Click here to Skip to main content
12,896,568 members (78,835 online)
Click here to Skip to main content
Add your own
alternative version


670 bookmarked
Posted 27 Dec 2005

Inject your code to a Portable Executable file

, 27 Dec 2005 GPL3
Rate this:
Please Sign up or sign in to vote.
This article demonstrates five steps to inject your code in a portable executable (EXE, DLL, OCX,...) file without recompiling source code.



0 Preface

It might be, you demand to comprehend the ways a virus program injects its procedure in to the interior of a portable executable file and corrupts it, or you are interested in implementing a packer or a protector for your specific intention to encrypt the data of your portable executable (PE) file. This article is committed to represent a brief intuition to realize the performance which is accomplished by EXE tools or some kind of mal-wares.

You can employ the source code of this article to create your custom EXE builder. It could be used to make an EXE protector in the right way, or with a wrong intention, to pullulate a virus. However, my purpose of writing this article has been to gaze on the first application, so I will not be responsible for the immoral usage of these methods.

1 Prerequisite

There are no specific mandatory prerequisites to follow the topics in this article. If you are familiar with debugger and also the portable file format, I suggest you to drop the sections 2 and 3, the whole of these sections have been made for people who don’t have any knowledge regarding the EXE file format and also debuggers.

2 Portable Executable file format

The Portable Executable file format was defined to provide the best way for the Windows Operating System to execute code and also to store the essential data which is needed to run a program, for example constant data, variable data, import library links, and resource data. It consists of MS-DOS file information, Windows NT file information, Section Headers, and Section images, Table 1.

2.1 The MS-DOS data

These data let you remember the first days of developing the Windows Operating System, the days. We were at the beginning of a way to achieve a complete Operating System like Windows NT 3.51 (I mean, Win3.1, Win95, Win98 were not perfect OSs). The MS-DOS data causes that your executable file calls a function inside MS-DOS and the MS-DOS Stub program lets it display: "This program can not be run in MS-DOS mode" or "This program can be run only in Windows mode", or some things like these comments when you try to run a Windows EXE file inside MS-DOS 6.0, where there is no footstep of Windows. Thus, this data is reserved for the code to indicate these comments in the MS-DOS operating system. The most interesting part of the MS-DOS data is "MZ"! Can you believe, it refers to the name of "Mark Zbikowski", one of the first Microsoft programmers?

To me, only the offset of the PE signature in the MS-DOS data is important, so I can use it to find the position of the Windows NT data. I just recommend you to take a look at Table 1, then observe the structure of IMAGE_DOS_HEADER in the <winnt.h> header in the <Microsoft Visual Studio .net path>\VC7\PlatformSDK\include\ folder or the <Microsoft Visual Studio 6.0 path>\VC98\include\ folder. I do not know why the Microsoft team has forgotten to provide some comment about this structure in the MSDN library!

typedef struct _IMAGE_DOS_HEADER { // DOS .EXE header <FONT color=green>"MZ"</FONT>
    WORD   e_magic;                // Magic number
    WORD   e_cblp;                 // Bytes on last page of file
    WORD   e_cp;                   // Pages in file
    WORD   e_crlc;                 // Relocations
    WORD   e_cparhdr;              // Size of header in paragraphs
    WORD   e_minalloc;             // Minimum extra paragraphs needed
    WORD   e_maxalloc;             // Maximum extra paragraphs needed
    WORD   e_ss;                   // Initial (relative) SS value
    WORD   e_sp;                   // Initial SP value
    WORD   e_csum;                 // Checksum
    WORD   e_ip;                   // Initial IP value
    WORD   e_cs;                   // Initial (relative) CS value
    WORD   e_lfarlc;               // File address of relocation table
    WORD   e_ovno;                 // Overlay number
    WORD   e_res[4];               // Reserved words
    WORD   e_oemid;                // OEM identifier (for e_oeminfo)
    WORD   e_oeminfo;              // OEM information; e_oemid specific
    WORD   e_res2[10];             // Reserved words
    LONG   <FONT color=red>e_lfanew</FONT>;               // File address of the new exe header

e_lfanew is the offset which refers to the position of the Windows NT data. I have provided a program to obtain the header information from an EXE file and to display it to you. To use the program, just try:

PE Viewer

This sample is useful for the whole of this article.

Table 1 - Portable Executable file format structure

00000000  ASCII <FONT color=green>"MZ"</FONT>
00000002  DW 0090
00000004  DW 0003
00000006  DW 0000
00000008  DW 0004
0000000A  DW 0000
0000000C  DW FFFF
0000000E  DW 0000
00000010  DW 00B8
00000012  DW 0000
00000014  DW 0000
00000016  DW 0000
00000018  DW 0040
0000001A  DW 0000
0000001C  DB 00
0000003B  DB 00
0000003C  DD <FONT color=red>000000F0</FONT>
…<BR>Reserved words<BR>…
Offset to PE signature
00000040  º.´.Í!¸\LÍ!<FONT color=green>This program canno</FONT>
00000060  <FONT color=green>t be run in DOS mode.</FONT>...$.......
Windows NT


SignaturePE signature (PE)
<FONT color=red>000000F0</FONT>  ASCII <FONT color=green>"PE"</FONT>
000000F4  DW 014C
000000F6  DW 0003
000000F8  DD 3B7D8410
000000FC  DD 00000000
00000100  DD 00000000
00000104  DW 00E0
00000106  DW 010F
00000108  DW 010B
0000010A  DB 07
0000010B  DB 00
0000010C  DD 00012800
00000110  DD 00009C00
00000114  DD 00000000
00000118  DD 00012475
0000011C  DD 00001000
00000120  DD 00014000
00000124  DD 01000000
00000128  DD 00001000
0000012C  DD 00000200
00000130  DW 0005
00000132  DW 0001
00000134  DW 0005
00000136  DW 0001
00000138  DW 0004
0000013A  DW 0000
0000013C  DD 00000000
00000140  DD 0001F000
00000144  DD 00000400
00000148  DD 0001D7FC
0000014C  DW 0002
0000014E  DW 8000
00000150  DD 00040000
00000154  DD 00001000
00000158  DD 00100000
0000015C  DD 00001000
00000160  DD 00000000
00000164  DD 00000010
Import Table
Resource Table
Exception Table
Certificate File
Relocation Table
Debug Data
Architecture Data
Global Ptr
TLS Table
Load Config Table
Bound Import Table
Import Address Table
Delay Import Descriptor
COM+ Runtime Header
000001E8  ASCII<FONT color=green>".text"</FONT>
000001F0  DD 000126B0
000001F4  DD 00001000
000001F8  DD 00012800
000001FC  DD 00000400
00000200  DD 00000000
00000204  DD 00000000
00000208  DW 0000
0000020A  DW 0000
0000020C  DD 60000020
00000210  ASCII<FONT color=green>".data"</FONT>; SECTION
00000218  DD 0000101C ; VirtualSize = 0x101C
0000021C  DD 00014000 ; VirtualAddress = 0x14000
00000220  DD 00000A00 ; SizeOfRawData = 0xA00
00000224  DD 00012C00 ; PointerToRawData = 0x12C00
00000228  DD 00000000 ; PointerToRelocations = 0x0
0000022C  DD 00000000 ; PointerToLineNumbers = 0x0
00000230  DW 0000     ; NumberOfRelocations = 0x0
00000232  DW 0000     ; NumberOfLineNumbers = 0x0
00000234  DD C0000040 ; Characteristics = 
00000238  ASCII<FONT color=green>".rsrc"</FONT>; SECTION
00000240  DD 00008960 ; VirtualSize = 0x8960
00000244  DD 00016000 ; VirtualAddress = 0x16000
00000248  DD 00008A00 ; SizeOfRawData = 0x8A00
0000024C  DD 00013600 ; PointerToRawData = 0x13600
00000250  DD 00000000 ; PointerToRelocations = 0x0
00000254  DD 00000000 ; PointerToLineNumbers = 0x0
00000258  DW 0000     ; NumberOfRelocations = 0x0
0000025A  DW 0000     ; NumberOfLineNumbers = 0x0
0000025C  DD 40000040 ; Characteristics = 
00000400  EA 22 DD 77 D7 23 DD 77  ê"Ýw×#Ýw
00000408  9A 18 DD 77 00 00 00 00  šÝw....
00000410  2E 1E C7 77 83 1D C7 77  .ÇwƒÇw
00000418  FF 1E C7 77 00 00 00 00  ÿÇw....
00000420  93 9F E7 77 D8 05 E8 77  "ŸçwØèw
00000428  FD A5 E7 77 AD A9 E9 77  ý¥çw­©éw
00000430  A3 36 E7 77 03 38 E7 77  £6çw8çw
00000438  41 E3 E6 77 60 8D E7 77  Aãæw`çw
00000440  E6 1B E6 77 2B 2A E7 77  ææw+*çw
00000448  7A 17 E6 77 79 C8 E6 77  zæwyÈæw
00000450  14 1B E7 77 C1 30 E7 77  çwÁ0çw
0001BF00  63 00 2E 00 63 00 68 00  c...c.h.
0001BF08  6D 00 0A 00 43 00 61 00  m...C.a.
0001BF10  6C 00 63 00 75 00 6C 00  l.c.u.l.
0001BF18  61 00 74 00 6F 00 72 00  a.t.o.r.
0001BF20  11 00 4E 00 6F 00 74 00  .N.o.t.
0001BF28  20 00 45 00 6E 00 6F 00   .E.n.o.
0001BF30  75 00 67 00 68 00 20 00  u.g.h. .
0001BF38  4D 00 65 00 6D 00 6F 00  M.e.m.o.
0001BF40  72 00 79 00 00 00 00 00  r.y.....
0001BF48  00 00 00 00 00 00 00 00  ........
0001BF50  00 00 00 00 00 00 00 00  ........
0001BF58  00 00 00 00 00 00 00 00  ........
0001BF60  00 00 00 00 00 00 00 00  ........
0001BF68  00 00 00 00 00 00 00 00  ........
0001BF70  00 00 00 00 00 00 00 00  ........
0001BF78  00 00 00 00 00 00 00 00  ........

