B1g_Mac

Problem

Here's a zip file. You can also find the file in /problems/b1g-mac_0_ac4b0dbedcd3b0f0097a5f056e04f97a.

Solution

  1. Unzip b1g_mac.zip with unzip b1g_mac.zip to get a folder called test (with 18 images inside) and a main.exe executable file.

  2. Running the main.exe file produces No flag found, please make sure this is run on the server, which is an interesting error message because this file cannot be run on the shell server since it is a Windows executable and the shell server runs Linux.

  3. Creating a fake flag file with echo picoCTF{fake_flag} > flag.txt and rerunning the program results in the following:

     Work is done!
 Wait for 5 seconds to exit.

  4. Since something happened lets see if the files are any different by re-extracting the test directory from the zip file and comparing it to the contents of the current test directory: diff -r test/ test_original/. Nothing appears to have changed. Lets reset the test directory and delete the "new" folder in case it actually changed something.

  5. Reverse the binary file using Ghidra (cheat sheet). Open it and in the symbol tree click on main. The decompiled main function will show on the left.

     int __cdecl _main(int _Argc,char **_Argv,char **_Env)

 {
 undefined4 local_60;
 undefined local_5a [50];
 undefined4 local_28;
 undefined4 local_24;
 undefined4 local_20;
 size_t local_1c;
 FILE *local_18;
 int local_14;

 ___main();
 _isOver = 0;
 local_28 = 0x65742f2e;
 local_24 = 0x7473;
 local_20 = 0;
 _folderName = &local_28;
 local_14 = 0;
 _pLevel = 0;
 local_18 = _fopen("flag.txt","r");
 if (local_18 == (FILE *)0x0) {
     _puts("No flag found, please make sure this is run on the server");
 }
 local_1c = _fread(local_5a,1,0x12,local_18);
 if ((int)local_1c < 1) {
                     /* WARNING: Subroutine does not return */
     _exit(0);
 }
 _flag = local_5a;
 _flag_size = 0x12;
 local_60 = 0;
 _flag_index = &local_60;
 _puts("Work is done!");
 _listdir(local_14,_folderName);
 _puts("Wait for 5 seconds to exit.");
 _sleep(5);
 return 2;
 }

  6. The program opens the flag file, reads 18 characters, initializes some globals and then calls _listdir (after printing "Work is done!" which is strange because nothing much is done before that message is printed). _listdir function:

     void __cdecl _listdir(int param_1,undefined4 param_2)

 {
 int iVar1;
 BOOL BVar2;
 CHAR local_958 [2048];
 _WIN32_FIND_DATAA local_158;
 HANDLE local_18;
 bool local_11;
 int local_10;

 local_18 = (HANDLE)0x0;
 _sprintf(local_958,"%s\\*.*",param_2);
 local_18 = FindFirstFileA(local_958,(LPWIN32_FIND_DATAA)&local_158);
 if (local_18 == (HANDLE)0xffffffff) {
     _printf("Path not found: [%s]\n",param_2);
 }
 else {
     local_10 = 1;
     local_11 = true;
     while (local_11 != false) {
     iVar1 = _strcmp(local_158.cFileName,".");
     if ((iVar1 != 0) && (iVar1 = _strcmp(local_158.cFileName,".."), iVar1 != 0)) {
         _sprintf(local_958,"%s\\%s",param_2,local_158.cFileName);
         if ((local_158.dwFileAttributes & 0x10) == 0) {
         if (local_10 == 1) {
             if (param_1 == 0) {
             _hideInFile(local_958);
             }
             else {
             if (param_1 == 1) {
                 _decodeBytes(local_958);
             }
             }
         }
         local_10 = 1 - local_10;
         }
         else {
         _printf("Folder: %s\n",local_958);
         _listdir(param_1,local_958);
         }
     }
     if (_isOver != '\0') break;
     BVar2 = FindNextFileA(local_18,(LPWIN32_FIND_DATAA)&local_158);
     local_11 = BVar2 != 0;
     }
     FindClose(local_18);
 }
 return;
 }

  7. _listdir is a recursive function which iterates over files in the folder (it was called with the folder "./test" by the main function) and, if param_1 if 0, it calls _hideInFile for every other file (only the 'Copy' files). If param_1 is set to 1, _decodeBytes is called on the file instead.

  8. Right click _listdir and go to "References > Find references to _listdir". One of the three options is an uncalled function labeled _decode:

     void UndefinedFunction_00401afe(void)

 {
 undefined4 uStack40;
 undefined4 uStack36;
 undefined4 uStack32;
 undefined4 uStack28;
 undefined4 uStack24;
 undefined4 uStack20;
 undefined4 uStack16;

 uStack36 = 0x616c6f68;
 uStack32 = 0x202020;
 uStack28 = 0;
 uStack24 = 0;
 uStack20 = 0;
 _buff_size = 0x12;
 uStack40 = 0;
 uStack16 = 1;
 _buff = &uStack36;
 _buff_index = &uStack40;
 _listdir(1,_folderName);
 _printf("value of DECODE %s \n",_buff);
 _puts("Wait for 5 seconds to exit.");
 _sleep(5);
                     /* WARNING: Subroutine does not return */
 _exit(0);
 }

  9. The _decode function uses the same _folderName global variable as the main function. We can call _decode_ using the debugger after _folderName gets initialized in the main function.

