[1]}\n2: {dllList[2]}")
    dllNumber = int(input("Choose DLL number: "))
    hashToResolve = input("Hash(hex) to Resolve: ")
 
    api_hash_resolve(dllList[dllNumber], hashToResolve)
 
if "__main__" == __name__:
    main()

Stage03 Extraction/Decryption from PNG file data

# stage03_extraction.py
# Author: ME :D
 
import urllib3
 
http = urllib3.PoolManager()
res = http.request('GET', 'https://i.ibb.co/KsfqHym/PNG-02-Copy.png')
 
xored_bytes_for_executable = bytearray()
 
for i in res.data[0x41110:]:
    xored_bytes_for_executable.append(i^0x61)
 
with open('stage03.bin', 'wb') as f:
    f.write(xored_bytes_for_executable)