2.2 The Windows NT data

As mentioned in the preceding section, e_lfanew storage in the MS-DOS data structure refers to the location of the Windows NT information. Hence, if you assume that the pMem pointer relates the start point of the memory space for a selected portable executable file, you can retrieve the MS-DOS header and also the Windows NT headers by the following lines, which you also can perceive in the PE viewer sample (pelib.cpp, PEStructure::OpenFileName()):

<PRE lang=c++>IMAGE_DOS_HEADER image_dos_header; IMAGE_NT_HEADERS image_nt_headers; PCHAR pMem; … memcpy(&image_dos_header, pMem, sizeof(IMAGE_DOS_HEADER)); memcpy(&image_nt_headers, pMem+image_dos_header.e_lfanew, sizeof(IMAGE_NT_HEADERS));

It seems to be very simple, the retrieval of the headers information. I recommend inspecting the MSDN library regarding the IMAGE_NT_HEADERS structure definition. It makes comprehensible to grasp what the image NT header maintains to execute a code inside the Windows NT OS. Now, you are conversant with the Windows NT structure, it consists of the "PE" Signature, the File Header, and the Optional Header. Do not forget to take a glimpse at their comments in the MSDN Library and besides in Table 1.

One the whole, I consider merely, on the most circumstances, the following cells of the IMAGE_NT_HEADERS structure:

<PRE lang=c++>FileHeader->NumberOfSections OptionalHeader->AddressOfEntryPoint OptionalHeader->ImageBase OptionalHeader->SectionAlignment OptionalHeader->FileAlignment OptionalHeader->SizeOfImage OptionalHeader-> DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT]->VirtualAddress OptionalHeader->DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT]->Size

You can observe clearly, the main purpose of these values, and their role when the internal virtual memory space allocated for an EXE file by the Windows OS is fully allocated, if you pay attention to their explanations in MSDN library, so I am not going to repeat the MSDN annotations here.

I should mention a brief comment regarding the PE data directories, or OptionalHeader-> DataDirectory[], as I think there are a few aspects of interest concerning them. When you come to survey the Optional header through the Windows NT information, you will find that there are 16 directories at the end of the Optional Header, where you can find the consecutive directories, including their Relative Virtual Address and Size. I just mention here, the notes from <winnt.h> to clarify these information:

<PRE lang=c++>#define IMAGE_DIRECTORY_ENTRY_EXPORT 0 // Export Directory #define IMAGE_DIRECTORY_ENTRY_IMPORT 1 // Import Directory #define IMAGE_DIRECTORY_ENTRY_RESOURCE 2 // Resource Directory #define IMAGE_DIRECTORY_ENTRY_EXCEPTION 3 // Exception Directory #define IMAGE_DIRECTORY_ENTRY_SECURITY 4 // Security Directory #define IMAGE_DIRECTORY_ENTRY_BASERELOC 5 // Base Relocation Table #define IMAGE_DIRECTORY_ENTRY_DEBUG 6 // Debug Directory #define IMAGE_DIRECTORY_ENTRY_ARCHITECTURE 7 // Architecture Specific Data #define IMAGE_DIRECTORY_ENTRY_GLOBALPTR 8 // RVA of GP #define IMAGE_DIRECTORY_ENTRY_TLS 9 // TLS Directory #define IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG 10 // Load Configuration Directory #define IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT 11 // Bound Import Directory in headers #define IMAGE_DIRECTORY_ENTRY_IAT 12 // Import Address Table #define IMAGE_DIRECTORY_ENTRY_DELAY_IMPORT 13 // Delay Load Import Descriptors #define IMAGE_DIRECTORY_ENTRY_COM_DESCRIPTOR 14 // COM Runtime descriptor

The last one (15) was reserved for use in future; I have not yet seen any purpose to use it even in PE64.

For instance, if you desire to perceive the relative virtual address (RVA) and the size of the resource data, it is enough to retrieve them by:

<PRE lang=c++>DWORD dwRVA = image_nt_headers.OptionalHeader-> DataDirectory[IMAGE_DIRECTORY_ENTRY_RESOURCE]->VirtualAddress; DWORD dwSize = image_nt_headers.OptionalHeader-> DataDirectory[IMAGE_DIRECTORY_ENTRY_RESOURCE]->Size;

To comprehend more regarding the significance of data directories, I forward you to section 3.4.3, Microsoft Portable Executable and the Common Object File Format Specification document by Microsoft, and furthermore section 6 of this document, where you discern the various types of sections and their applications. We will discuss the section's advantage subsequently.

2.3 The Section Headers and Sections

We currently observe how the portable executable files declare the location and the size of a section on a disk storage file and inside the virtual memory space allocated for the program with IMAGE_NT_HEADERS-> OptionalHeader->SizeOfImage by the Windows task manager, as well the characteristics to demonstrate the type of the section. To understand better the Section header as my previous declaration, I suggest having a short gape on the IMAGE_SECTION_HEADER structure definition in the MSDN library. For an EXE packer developer, VirtualSize, VirtualAddress, SizeOfRawData, PointerToRawData, and Characteristics cells have significant rules. While developing an EXE packer, you should be clever enough to play with them. There are somethings to be noted while you modify them; you should take care to align the VirtualSize and VirtualAddress according to OptionalHeader->SectionAlignment, as well as SizeOfRawData and PointerToRawData in line with OptionalHeader->FileAlignment. Otherwise, you will corrupt your target EXE file and it will never run. Regarding Characteristics, I pay attention mostly to establish a section by IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE | IMAGE_SCN_CNT_INITIALIZED_DATA, I prefer my new section has ability to initialize such data during running process; such as import table; besides, I need it to be able to modify itself by the loader with my settings in the section characteristics to read- and writeable.

Moreover, you should pay attention to the section names, you can know the purpose of each section by its name. I will just forward you to section 6: Microsoft Portable Executable and the Common Object File Format Specification documents. I believe, it represents the totality of sections by their names, Table 2.

Table 2 - Section names

".text"Code Section
"CODE"Code Section of file linked by Borland Delphi or Borland Pascal
".data"Data Section
"DATA"Data Section of file linked by Borland Delphi or Borland Pascal
".rdata"Section for Constant Data
".idata"Import Table
".edata" Export Table
".tls"TLS Table
".reloc"Relocation Information
".rsrc"Resource Information

To comprehend the section headers and also the sections, you can run the sample PE viewer. By this PE viewer, you only can realize the application of the section headers in a file image, so to observe the main significance in the Virtual Memory, you should try to load a PE file by a debugger, and the next section represents the main idea of using the virtual address and –size in the virtual memory by using a debugger. The last note is about IMAGE_NT_HEADERS-> FileHeader-><CODE>NumberOfSections, that provides a number of sections in a PE file, do not forget to adjust it whenever you remove or add some sections to a PE file, I am talking about section injection!

3 Debugger, Disassembler and some Useful Tools

In this part, you will become familiar with the necessary and essential equipments to develop your PE tools.

3.1 Debuggers

The first essential prerequisite, to become a PE tools developer, is to have enough experience with bug tracer tools. Furthermore, you should know most of the assembly instructions. To me, the Intel documents are the best references. You can obtain them from the Intel site for IA-32, and on top of that IA-64; the future belongs to IA-64 CPUs, Windows XP 64-bit, and also PE64!

To trace a PE file, SoftICE by Compuware Corporation, I knew it also as named NuMega when I was at high school, is the best debugger in the world. It implements process tracing by using kernel mode method debugging without applying Windows debugging application programming interface (API) functions. In addition, I am going to introduce one perfect debugger in user mode level. It utilizes the Windows debugging API to trace a PE file and also attaches itself to an active process. These API functions have been provided by Microsoft teams, inside the Windows Kernel32 library, to trace a specific process, by using Microsoft tools, or perhaps, to make your own debugger! Some of those API functions inlude: CreateThread(), CreateProcess(), OpenProcess(), DebugActiveProcess(), GetThreadContext(), SetThreadContext(), ContinueDebugEvent(), DebugBreak(), ReadProcessMemory(), WriteProcessMemory(), SuspendThread()</A>, and ResumeThread().

3.1.1 SoftICE

It was in 1987; Frank Grossman and Jim Moskun decided to establish a company called NuMega Technologies in Nashua, NH, in order to develop some equipments to trace and test the reliability of Microsoft Windows software programs. Now, it is a part of Compuware Corporation and its product has participated to accelerate the reliability in Windows software, and additionally in Windows driver developments. Currently, everyone knows the Compuware DriverStudio which is used to establish an environment for implementing the elaboration of a kernel driver or a system file by aiding the Windows Driver Development Kit (DDK). It bypasses the involvement of DDK to implement a portable executable file of kernel level for a Windows system software developer. For us, only one instrument of DriverStudio is important, SoftICE, this debugger can be used to trace every portable executable file, a PE file for user mode level or a PE file for kernel mode level.

Figure 1 - SoftICE Window

EDI=7C919738 EBP=0007FFF0 ESP=0007FFC4 EIP=010119E0 o d i s z a p c
CS=0008 DS=0023 SS=0010 ES=0023 FS=0030 GS=0000
0023:01013000 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0023:01013010 01 00 00 00 20 00 00 00-0A 00 00 00 0A 00 00 00 ................
0023:01013020 20 00 00 00 00 00 00 00-53 63 69 43 61 6C 63 00 ........SciCalc.
0023:01013030 00 00 00 00 00 00 00 00-62 61 63 6B 67 72 6F 75 ........backgrou
0023:01013040 6E 64 00 00 00 00 00 00-2E 00 00 00 00 00 00 00 nd..............
0010:0007FFC4 4F 6D 81 7C 38 07 91 7C-FF FF FF FF 00 90 FD 7F Om |8 ‘| .
0010:0007FFD4 ED A6 54 80 C8 FF 07 00-E8 B4 F5 81 FF FF FF FF T .
0010:0007FFE4 F3 99 83 7C 58 6D 81 7C-00 00 00 00 00 00 00 00 Xm |........
0010:0007FFF4 00 00 00 00 E0 19 01 01-00 00 00 00 00 00 00 00 .... ....
010119E0 PUSH EBP
010119E1 MOV EBP,ESP
010119E3 PUSH -1
010119E5 PUSH 01001570
010119EA PUSH 01011D60
010119F5 PUSH EAX
010119FD ADD ESP,-68
01011A00 PUSH EBX
01011A01 PUSH ESI
01011A02 PUSH EDI
01011A06 MOV DWORD PTR SS:[EBP-4],0

3.1.2 OllyDbg

It was about 4 years ago, that I first saw this debugger by chance. For me, it was the best choice, I was not so wealthy to purchase SoftICE, and at that time, SoftICE only had good functions for DOS, Windows 98, and Windows 2000. I found that this debugger supported all kinds of Windows versions. Therefore, I started to learn it very fast, and now it is my favorite debugger for the Windows OS. It is a debugger that can be used to trace all kinds of portable executable files except a Common Language Infrastructure (CLI) file format in user mode level, by using the Windows debugging API. Oleh Yuschuk, the author, is one of worthiest software developers I have seen in my life. He is a Ukrainian who now lives in Germany. I should mention here that his debugger is the best choice for hacker and cracker parties around the world! It is a freeware! You can try it from OllyDbg Homepage.