  10. Get location in main right after initializing _folderName: 0x00401bda

    00401bc1 c7 44 24        MOV        dword ptr [ESP + local_20],0x0
            50 00 00 
            00 00
00401bc9 8d 44 24 48     LEA        EAX=>local_28,[ESP + 0x48]
00401bcd a3 34 54        MOV        [_folderName],EAX                                = ??
            40 00
00401bd2 c7 44 24        MOV        dword ptr [ESP + local_14],0x0
            5c 00 00 
            00 00
00401bda c7 05 48        MOV        dword ptr [_pLevel],0x31                         = ??
            54 40 00 
            31 00 00 00

  11. Get address of _decode: 0x00401afe

                                 LAB_00401afc                                    XREF[1]:     004019bd(j)  
    00401afc c9              LEAVE
    00401afd c3              RET
                         _decode
    00401afe 55              PUSH       EBP
    00401aff 89 e5           MOV        EBP,ESP
    00401b01 83 ec 38        SUB        ESP,0x38
    00401b04 c7 45 e0        MOV        dword ptr [EBP + -0x20],0x616c6f68
             68 6f 6c 61
    00401b0b c7 45 e4        MOV        dword ptr [EBP + -0x1c],0x202020
             20 20 20 00

  12. Debugger and Get Flag 1. Linux (winedbg - did not work): Open in GDB with winedbg main.exe then set a breakpoint at 0x00401bda with break *0x00401bda (the * means "address" instead of function) then next then set $eip = 0x00401afe then next and get the flag. 2. Windows (x32dbg - Successful): It is important that the file times in the zip are not changed. On Windows, use 7-zip to open and extract these files without modifying their file times. The built-in to Windows extraction option does not work. Run x96dbg from x64dbg and select x32dbg. Then open main.exe in x32dbg and run SetBPX 00401bda to set a breakpoint at 0x00401bda. Next, click through "run" until this breakpoint is reached. When the breakpoint is hit, run EIP=00401afe to change the EIP to _decode. "Run" one more time and get the flag.

    value of DECODE picoCTF{M4cTim35!}
Wait for 5 seconds to exit.

  13. _hideInFile function:

    void __cdecl _hideInFile(LPCSTR param_1)

{
BOOL BVar1;
FILETIME local_2c;
FILETIME local_24;
FILETIME local_1c;
char local_12;
char local_11;
HANDLE local_10;

local_10 = CreateFileA(param_1,0x100,0,(LPSECURITY_ATTRIBUTES)0x0,3,0,(HANDLE)0x0);
_DoNotUpdateLastAccessTime(local_10);
if (local_10 == (HANDLE)0xffffffff) {
    _printf("Error:INVALID_HANDLED_VALUE");
}
else {
    BVar1 = GetFileTime(local_10,(LPFILETIME)&local_1c,(LPFILETIME)&local_24,(LPFILETIME)&local_2c);
    if (BVar1 == 0) {
    _printf("Error: C-GFT-01");
    }
    else {
    local_11 = *(char *)(*_flag_index + _flag);
    *_flag_index = *_flag_index + 1;
    local_12 = *(char *)(*_flag_index + _flag);
    *_flag_index = *_flag_index + 1;
    _encodeBytes(local_11,local_12,(uint *)&local_2c);
    if (0 < _pLevel) {
        local_11 = *(char *)(*_flag_index + _flag);
        *_flag_index = *_flag_index + 1;
        local_12 = *(char *)(*_flag_index + _flag);
        *_flag_index = *_flag_index + 1;
        _encodeBytes(local_11,local_12,(uint *)&local_1c);
    }
    if (_pLevel == 2) {
        local_11 = *(char *)(*_flag_index + _flag);
        *_flag_index = *_flag_index + 1;
        local_12 = *(char *)(*_flag_index + _flag);
        *_flag_index = *_flag_index + 1;
        _encodeBytes(local_11,local_12,(uint *)&local_24);
    }
    BVar1 = SetFileTime(local_10,&local_1c,&local_24,&local_2c);
    if (BVar1 == 0) {
        _printf("Error: C-SFT-01");
    }
    else {
        if (_flag_size <= *_flag_index) {
        _isOver = 1;
        }
        CloseHandle(local_10);
    }
    }
}
return;
}

    We can see that the function is using GetFileTime and SetFileTime, and this also explains the challenge name (MAC stands for "Modification, Access, Creation"). The Windows user interface shows us the date and H:M:S, but NTFS file systems have a resolution of 100 Nanoseconds for these fields (The FILETIME structure represents the time in 100-nanosecond intervals since January 1, 1601). More info about how the flag was encoded and how to write a script to decode it: Dvd848 (Archive) and AMACB (Archive)

Flag

picoCTF{M4cTim35!}

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