Figure 2 - OllyDbg CPU Window

<FONT color=gray><FONT color=red>010119E0</FONT>
<FONT color=red>$ 55</FONT>
. 8BEC
. 6A FF
. 68 70150001
. 68 601D0101
. 64:A1 00000000
. 50
. 64:8925 000000
. 83C4 98
. 53
. 56
. 57
. 8965 E8
. C745 FC 0000000
. 6A 02
. FF15 B8100001
. 83C4 04
. C705 783F0101 F
. C705 7C3F0101 F
. FF15 BC100001
. 8B0D 743F0101
. 8908
. FF15 C0100001
. 8B15 703F0101
. 8910
. A1 C4100001
. 8B08
. 890D 803F0101
. E8 F6020000
. A1 F0390101
. 85C0
. 75 0E
<FONT color=red>PUSH EBP</FONT>
PUSH CALC.01001570
PUSH JMP.&MSVCRT._except_handler3 
MOV DWORD PTR DS:[1013F78],-1
CALL CALC.01011D50
<FONT color=gray>EAX<FONT color=red> 00000000</FONT> 
ECX<FONT color=red> 0007FFB0</FONT>
EDX<FONT color=red> 7C90EB94</FONT> ntdll.KiFastSystemCallRet
EBX 7FFD8000
ESP<FONT color=red> 0007FFC4</FONT>
EBP<FONT color=red> 0007FFF0</FONT>
EDI 7C910738 ntdll.7C910738
EIP<FONT color=red> 010119E0</FONT> CALC.ModuleEntryPoint
C 0  ES 0023 32bit 0(FFFFFFFF)
P <FONT color=red>1</FONT>  CS 001B 32bit 0(FFFFFFFF)
A 0  SS 0023 32bit 0(FFFFFFFF)
Z <FONT color=red>1</FONT>  DS 0023 32bit 0(FFFFFFFF)
S 0  FS <FONT color=red>003B</FONT> 32bit 7FFDF000(FFF)
T 0  GS 0000 NULL
D 0
O 0  LastErr <FONT color=red>ERROR_ALREADY_EXISTS (000000B7)</FONT>
EFL <FONT color=red>00000246</FONT> (NO,NB,E,BE,NS,PE,GE,LE)
ST0 empty -UNORM BBB0 01050104 00000000
ST1 empty 0.0
ST2 empty 0.0
ST3 empty 0.0
ST4 empty 0.0
ST5 empty 0.0
ST6 empty <FONT color=red>1.0000000000000000000</FONT>
ST7 empty <FONT color=red>1.0000000000000000000</FONT> 
               3 2 1 0      E S P U O Z D I
FST <FONT color=red>4020</FONT>  Cond <FONT color=red>1</FONT> 0 0 0  Err 0 0 <FONT color=red>1</FONT> 0 0 0 0 0
FCW 027F  Prec NEAR,53  Mask    1 1 1 1 1 1</FONT>
<FONT color=gray>01013000  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
01013010  01 00 00 00 20 00 00 00 0A 00 00 00 0A 00 00 00  ................
01013020  20 00 00 00 00 00 00 00 53 63 69 43 61 6C 63 00  ........SciCalc.
01013030  00 00 00 00 00 00 00 00 62 61 63 6B 67 72 6F 75  ........backgrou
01013040  6E 64 00 00 00 00 00 00 2E 00 00 00 00 00 00 00  nd..............
01013050  30 00 00 00 FF FF FF FF 01 00 00 00 00 00 00 00  0...     .......
01013060  00 00 57 00 58 00 56 01 5C 02 5D 02 07 03 59 03  ..W.X.V \ ]   Y 
01013070  5E 03 5A 03 5B 03 5F 04 00 00 00 00 FF FF FF FF    Z [ _ ....
01013080  FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF                  
01013090  00 00 00 00 00 00 00 00 84 15 00 01 00 00 00 00  ........  . ....
010130A0  2E 4B 00 00 00 00 00 00 00 00 FF 00 50 00 00 00  .K........ .P...
010130B0  FF 00 00 00 51 00 00 00 FF 00 00 00 52 00 00 00   ...Q... ...R...
010130C0  FF 00 00 00 53 00 00 00 00 00 FF 00 54 00 00 00   ...S..... .T...
010130D0  00 00 FF 00 55 00 00 00 FF 00 00 00 56 00 00 00  .. .U... ...V...
010130E0  FF 00 00 00 57 00 00 00 FF 00 00 00 58 00 00 00   ...W... ...X...</FONT>
<FONT color=gray>0007FFC4  7C816D4F  RETURN to kernel32.7C816D4F
0007FFC8  7C910738  ntdll.7C910738
0007FFD0  7FFD8000
0007FFD4  8054A6ED
0007FFD8  0007FFC8
0007FFDC  82574DA8
0007FFE0  FFFFFFFF  End of SEH chain
0007FFE4  7C8399F3  SE handler
0007FFE8  7C816D58  kernel32.7C816D58
0007FFEC  00000000
0007FFF0  00000000
0007FFF4  00000000
0007FFF8  010119E0  CALC.ModuleEntryPoint
0007FFFC  00000000</FONT>

3.1.3 Which parts are important in a debugger interface?

I have introduced two debuggers without talking about how you can employ them, and also which parts you should pay attention more. Regarding using debuggers, I refer you to their instructions in help documents. However, I want to explain shortly the important parts of a debugger; of course, I am talking about low-level debuggers, or in other words, machine-language debuggers of the x86 CPU families.

All of low-level debuggers consist of the following subdivisions:

  1. Registers viewer.


    o d t s z a p c

  2. Disassembler or Code viewer.

    010119E0 PUSH EBP
    010119E1 MOV EBP,ESP
    010119E3 PUSH -1
    010119E5 PUSH 01001570
    010119EA PUSH 01011D60
    010119EF MOV EAX,DWORD PTR FS:[0]
    010119F5 PUSH EAX
    010119F6 MOV DWORD PTR FS:[0],ESP
    010119FD ADD ESP,-68
    01011A00 PUSH EBX
    01011A01 PUSH ESI
    01011A02 PUSH EDI
    01011A03 MOV DWORD PTR SS:[EBP-18],ESP
    01011A06 MOV DWORD PTR SS:[EBP-4],0 

  3. Memory watcher.

    0023:01013000 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
    0023:01013010 01 00 00 00 20 00 00 00-0A 00 00 00 0A 00 00 00 ................
    0023:01013020 20 00 00 00 00 00 00 00-53 63 69 43 61 6C 63 00 ........SciCalc.
    0023:01013030 00 00 00 00 00 00 00 00-62 61 63 6B 67 72 6F 75 ........backgrou
    0023:01013040 6E 64 00 00 00 00 00 00-2E 00 00 00 00 00 00 00 nd..............

  4. Stack viewer.

    0010:0007FFC4 4F 6D 81 7C 38 07 91 7C-FF FF FF FF 00 90 FD 7F Om |8 ‘| .
    0010:0007FFD4 ED A6 54 80 C8 FF 07 00-E8 B4 F5 81 FF FF FF FF T .
    0010:0007FFE4 F3 99 83 7C 58 6D 81 7C-00 00 00 00 00 00 00 00 Xm |........
    0010:0007FFF4 00 00 00 00 E0 19 01 01-00 00 00 00 00 00 00 00 .... ....

  5. Command line, command buttons, or shortcut keys to follow the debugging process.

    Step IntoF11F7
    Step OverF10F8
    Set Break PointF8F2

You can compare Figure 1 and Figure 2 to distinguish the difference between SoftICE and OllyDbg. When you want to trace a PE file, you should mostly consider these five subdivisions. Furthermore, every debugger comprises of some other useful parts; you should discover them by yourself.

3.2 Disassembler

We can consider OllyDbg and SoftICE as excellent disassemblers, but I also want to introduce another disassembler tool which is famous in the reverse engineering world.

3.2.1 Proview disassembler

Proview or PVDasm is an admirable disassembler by the Reverse-Engineering-Community; it is still under development and bug fixing. You can find its disassmbler source engine and employ it to create your own disassembler.

3.2.2 W32Dasm

W32DASM can disassemble both 16 and 32 bit executable file formats. In addition to its disassembling ability, you can employ it to analyze import, export and resource data directories data.

3.2.3 IDA Pro

All reverse-engineering experts know that IDA Pro can be used to investigate, not only x86 instructions, but that of various kinds of CPU types like AVR, PIC, and etc. It can illustrate the assembly source of a portable executable file by using colored graphics and tables, and is very useful for any newbie in this area. Furthermore, it has the capability to trace an executable file inside the user mode level in the same way as OllyDbg.

3.3 Some Useful Tools

A good PE tools developer is conversant with the tools which save his time, so I recommend to select some appropriate instruments to investigate the base information under a portable executable file.

3.3.1 LordPE

LordPE by y0da is still the first choice to retrieve PE file information with the possibility to modify them.

3.3.2 PEiD

PE iDentifier is valuable to identify the type of compilers, packers, and cryptors of PE files. As of now, it can detect more than 500 different signature types of PE files.

3.3.3 Resource Hacker

Resource Hacker can be employed to modify resource directory information; icon, menu, version info, string table, and etc.

3.3.4 WinHex

WinHex, it is clear what you can do with this tool.

3.3.5 CFF Explorer

Eventually, CFF Explorer by Ntoskrnl is what you wish to have as a PE Utility tool in your dream; it supports PE32/64, PE rebuild included Common Language Infrastructure (CLI) file, in other words, the .NET file, a resource modifier, and much more facilities which can not be found in others, just try and discover every unimaginable option by hand.

4 Add new section and Change OEP

We are ready to do the first step of making our project. So I have provided a library to add a new section and rebuild the portable executable file. Before starting, I like you get familiar with the headers of a PE file, by using OllyDbg. You should first open a PE file, that pops up a menu, View->Executable file, again get a popup menu Special->PE header. And you will observe a scene similar to Figure 3. Now, come to Main Menu View->Memory, try to distinguish the sections inside the Memory map window.

Figure 3

4D 5A
ASCII <FONT color=green>"MZ"</FONT>
DW 0090
DW 0003
DW 0000
DW 0004
DW 0000
DW 0000
DW 00B8
DW 0000
DW 0000
DW 0000
DW 0040
DW 0000
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DB 00
DD <FONT color=red>000000F0</FONT>
DOS EXE Signature
DOS_PartPag = 90 (144.)
DOS_PageCnt = 3
DOS_ReloCnt = 0
DOS_HdrSize = 4
DOS_MinMem = 0
DOS_MaxMem = FFFF (65535.)
DOS_ReloSS = 0
DOS_ExeSP = B8
DOS_ChkSum = 0
DOS_ExeIP = 0
DOS_ReloCS = 0
DOS_TablOff = 40
DOS_Overlay = 0

Offset to PE signature

I want to explain how we can plainly change the Offset of Entry Point (OEP) in our sample file, CALC.EXE of Windows XP. First, by using a PE Tool, and also using our PE Viewer, we find OEP, 0x00012475, and Image Base, 0x01000000. This value of OEP is the Relative Virtual Address, so the Image Base value is used to convert it to the Virtual Address.

Virtual_Address = Image_Base + Relative_Virtual_Address

<PRE lang=c++>DWORD OEP_RVA = image_nt_headers->OptionalHeader.AddressOfEntryPoint ; // OEP_RVA = 0x00012475 DWORD OEP_VA = image_nt_headers->OptionalHeader.ImageBase + OEP_RVA ; // OEP_VA = 0x01000000 + 0x00012475 = 0x01012475

PE Maker - Step 1

CALC.EXE - test file

DynLoader(), in loader.cpp, is reserved for the data of the new section, in other words, the Loader.

DynLoader Step 1

<PRE lang=c++>__stdcall void DynLoader() { _asm { //---------------------------------- DWORD_TYPE(DYN_LOADER_START_MAGIC) //---------------------------------- MOV EAX,01012475h // << Original OEP JMP EAX //---------------------------------- DWORD_TYPE(DYN_LOADER_END_MAGIC) //---------------------------------- } }

Unfortunately, this source can only be applied for the sample test file. We should complete it by saving the value of the original OEP in the new section, and use it to reach the real OEP. I have accomplished it in Step 2 (Section 5).

4.1 Retrieve and Rebuild PE file

I have made a simple class library to recover PE information and to use it in a new PE file.

CPELibrary Class Step 1

<PRE lang=c++>//---------------------------------------------------------------- class CPELibrary { private: //----------------------------------------- PCHAR pMem; DWORD dwFileSize; //----------------------------------------- protected: //----------------------------------------- PIMAGE_DOS_HEADER image_dos_header; PCHAR pDosStub; DWORD dwDosStubSize, dwDosStubOffset; PIMAGE_NT_HEADERS image_nt_headers; PIMAGE_SECTION_HEADER image_section_header[MAX_SECTION_NUM]; PCHAR image_section[MAX_SECTION_NUM]; //----------------------------------------- protected: //----------------------------------------- DWORD PEAlign(DWORD dwTarNum,DWORD dwAlignTo); void AlignmentSections(); //----------------------------------------- DWORD Offset2RVA(DWORD dwRO); DWORD RVA2Offset(DWORD dwRVA); //----------------------------------------- PIMAGE_SECTION_HEADER ImageRVA2Section(DWORD dwRVA); PIMAGE_SECTION_HEADER ImageOffset2Section(DWORD dwRO); //----------------------------------------- DWORD ImageOffset2SectionNum(DWORD dwRVA); PIMAGE_SECTION_HEADER AddNewSection(char* szName,DWORD dwSize); //----------------------------------------- public: //----------------------------------------- CPELibrary(); ~CPELibrary(); //----------------------------------------- void OpenFile(char* FileName); void SaveFile(char* FileName); //----------------------------------------- };

By Table 1, the usage of image_dos_header, pDosStub, image_nt_headers, image_section_header [MAX_SECTION_NUM], and image_section[MAX_SECTION_NUM] is clear. We use OpenFile() and SaveFile() to retrieve and rebuild a PE file. Furthermore, AddNewSection() is employed to create the new section, the important step.

4.2 Create Data for new Section

In pecrypt.cpp, I have represented another class, CPECryptor, to comprise the data of the new section. Nevertheless, the data of the new section is created by DynLoader() in loader.cpp, DynLoader Step 1. We use the CPECryptor class to enter this data in to the new section, and also some other stuff.

CPECryptor Class Step 1

<PRE lang=c++>//---------------------------------------------------------------- class CPECryptor: public CPELibrary { private: //---------------------------------------- PCHAR pNewSection; //---------------------------------------- DWORD GetFunctionVA(void* FuncName); void* ReturnToBytePtr(void* FuncName, DWORD findstr); //---------------------------------------- protected: //---------------------------------------- public: //---------------------------------------- void CryptFile(int(__cdecl *callback) (unsigned int, unsigned int)); //---------------------------------------- }; //----------------------------------------------------------------

4.3 Some notes regarding creating a new PE file

  • Align the VirtualAddress and the VirtualSize of each section by SectionAlignment: <PRE lang=c++>image_section_header[i]->VirtualAddress= PEAlign(image_section_header[i]->VirtualAddress, image_nt_headers->OptionalHeader.SectionAlignment); image_section_header[i]->Misc.VirtualSize= PEAlign(image_section_header[i]->Misc.VirtualSize, image_nt_headers->OptionalHeader.SectionAlignment);
  • Align the PointerToRawData and the SizeOfRawData of each section by FileAlignment: <PRE lang=c++>image_section_header[i]->PointerToRawData = PEAlign(image_section_header[i]->PointerToRawData, image_nt_headers->OptionalHeader.FileAlignment); image_section_header[i]->SizeOfRawData = PEAlign(image_section_header[i]->SizeOfRawData, image_nt_headers->OptionalHeader.FileAlignment);
  • Correct the SizeofImage by the virtual size and the virtual address of the last section: <PRE lang=c++>image_nt_headers->OptionalHeader.SizeOfImage = image_section_header[LastSection]->VirtualAddress + image_section_header[LastSection]->Misc.VirtualSize;
  • Set the Bound Import Directory header to zero, as this directory is not very important to execute a PE file: <PRE lang=c++>image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT]. VirtualAddress = 0; image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT].Size = 0;

4.4 Some notes regarding linking this VC Project

  • Set Linker->General->Enable Incremental Linking to No (/INCREMENTAL:NO).

    You can comprehend the difference between incremental link and no-incremental link by looking at the following picture:

    To acquire the virtual address of DynLoader(), we obtain the virtual address of JMP pemaker.DynLoader in the incremental link, but by no-incremental link, the real virtual address is gained by the following code:

    <PRE lang=c++>DWORD dwVA= (DWORD) DynLoader;

    This setting is more critical in the incremental link when you try to find the beginning and ending of the Loader, DynLoader(), by CPECryptor::ReturnToBytePtr():

    <PRE lang=c++>void* CPECryptor::ReturnToBytePtr(void* FuncName, DWORD findstr) { void* tmpd; __asm { mov eax, FuncName jmp df hjg: inc eax df: mov ebx, [eax] cmp ebx, findstr jnz hjg mov tmpd, eax } return tmpd; }

5 Store Important Data and Reach Original OEP

Right now, we save the Original OEP and also the Image Base in order to reach to the virtual address of OEP. I have reserved a free space at the end of DynLoader() to store them, DynLoader Step 2.

PE Maker - Step 2

DynLoader Step 2

<PRE lang=c++>__stdcall void DynLoader() { _asm { //---------------------------------- DWORD_TYPE(DYN_LOADER_START_MAGIC) //---------------------------------- Main_0: PUSHAD // get base ebp CALL Main_1 Main_1: POP EBP SUB EBP,OFFSET Main_1 MOV EAX,DWORD PTR [EBP+_RO_dwImageBase] ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint] PUSH EAX RETN // >> JMP to Original OEP //---------------------------------- DWORD_TYPE(DYN_LOADER_START_DATA1) //----------------------------------<FONT color=red> _RO_dwImageBase: DWORD_TYPE(0xCCCCCCCC) _RO_dwOrgEntryPoint: DWORD_TYPE(0xCCCCCCCC)</FONT> //---------------------------------- DWORD_TYPE(DYN_LOADER_END_MAGIC) //---------------------------------- } }

The new function, CPECryptor::CopyData1(), will implement the copy of the Image Base value and the Offset of Entry Point value into 8 bytes of free space in the loader.

5.1 Restore the first Registers Context

It is important to recover the Original Context of the thread. We have not yet done it in the DynLoader Step 2 source code. We can modify the source of DynLoader() to repossess the first Context.

<PRE lang=c++>__stdcall void DynLoader() { _asm { //---------------------------------- DWORD_TYPE(DYN_LOADER_START_MAGIC) //---------------------------------- Main_0: <FONT color=red>PUSHAD// Save the registers context in stack</FONT> CALL Main_1 Main_1: POP EBP// Get Base EBP SUB EBP,OFFSET Main_1 MOV EAX,DWORD PTR [EBP+_RO_dwImageBase] ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint] MOV DWORD PTR [ESP+1Ch],EAX // pStack.Eax <- EAX <FONT color=red>POPAD // Restore the first registers context from stack</FONT> PUSH EAX XOR EAX, EAX RETN // >> JMP to Original OEP //---------------------------------- DWORD_TYPE(DYN_LOADER_START_DATA1) //---------------------------------- _RO_dwImageBase: DWORD_TYPE(0xCCCCCCCC) _RO_dwOrgEntryPoint: DWORD_TYPE(0xCCCCCCCC) //---------------------------------- DWORD_TYPE(DYN_LOADER_END_MAGIC) //---------------------------------- } }

5.2 Restore the Original Stack

We can also recover the original stack by setting the value of the beginning stack + 0x34 to the Original OEP, but it is not very important. Nevertheless, in the following code, I have accomplished the loader code by a simple trick to reach OEP in addition to redecorating the stack. You can observe the implementation by tracing using OllyDbg or SoftICE.

<PRE lang=c++>__stdcall void DynLoader() { _asm { //---------------------------------- DWORD_TYPE(DYN_LOADER_START_MAGIC) //---------------------------------- Main_0: PUSHAD // Save the registers context in stack CALL Main_1 Main_1: POP EBP SUB EBP,OFFSET Main_1 MOV EAX,DWORD PTR [EBP+_RO_dwImageBase] ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint] MOV DWORD PTR [ESP+54h],EAX // pStack.Eip <- EAX POPAD // Restore the first registers context from stack CALL _OEP_Jump DWORD_TYPE(0xCCCCCCCC) _OEP_Jump: PUSH EBP MOV EBP,ESP MOV EAX,DWORD PTR [ESP+3Ch] // EAX <- pStack.Eip MOV DWORD PTR [ESP+4h],EAX // _OEP_Jump RETURN pointer <- EAX XOR EAX,EAX LEAVE RETN //---------------------------------- DWORD_TYPE(DYN_LOADER_START_DATA1) //---------------------------------- _RO_dwImageBase: DWORD_TYPE(0xCCCCCCCC) _RO_dwOrgEntryPoint: DWORD_TYPE(0xCCCCCCCC) //---------------------------------- DWORD_TYPE(DYN_LOADER_END_MAGIC) //---------------------------------- } }

5.3 Approach OEP by Structured Exception Handling

An exception is generated when a program falls into a fault code execution and an error happens, so in such a special condition, the program immediately jumps to a function called the exception handler from exception handler list of the Thread Information Block.

The next example of a try-except statement in C++ clarifies the operation of structured exception handling. Besides the assembly code of this code, it elucidates the structured exception handler installation, the raise of an exception, and the exception handler function.

<PRE lang=c++>#include "stdafx.h" #include "windows.h" void RAISE_AN_EXCEPTION() { _asm { INT 3 INT 3 INT 3 INT 3 } } int _tmain(int argc, _TCHAR* argv[]) { __try { __try{ printf("1: Raise an Exception\n"); RAISE_AN_EXCEPTION(); } __finally { printf("2: In Finally\n"); } } __except( printf("3: In Filter\n"), EXCEPTION_EXECUTE_HANDLER ) { printf("4: In Exception Handler\n"); } return 0; }
<FONT color=black>; main()</FONT><FONT color=gray>
00401000: PUSH EBP
00401001: MOV EBP,ESP
00401003: PUSH -1
00401005: PUSH 00407160
<FONT color=black>; __try {</FONT>
<FONT color=green>; the structured exception handler (SEH) installation </FONT><FONT color=blue>
0040100A: PUSH _except_handler3  
0040100F: MOV EAX,DWORD PTR FS:[0]
00401015: PUSH EAX
00401016: MOV DWORD PTR FS:[0],ESP</FONT>
0040101D: SUB ESP,8
00401020: PUSH EBX
00401021: PUSH ESI
00401022: PUSH EDI
00401023: MOV DWORD PTR SS:[EBP-18],ESP
<FONT color=black>;     __try {</FONT>
00401026: XOR ESI,ESI
00401028: MOV DWORD PTR SS:[EBP-4],ESI
0040102B: MOV DWORD PTR SS:[EBP-4],1
00401032: PUSH OFFSET <FONT color=brown>"1: Raise an Exception"</FONT>
00401037: CALL printf
0040103C: ADD ESP,4
<FONT color=green>; the raise a exception, INT 3 exception</FONT>
0040103F: INT3      
00401040: INT3
00401041: INT3
00401042: INT3</FONT>
<FONT color=black>;     } __finally {</FONT>
00401043: MOV DWORD PTR SS:[EBP-4],ESI
00401046: CALL 0040104D
0040104B: JMP 00401080
0040104D: PUSH OFFSET <FONT color=brown>"2: In Finally"</FONT>
00401052: CALL printf
00401057: ADD ESP,4
0040105A: RETN
<FONT color=black>;     }</FONT>
<FONT color=black>; }</FONT>
<FONT color=black>; __except( </FONT>
0040105B: JMP 00401080
0040105D: PUSH OFFSET <FONT color=brown>"3: In Filter"</FONT>
00401062: CALL printf
00401067: ADD ESP,4
0040106F: RETN
<FONT color=black>; {</FONT>
<FONT color=green>; the exception handler funtion</FONT><FONT color=blue>
00401070: MOV ESP,DWORD PTR SS:[EBP-18]
00401073: PUSH OFFSET <FONT color=brown>"4: In Exception Handler"</FONT>
00401078: CALL printf
0040107D: ADD ESP,4</FONT>
<FONT color=black>; }</FONT>
00401080: MOV DWORD PTR SS:[EBP-4],-1
0040108C: XOR EAX,EAX
<FONT color=green>; restore previous SEH</FONT><FONT color=blue>
00401091: MOV DWORD PTR FS:[0],ECX</FONT>
00401098: POP EDI
00401099: POP ESI
0040109A: POP EBX
0040109B: MOV ESP,EBP
0040109D: POP EBP
0040109E: RETN</FONT>

Make a Win32 console project, and link and run the preceding C++ code, to perceive the result:

1: Raise an Exception
3: In Filter
2: In Finally
4: In Exception Handler

This program runs the exception expression, printf("3: In Filter\n");, when an exception happens, in this example the INT 3 exception. You can employ other kinds of exception too. In OllyDbg, Debugging options->Exceptions, you can see a short list of different types of exceptions.

5.3.1 Implement Exception Handler

We desire to construct a structured exception handler in order to reach OEP. Now, I think you have distinguished the SEH installation, the exception raise, and the exception expression filter, by foregoing the assembly code. To establish our exception handler approach, we need to comprise the following codes:

  • SEH installation:
    <FONT color=gray>    LEA EAX,[EBP+_except_handler1_OEP_Jump]
        PUSH EAX
        PUSH DWORD PTR FS:[0]
  • An Exception Raise:
    <FONT color=gray>    INT 3</FONT>
  • Exception handler expression filter:
    <FONT color=gray>_except_handler1_OEP_Jump:
        PUSH EBP
        MOV EBP,ESP

So we yearn for making the ensuing C++ code in assembly language to inaugurate our engine to approach the Offset of Entry Point by SEH.

<PRE lang=c++>__try // SEH installation { __asm { INT 3 // An Exception Raise } } __except( ..., EXCEPTION_CONTINUE_SEARCH ){} // Exception handler expression filter

In assembly code...

<FONT color=gray>    <FONT color=green>; ----------------------------------------------------
    ; the structured exception handler (SEH) installation
    <FONT color=black>; __try {</FONT></FONT>
    LEA EAX,[EBP+_except_handler1_OEP_Jump]
    <FONT color=green>; ----------------------------------------------------
    ; the raise a INT 3 exception</FONT>
    INT 3
    INT 3
    INT 3
    INT 3
    <FONT color=black>; }
    ; __except( ... </FONT>
    <FONT color=green>; ----------------------------------------------------
    ; exception handler expression filter</FONT>
    <FONT color=black>; , EXCEPTION_CONTINUE_SEARCH ) { }</FONT></FONT>

The exception value, __except(..., Value), determines how the exception is handled, it can have three values, 1, 0, -1. To understand them, refer to the try-except statement description in the MSDN library. We set it to EXCEPTION_CONTINUE_SEARCH (0), not to run the exception handler function, therefore by this value, the exception is not recognized, is simply ignored, and the thread continues its code-execution.

How the SEH installation is implemented

As you perceived from the illustrated code, the SEH installation is done by the FS segment register. Microsoft Windows 32 bit uses the FS segment register as a pointer to the data block of the main thread. The first 0x1C bytes comprise the information of the Thread Information Block (TIB). Therefore, FS:[00h] refers to ExceptionList of the main thread, Table 3. In our code, we have pushed the pointer to _except_handler1_OEP_Jump in the stack and changed the value of ExceptionList, FS:[00h], to the beginning of the stack, ESP.

Thread Information Block (TIB)

<PRE lang=c++>typedef struct _NT_TIB32 { DWORD ExceptionList; DWORD StackBase; DWORD StackLimit; DWORD SubSystemTib; union { DWORD FiberData; DWORD Version; }; DWORD ArbitraryUserPointer; DWORD Self; } NT_TIB32, *PNT_TIB32;

Table 3 - FS segment register and Thread Information Block

DWORD PTR FS:[00h]ExceptionList
DWORD PTR FS:[04h]StackBase
DWORD PTR FS:[08h]StackLimit
DWORD PTR FS:[0Ch]SubSystemTib
DWORD PTR FS:[10h]FiberData / Version
DWORD PTR FS:[14h]ArbitraryUserPointer
DWORD PTR FS:[18h]Self

5.3.2 Attain OEP by adjusting the Thread Context

In this part, we effectuate our performance by accomplishing the OEP approach. We change the Context of the thread and ignore every simple exception handling, and let the thread continue the execution, but in the original OEP!

When an exception happens, the context of the processor during the time of the exception is saved in the stack. By EXCEPTION_POINTERS, we have access to the pointer of ContextRecord. The ContextRecord has the CONTEXT data structure, Table 4, this is the thread context during the exception time. When we ignore the exception by EXCEPTION_CONTINUE_SEARCH (0), the instruction pointer as well the context will be set to ContextRecord in order to return to the previous condition. Therefore, if we change the Eip of the Win32 Thread Context to the Original Offset of Entry Point, it will come clearly into OEP.

MOV EAX, ContextRecord
MOV EDI, dwOEP                   ; EAX <- dwOEP
MOV DWORD PTR DS:[EAX+0B8h], EDI ; pContext.Eip <- EAX

Win32 Thread Context structure

<PRE lang=c++>#define MAXIMUM_SUPPORTED_EXTENSION 512 typedef struct _CONTEXT { //----------------------------------------- DWORD ContextFlags; //----------------------------------------- DWORD Dr0; DWORD Dr1; DWORD Dr2; DWORD Dr3; DWORD Dr6; DWORD Dr7; //----------------------------------------- FLOATING_SAVE_AREA FloatSave; //----------------------------------------- DWORD SegGs; DWORD SegFs; DWORD SegEs; DWORD SegDs; //----------------------------------------- DWORD Edi; DWORD Esi; DWORD Ebx; DWORD Edx; DWORD Ecx; DWORD Eax; //----------------------------------------- DWORD Ebp; DWORD Eip; DWORD SegCs; DWORD EFlags; DWORD Esp; DWORD SegSs; //----------------------------------------- BYTE ExtendedRegisters[MAXIMUM_SUPPORTED_EXTENSION]; //---------------------------------------- } CONTEXT, *LPCONTEXT;

Table 4 - CONTEXT

Context Flags0x00000000ContextFlags

Context Debug Registers


Context Floating Point

RegisterArea [0x50]
Context Segments0x0000008CSegGs
Context Integer0x0000009CEdi
Context Control0x000000B4Ebp
Context Extended Registers



By the following code, we have accomplished the main purpose of coming to OEP by the structured exception handler:

<PRE lang=c++>__stdcall void DynLoader() { _asm { //---------------------------------- DWORD_TYPE(DYN_LOADER_START_MAGIC) //---------------------------------- Main_0: PUSHAD // Save the registers context in stack CALL Main_1 Main_1: POP EBP SUB EBP,OFFSET Main_1 // Get Base EBP MOV EAX,DWORD PTR [EBP+_RO_dwImageBase] ADD EAX,DWORD PTR [EBP+_RO_dwOrgEntryPoint] MOV DWORD PTR [ESP+10h],EAX // pStack.Ebx <- EAX LEA EAX,[EBP+_except_handler1_OEP_Jump] MOV DWORD PTR [ESP+1Ch],EAX // pStack.Eax <- EAX POPAD // Restore the first registers context from stack //---------------------------------------------------- // the structured exception handler (SEH) installation PUSH EAX XOR EAX, EAX PUSH DWORD PTR FS:[0] // NT_TIB32.ExceptionList MOV DWORD PTR FS:[0],ESP // NT_TIB32.ExceptionList <-ESP //---------------------------------------------------- // the raise a INT 3 exception DWORD_TYPE(0xCCCCCCCC) //-------------------------------------------------------- // -------- exception handler expression filter ---------- _except_handler1_OEP_Jump: PUSH EBP MOV EBP,ESP //------------------------------ MOV EAX,DWORD PTR SS:[EBP+010h] // PCONTEXT: pContext <- EAX //============================== PUSH EDI // restore original SEH MOV EDI,DWORD PTR DS:[EAX+0C4h] // pContext.Esp PUSH DWORD PTR DS:[EDI] POP DWORD PTR FS:[0] ADD DWORD PTR DS:[EAX+0C4h],8 // pContext.Esp //------------------------------ // set the Eip to the OEP MOV EDI,DWORD PTR DS:[EAX+0A4h] // EAX <- pContext.Ebx MOV DWORD PTR DS:[EAX+0B8h],EDI // pContext.Eip <- EAX //------------------------------ POP EDI //============================== MOV EAX, EXCEPTION_CONTINUE_SEARCH LEAVE RETN //---------------------------------- DWORD_TYPE(DYN_LOADER_START_DATA1) //---------------------------------- _RO_dwImageBase: DWORD_TYPE(0xCCCCCCCC) _RO_dwOrgEntryPoint: DWORD_TYPE(0xCCCCCCCC) //---------------------------------- DWORD_TYPE(DYN_LOADER_END_MAGIC) //---------------------------------- } }

6 Build an Import Table and Reconstruct the Original Import Table

To use the Windows dynamic link library (DLL) in Windows application programming, there are two ways:

  • Using Windows libraries by additional dependencies:

  • Using Windows dynamic link libraries in run-time: <PRE lang=c++>// DLL function signature typedef HGLOBAL (*importFunction_GlobalAlloc)(UINT, SIZE_T); ... importFunction_GlobalAlloc __GlobalAlloc; // Load DLL file HINSTANCE hinstLib = LoadLibrary("Kernel32.dll"); if (hinstLib == NULL) { // Error - unable to load DLL } // Get function pointer __GlobalAlloc = (importFunction_GlobalAlloc)GetProcAddress(hinstLib, "GlobalAlloc"); if (addNumbers == NULL) { // Error - unable to find DLL function } FreeLibrary(hinstLib);

When you make a Windows application project, the linker includes at least kernel32.dll in the base dependencies of your project. Without LoadLibrary() and GetProcAddress() of Kernel32.dll, we can not load a DLL in run-time. The dependencies information is stored in the import table section. By Dependency Walker, it is not so difficult to observe the DLL module and the functions which are imported into a PE file.

We attempt to establish our custom import table to conduct our project. Furthermore, we have to fix up the original import table at the end in order to run the real code of the program.

PE Maker - Step 3

6.1 Construct the Client Import Table

I strongly advise you to read the section 6.4 of the Microsoft Portable Executable and the Common Object File Format Specification document. This section contains the principal information to comprehend the import table performance.

The import table data is accessible by a second data directory of the optional header from PE headers, so you can access it by using the following code:

<PRE lang=c++>DWORD dwVirtualAddress = image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress; DWORD dwSize = image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].Size;

The VirtualAddress refers to structures by IMAGE_IMPORT_DESCRIPTOR. This structure contains the pointer to the imported DLL name and the relative virtual address of the first thunk.

<PRE lang=c++>typedef struct _IMAGE_IMPORT_DESCRIPTOR { union { DWORD Characteristics; DWORD OriginalFirstThunk; }; DWORD TimeDateStamp; DWORD ForwarderChain; DWORD <FONT color=red>Name</FONT>; // the imported DLL name DWORD <FONT color=red>FirstThunk</FONT>; // the relative virtual address of the first thunk } IMAGE_IMPORT_DESCRIPTOR, *PIMAGE_IMPORT_DESCRIPTOR;

When a program is running, the Windows task manager sets the thunks by the virtual address of the function. The virtual address is found by the name of the function. At first, the thunks hold the relative virtual address of the function name, Table 5; during execution, they are fixed up by the virtual address of the functions, Table 6.

Table 5 - The Import Table in file image


Table 6 - The Import Table in virtual memory


We want to make a simple import table to import LoadLibrary(), and GetProcAddress() from Kernel32.dll. We need these two essential API functions to cover other API functions in run-time. The following assembly code shows how easily we can reach our solution:

<FONT color=gray>0101F000: 
<FONT color=blue>00000000</FONT> ; OriginalFirstThunk
0101F004: <FONT color=blue>00000000</FONT> ; TimeDateStamp
0101F008: <FONT color=blue>00000000</FONT> ; ForwarderChain
0101F00C: <FONT color=blue>0001F034</FONT> ; Name;       ImageBase + 0001F034 -> 0101F034 -> "Kernel32.dll",0
0101F010: <FONT color=blue>0001F028</FONT> ; FirstThunk; ImageBase + 0001F028 -> 0101F028
0101F014: <FONT color=blue>00000000</FONT>
0101F018: <FONT color=blue>00000000</FONT>
0101F01C: <FONT color=blue>00000000</FONT>
0101F020: <FONT color=blue>00000000</FONT>
0101F024: <FONT color=blue>00000000</FONT>
0101F028: <FONT color=blue>0001F041</FONT> ; ImageBase + 0001F041 -> 0101F041 -> 0,0,"LoadLibraryA",0
0101F02C: <FONT color=blue>0001F050</FONT> ; ImageBase + 0001F050 -> 0101F050 -> 0,0,"GetProcAddress",0
0101F030: <FONT color=blue>00000000</FONT>
0101F034: <FONT color=brown>'K' 'e' 'r' 'n' 'e' 'l' '3' '2' '.' 'd' 'l' 'l' </FONT><FONT color=blue>00</FONT>
0001F041: <FONT color=blue>00 00</FONT> <FONT color=brown>'L' 'o' 'a' 'd' 'L' 'i' 'b' 'r' 'a' 'r' 'y' 'A'</FONT> 
<FONT color=blue>00</FONT>
0001F050: <FONT color=blue>00 00</FONT> <FONT color=brown>'G' 'e' 't' 'P' 'r' 'o' 'c' 'A' 'd' 'd' 'r' 'e' 's' 's'
</FONT> <FONT color=blue>00</FONT></FONT>

After running...

<FONT color=gray>0101F000: 
<FONT color=blue>00000000</FONT> ; OriginalFirstThunk
0101F004: <FONT color=blue>00000000</FONT> ; TimeDateStamp
0101F008: <FONT color=blue>00000000</FONT> ; ForwarderChain
0101F00C: <FONT color=blue>0001F034</FONT> ; Name;       ImageBase + 0001F034 -> 0101F034 -> "Kernel32.dll",0
0101F010: <FONT color=blue>0001F028</FONT> ; FirstThunk; ImageBase + 0001F028 -> 0101F028
0101F014: <FONT color=blue>00000000</FONT>
0101F018: <FONT color=blue>00000000</FONT>
0101F01C: <FONT color=blue>00000000</FONT>
0101F020: <FONT color=blue>00000000</FONT>
0101F024: <FONT color=blue>00000000</FONT>
0101F028: <FONT color=red>7C801D77</FONT> ; -> Kernel32.LoadLibrary()
0101F02C: <FONT color=red>7C80AC28</FONT> ; -> Kernel32.GetProcAddress()
0101F030: <FONT color=blue>00000000</FONT>
0101F034: <FONT color=brown>'K' 'e' 'r' 'n' 'e' 'l' '3' '2' '.' 'd' 'l' 'l' </FONT>
<FONT color=blue>00</FONT>
0001F041: <FONT color=blue>00 00</FONT> <FONT color=brown>'L' 'o' 'a' 'd' 'L' 'i' 'b' 'r' 'a' 'r' 'y' 'A'
</FONT> <FONT color=blue>00</FONT>
0001F050: <FONT color=blue>00 00</FONT> <FONT color=brown>'G' 'e' 't' 'P' 'r' 'o' 'c' 'A' 'd' 'd' 'r' 'e' 's' 's'
</FONT> <FONT color=blue>00</FONT></FONT>

I have prepared a class library to make every import table by using a client string table. The CITMaker class library in itmaker.h, it will build an import table by sz_IT_EXE_strings and also the relative virtual address of the import table.

<PRE lang=c++>static const char *sz_IT_EXE_strings[]= { "Kernel32.dll", "LoadLibraryA", "GetProcAddress", 0,, 0, };

We subsequently employ this class library to establish an import table to support DLLs and OCXs, so this is a general library to present all possible import tables easily. The next step is clarified in the following code.

<PRE lang=c++>CITMaker *<FONT color=red>ImportTableMaker</FONT> = new CITMaker( IMPORT_TABLE_EXE ); ... pimage_section_header=AddNewSection( ".xxx", dwNewSectionSize ); // build import table by the current virtual address <FONT color=red>ImportTableMaker</FONT>-><FONT color=green>Build</FONT> ( <FONT color=blue>pimage_section_header->VirtualAddress</FONT> ); memcpy( pNewSection, <FONT color=red>ImportTableMaker</FONT>-><FONT color=green>pMem</FONT>, <FONT color=red>ImportTableMaker</FONT>-><FONT color=green>dwSize</FONT> ); ... memcpy( image_section[image_nt_headers->FileHeader.NumberOfSections-1], pNewSection, dwNewSectionSize ); ... image_nt_headers->OptionalHeader. DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress = <FONT color=blue>pimage_section_header->VirtualAddress</FONT>; image_nt_headers->OptionalHeader. DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].Size = <FONT color=red>ImportTableMaker</FONT>-><FONT color=green>dwSize</FONT>; ... delete <FONT color=red>ImportTableMaker</FONT>;

The import table is copied at the beginning of the new section, and the relevant data directory is adjusted to the relative virtual address of the new section and the size of the new import table

6.2 Using other API functions in run-time

At this time, we can load other DLLs and find the process address of other functions by using LoadLibrary() and GetProcAddress():

<FONT color=gray>lea edi, <FONT color=red>@</FONT><FONT color=brown>"Kernel32.dll"</FONT>
<FONT color=blue>push edi
mov eax,offset _p_LoadLibrary
call [ebp+eax] //LoadLibrary(lpLibFileName);</FONT>
mov esi,eax    // esi -> hModule
lea edi, <FONT color=red>@</FONT><FONT color=brown>"GetModuleHandleA"</FONT>
<FONT color=blue>push edi
push esi
mov eax,offset _p_GetProcAddress
call [ebp+eax] //GetModuleHandle=GetProcAddress(hModule, lpProcName);</FONT>

I want to have a complete imported function table similar in performance done in a real EXE file. If you look inside a PE file, you will discover that an API call is done by an indirection jump through the virtual address of the API function:


<FONT color=gray>...
0101F028: <FONT color=red>7C801D77</FONT>      ; Virtual Address of kernel32.LoadLibrary()
0101F120: JMP DWORD PTR [<FONT color=red>0101F028</FONT>]
0101F230: CALL <FONT color=red>0101F120</FONT> ;  JMP to kernel32.LoadLibrary

It makes it easy to expand the other part of our project by this performance, so we construct two data tables: first for API virtual addresses, and the second for the JMP [XXXXXXXX].

<PRE lang=c++>#define __jmp_api byte_type(0xFF) byte_type(0x25) __asm { ... //---------------------------------------------------------------- _p_GetModuleHandle: dword_type(0xCCCCCCCC) _p_VirtualProtect: dword_type(0xCCCCCCCC) _p_GetModuleFileName: dword_type(0xCCCCCCCC) _p_CreateFile: dword_type(0xCCCCCCCC) _p_GlobalAlloc: dword_type(0xCCCCCCCC) //---------------------------------------------------------------- _jmp_GetModuleHandle: __jmp_api dword_type(0xCCCCCCCC) _jmp_VirtualProtect: __jmp_api dword_type(0xCCCCCCCC) _jmp_GetModuleFileName: __jmp_api dword_type(0xCCCCCCCC) _jmp_CreateFile: __jmp_api dword_type(0xCCCCCCCC) _jmp_GlobalAlloc: __jmp_api dword_type(0xCCCCCCCC) //---------------------------------------------------------------- ... }

In the succeeding code, we has concluded our ambition to install a custom internal import table! (We can not call it import table.)

<FONT color=gray>    ...
    lea edi,[ebp+_p_szKernel32]
    lea ebx,[ebp+_p_GetModuleHandle]
    lea ecx,[ebp+_jmp_GetModuleHandle]
    add ecx,02h
        push ecx
        <FONT color=blue>push edi
        mov eax,offset _p_LoadLibrary
        call [ebp+eax]    //LoadLibrary(lpLibFileName);</FONT>
        pop ecx
        mov esi,eax       // esi -> hModule
        push edi
        call __strlen
        add esp,04h
        add edi,eax
            push ecx
            <FONT color=blue>push edi
            push esi
            mov eax,offset _p_GetProcAddress
            call [ebp+eax]//GetModuleHandle=GetProcAddress(hModule, lpProcName);</FONT>
            pop ecx
            <FONT color=green>mov [ebx],eax
            mov [ecx],ebx // JMP DWORD PTR [XXXXXXXX] </FONT>
            add ebx,04h
            add ecx,06h
            push edi
            call __strlen
            add esp,04h
            add edi,eax
            mov al,byte ptr [edi]
        test al,al
        jnz _api_get_proc_address_loop
        inc edi
        mov al,byte ptr [edi]
    test al,al
    jnz _api_get_lib_address_loop

6.3 Fix up the Original Import Table

In order to run the program again, we should fix up the thunks of the actual import table, otherwise we have a corrupted target PE file. Our code must correct all of the thunks the same as Table 5 to Table 6. Once more, LoadLibrary() and GetProcAddress() aid us in our effort to reach our intention.

<FONT color=gray>    ...
    mov ebx,[ebp+<FONT color=red>_p_dwImportVirtualAddress</FONT>]
    test ebx,ebx
    jz _it_fixup_end
    mov esi,[ebp+<FONT color=red>_p_dwImageBase</FONT>]
    add ebx,esi                   // dwImageBase + dwImportVirtualAddress
        mov eax,[ebx+00Ch]        // image_import_descriptor.Name
        test eax,eax
        jz _it_fixup_end
        mov ecx,[ebx+010h]        // image_import_descriptor.FirstThunk
        add ecx,esi
        mov [ebp+<FONT color=red>_p_dwThunk</FONT>],ecx // dwThunk
        mov ecx,[ebx]             // image_import_descriptor.Characteristics
        test ecx,ecx
        jnz _it_fixup_table
            mov ecx,[ebx+010h]
        add ecx,esi
        mov [ebp+<FONT color=red>_p_dwHintName</FONT>],ecx // dwHintName
        add eax,esi  // image_import_descriptor.Name + dwImageBase = ModuleName
        <FONT color=blue>push eax  // lpLibFileName
        mov eax,offset _p_LoadLibrary
        call [ebp+eax]             // LoadLibrary(lpLibFileName);</FONT>

        test eax,eax
        jz _it_fixup_end
        mov edi,eax
            mov ecx,[ebp+<FONT color=red>_p_dwHintName</FONT>] // dwHintName
            mov edx,[ecx]            // image_thunk_data.Ordinal
            test edx,edx
            jz _it_fixup_next_module
            test edx,080000000h      // .IF( import by ordinal )
            jz _it_fixup_by_name
                and edx,07FFFFFFFh   // get ordinal
                jmp _it_fixup_get_addr
            add edx,esi  // image_thunk_data.Ordinal + dwImageBase = OrdinalName
            inc edx
            inc edx                  // OrdinalName.Name
            <FONT color=blue>push edx //lpProcName
            push edi                 // hModule                        
            mov eax,offset _p_GetProcAddress
            call [ebp+eax] // GetProcAddress(hModule, lpProcName);</FONT>

            <FONT color=green>mov ecx,[ebp+<FONT color=red>_p_dwThunk</FONT>] // dwThunk
            mov [ecx],eax  // correction the thunk</FONT>
            // dwThunk => next dwThunk
            add dword ptr [ebp+<FONT color=red>_p_dwThunk</FONT>], <FONT color=blue>004h</FONT>
            // dwHintName => next dwHintName
            add dword ptr [ebp+<FONT color=red>_p_dwHintName</FONT>],<FONT color=blue>004h</FONT>
        jmp _it_fixup_get_proc_address_loop
        add ebx,014h      // sizeof(IMAGE_IMPORT_DESCRIPTOR)
    jmp _it_fixup_get_lib_address_loop

7 Support DLL and OCX

Now, we intend to include the dynamic link library (DLL) and OLE-ActiveX Control in our PE builder project. Supporting them is very easy if we pay attention to the two time arrival into the Offset of Entry Point, the relocation table implementation, and the client import table.

PE Maker - Step 4

7.1 Twice OEP approach

The Offset of Entry Point of a DLL file or an OCX file is touched by the main program atleast twice:

  • Constructor:

    When a DLL is loaded by LoadLibrary(), or an OCX is registered by using LoadLibrary() and GetProcAddress() through calling DllRegisterServer(), the first of the OEP arrival is done.

    <PRE lang=c++>hinstDLL = LoadLibrary( "test1.dll" );<PRE lang=c++>hinstOCX = LoadLibrary( "test1.ocx" ); _DllRegisterServer = GetProcAddress( hinstOCX, "DllRegisterServer" ); _DllRegisterServer(); // ocx register
  • Destructor:

    When the main program frees the library usage by FreeLibrary(), the second OEP arrival happens.

    <PRE lang=c++>FreeLibrary( hinstDLL );<PRE lang=c++>FreeLibrary( hinstOCX );

To perform this, I have employed a trick, that causes in the second time again, the instruction pointer (EIP) traveling towards the original OEP by the structured exception handler.

<FONT color=gray><FONT color=black>_main_0:
    pushad  // save the registers context in stack
    call _main_1
    pop ebp
    sub ebp,offset _main_1 // get base ebp
    //---------------- support dll, ocx  -----------------
    jmp _support_dll_1 // <FONT color=red>nop; nop; // << trick</FONT> // in the second time OEP
    <FONT color=black>jmp _support_dll_2</FONT>
    //---------------- support dll, ocx  1 ---------------
    mov edi,[ebp+_p_dwImageBase]
    add edi,[edi+03Ch]// edi -> IMAGE_NT_HEADERS
    mov ax,word ptr [edi+016h]// edi -> image_nt_headers->FileHeader.Characteristics
    test ax,<FONT color=green>IMAGE_FILE_DLL</FONT>
    jz _support_dll_2
        mov ax, <FONT color=red>9090h // << trick</FONT>
        mov word ptr [ebp+_support_dll_0],ax</FONT>
<FONT color=black>_support_dll_2:
    into OEP by SEH ...</FONT>

I hope you have caught the trick in the preceding code, but this is not all of it, we have problem in ImageBase, when the library has been loaded in different image bases by the main program. We should write some code to find the real image base and store it to use forward.

    <FONT color=gray>mov eax,<FONT color=green>[esp+24h]</FONT> // the real imagebase
    mov ebx,<FONT color=green>[esp+30h]</FONT> // oep
    cmp eax,ebx
    ja _no_dll_pe_file_0
        cmp word ptr [eax],IMAGE_DOS_SIGNATURE
        jne _no_dll_pe_file_0
            mov [ebp+_p_dwImageBase],eax

This code finds the real image base by investigating the stack information. By using the real image base and the formal image base, we should correct all memory calls inside the image program!! Don't be afraid, it will be done simply by the relocating the table information.

7.2 Implement Relocation Table

To understand the relocation table better, you can take a look at the section 6.6 of Microsoft Portable Executable and Common Object File Format Specification document. The relocation table contains many packages to relocate the information related to the virtual address inside the virtual memory image. Each package comprise of a 8 bytes header to exhibit the base virtual address and the number of data, demonstrated by the IMAGE_BASE_RELOCATION data structure.


Table 7 - The Relocation Table




Table 7 illustrates the main idea of the relocation table. Furthermore, you can upload a DLL or an OCX file in OllyDbg to observe the relocation table, the ".reloc" section through Memory map window. By the way, we find the position of the relocation table by using the following code in our project:

<PRE lang=c++>DWORD dwVirtualAddress = image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC]. VirtualAddress; DWORD dwSize = image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].Size;

By OllyDbg, we have the same as the following for the ".reloc" section, by using the Long Hex viewer mode. In this example, the base virtual address is 0x1000 and the size of the block is 0x184.

008E1000 : 00001000  00000184  30163000  30403028
008E1010 : 30683054  308C3080  30AC309C  30D830CC
008E1020 : 30E030DC  30E830E4  30F030EC  310030F4
008E1030 : 3120310D  315F3150  31A431A0  31C031A8
008E1040 : 31D031CC  31F431EC  31FC31F8  32043200
008E1050 : 320C3208  32143210  324C322C  32583254
008E1060 : 3260325C  32683264  3270326C  32B03274

It relocates the data in the subsequent virtual addresses:

<PRE lang=c++>0x1000 + 0x0000 = 0x1000 0x1000 + 0x0016 = 0x1016 0x1000 + 0x0028 = 0x1028 0x1000 + 0x0040 = 0x1040 0x1000 + 0x0054 = 0x1054 ...

Each package performs the relocation by using consecutive 4 bytes form its internal information. The first byte refers to the type of relocation and the next three bytes are the offset which must be used with the base virtual address and the image base to correct the image information.


What is the type

The type can be one of the following values:

  • IMAGE_REL_BASED_ABSOLUTE (0): no effect.
  • IMAGE_REL_BASED_HIGH (1): relocate by the high 16 bytes of the base virtual address and the offset.
  • IMAGE_REL_BASED_LOW (2): relocate by the low 16 bytes of the base virtual address and the offset.
  • IMAGE_REL_BASED_HIGHLOW (3): relocate by the base virtual address and the offset.

What is done in the relocation?

By relocation, some values inside the virtual memory are corrected according to the current image base by the ".reloc" section packages.

delta_ImageBase = current_ImageBase - image_nt_headers->OptionalHeader.ImageBase
<PRE lang=c++>mem[ current_ImageBase + 0x1000 ] = mem[ current_ImageBase + 0x1000 ] + delta_ImageBase ; mem[ current_ImageBase + 0x1016 ] = mem[ current_ImageBase + 0x1016 ] + delta_ImageBase ; mem[ current_ImageBase + 0x1028 ] = mem[ current_ImageBase + 0x1028 ] + delta_ImageBase ; mem[ current_ImageBase + 0x1040 ] = mem[ current_ImageBase + 0x1040 ] + delta_ImageBase ; mem[ current_ImageBase + 0x1054 ] = mem[ current_ImageBase + 0x1054 ] + delta_ImageBase ; ...

I have employed the following code from Morphine packer to implement the relocation.

    <FONT color=gray>...
    mov eax,[ebp+_p_dwImageBase]
    mov edx,eax
    mov ebx,eax
    add ebx,[ebx+3Ch] // edi -> IMAGE_NT_HEADERS
    mov ebx,[ebx+034h]// edx ->image_nt_headers->OptionalHeader.ImageBase
    <FONT color=red>sub edx,ebx // edx -> reloc_correction // delta_ImageBase</FONT>
    je _reloc_fixup_end
    mov ebx,[ebp+_p_dwRelocationVirtualAddress]
    test ebx,ebx
    jz _reloc_fixup_end
    add ebx,eax
    mov eax,[ebx+004h]          //ImageBaseRelocation.SizeOfBlock
    test eax,eax
    jz _reloc_fixup_end
    lea ecx,[eax-008h]
    shr ecx,001h
    lea edi,[ebx+008h]
        movzx eax,word ptr [edi]//Entry
        push edx
        mov edx,eax
        shr eax,00Ch            //Type = Entry >> 12
        mov esi,[ebp+_p_dwImageBase]//ImageBase
        and dx,00FFFh
        add esi,[ebx]
        add esi,edx
        pop edx
_reloc_fixup_HIGH:              // IMAGE_REL_BASED_HIGH  
        dec eax
        jnz _reloc_fixup_LOW
            mov eax,edx
            shr eax,010h        //HIWORD(Delta)
            jmp _reloc_fixup_LOW_fixup        
_reloc_fixup_LOW:               // IMAGE_REL_BASED_LOW 
            dec eax
        jnz _reloc_fixup_HIGHLOW
        movzx eax,dx            //LOWORD(Delta)
            <FONT color=red>add word ptr [esi],ax// mem[x] = mem[x] + delta_ImageBase</FONT>
        jmp _reloc_fixup_next_entry
_reloc_fixup_HIGHLOW:           // IMAGE_REL_BASED_HIGHLOW
            dec eax
        jnz _reloc_fixup_next_entry
        <FONT color=red>add [esi],edx           // mem[x] = mem[x] + delta_ImageBase</FONT>
        inc edi
        inc edi                 //Entry++
        loop _reloc_fixup_do_entry
    add ebx,[ebx+004h]
    jmp _reloc_fixup_block

7.3 Build a Special Import table

In order to support the OLE-ActiveX Control registration, we should present an appropriate import table to our target OCX and DLL file.

Therefore, I have established an import table by the following string:

<PRE lang=c++>const char *sz_IT_OCX_strings[]= { "Kernel32.dll", "LoadLibraryA", "GetProcAddress", "GetModuleHandleA", 0, "User32.dll", "GetKeyboardType", "WindowFromPoint", 0, "AdvApi32.dll", "RegQueryValueExA", "RegSetValueExA", "StartServiceA", 0, "Oleaut32.dll", "SysFreeString", "CreateErrorInfo", "SafeArrayPtrOfIndex", 0, "Gdi32.dll", "UnrealizeObject", 0, "Ole32.dll", "CreateStreamOnHGlobal", "IsEqualGUID", 0, "ComCtl32.dll", "ImageList_SetIconSize", 0, 0, };

Without these API functions, the library can not be loaded, and moreover the DllregisterServer() and DllUregisterServer() will not operate. In CPECryptor::CryptFile, I have distinguished between EXE files and DLL files in the initialization of the new import table object during creation:

<PRE lang=c++>if(( image_nt_headers->FileHeader.Characteristics & IMAGE_FILE_DLL ) == IMAGE_FILE_DLL ) { ImportTableMaker = new CITMaker( IMPORT_TABLE_OCX ); } else { ImportTableMaker = new CITMaker( IMPORT_TABLE_EXE ); }

8 Preserve the Thread Local Storage

By using Thread Local Storage (TLS), a program is able to execute a multithreaded process, this performance mostly is used by Borland linkers: Delphi and C++ Builder. When you pack a PE file, you should take care to keep clean the TLS, otherwise, your packer will not support Borland Delphi and C++ Builder linked EXE files. To comprehend TLS, I refer you to section 6.7 of the Microsoft Portable Executable and Common Object File Format Specification document, you can observe the TLS structure by IMAGE_TLS_DIRECTORY32 in winnt.h.

<PRE lang=c++>typedef struct _IMAGE_TLS_DIRECTORY32 { DWORD StartAddressOfRawData; DWORD EndAddressOfRawData; DWORD AddressOfIndex; DWORD AddressOfCallBacks; DWORD SizeOfZeroFill; DWORD Characteristics; } IMAGE_TLS_DIRECTORY32, * PIMAGE_TLS_DIRECTORY32;

To keep safe the TLS directory, I have copied it in a special place inside the loader:

<FONT color=gray>...
_tls_dwStartAddressOfRawData:   dword_type(0xCCCCCCCC)
_tls_dwEndAddressOfRawData:     dword_type(0xCCCCCCCC)
_tls_dwAddressOfIndex:          dword_type(0xCCCCCCCC)
_tls_dwAddressOfCallBacks:      dword_type(0xCCCCCCCC)
_tls_dwSizeOfZeroFill:          dword_type(0xCCCCCCCC)
_tls_dwCharacteristics:         dword_type(0xCCCCCCCC)

It is necessary to correct the TLS directory entry in the Optional Header:

<PRE lang=c++>if(image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_TLS]. VirtualAddress!=0) { memcpy(&pDataTable->image_tls_directory, image_tls_directory, sizeof(IMAGE_TLS_DIRECTORY32)); dwOffset=DWORD(pData1)-DWORD(pNewSection); dwOffset+=sizeof(t_DATA_1)-sizeof(IMAGE_TLS_DIRECTORY32); image_nt_headers-> OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_TLS]. VirtualAddress=dwVirtualAddress + dwOffset; }

9 Inject your code

We are ready to place our code inside the new section. Our code is a "Hello World!" message by MessageBox() from user32.dll.

<FONT color=gray>...
lea eax,[ebp+_p_szCaption]
push eax
lea eax,[ebp+_p_szText]
push eax
push NULL
call _jmp_MessageBox
// MessageBox(NULL, szText, szCaption, MB_OK | MB_ICONINFORMATION) ;

PE Maker - Step 5

10 Conclusion

By this article, you have perceived how easily we can inject code to a portable executable file. You can complete the code by using the source of other packers, create a packer in the same way as Yoda's Protector, and make your packer undetectable by mixing up with Morphine source code. I hope that you have enjoyed this brief discussion of one part of the reverse engineering field. See you again in the next discussion!


This article, along with any associated source code and files, is licensed under The GNU General Public License (GPLv3)


About the Author

No Biography provided

You may also be interested in...

Comments and Discussions

QuestionHow does AddNewSection work? Pin
adhesive29-Jul-10 22:28
memberadhesive29-Jul-10 22:28 
AnswerRe: How does AddNewSection work? Pin
tafdasfasf23-Oct-10 3:31
membertafdasfasf23-Oct-10 3:31 
Questionanyone explain why? Pin
minkun25-May-10 19:27
memberminkun25-May-10 19:27 
GeneralInjected EXE can't run Pin
John Kenedy S.Kom12-May-10 23:30
memberJohn Kenedy S.Kom12-May-10 23:30 
GeneralRe: Injected EXE can't run Pin
John Kenedy S.Kom12-May-10 23:58
memberJohn Kenedy S.Kom12-May-10 23:58 
GeneralRe: Injected EXE can't run Pin
Member 462691819-Nov-10 11:15
memberMember 462691819-Nov-10 11:15 
GeneralMaybe the wrong with Offset in Data viewer Pin
wk1983031831-Jan-10 3:03
memberwk1983031831-Jan-10 3:03 
GeneralRe: Maybe the wrong with Offset in Data viewer Pin
wk198303181-Feb-10 20:20
memberwk198303181-Feb-10 20:20 
The project: peviewer
file: main.cpp
method: void ShowComment(HWND hwnd, int iType)

InsertTable(0,PEfile->image_dos_header->e_magic, 4,
"e_magic", "Magic number",0);

"&PEfile->image_dos_header" should change to:"PEfile->image_dos_header".
Because the "image_dos_header" is a pointer, not a struct.
Generalwell Pin
Nguyen Sy Bang18-Jan-10 21:27
memberNguyen Sy Bang18-Jan-10 21:27 
GeneralExcellent Pin
eg_Anubhava15-Dec-09 22:00
membereg_Anubhava15-Dec-09 22:00 
Generalgreat! Pin
newine10-Sep-09 19:53
membernewine10-Sep-09 19:53 
GeneralGreat article Pin
JoeJiao4-Aug-09 6:31
memberJoeJiao4-Aug-09 6:31 
GeneralProblem related to virtual directory Pin
Nazneen.Insignia29-Jul-09 0:31
memberNazneen.Insignia29-Jul-09 0:31 
Generalnice article Pin
Li Shu14-Apr-09 22:40
memberLi Shu14-Apr-09 22:40 
Generalnice article! Pin
whitehat.c5-Mar-09 19:32
memberwhitehat.c5-Mar-09 19:32 
Questionhelp me please!... [modified] Pin
Ikram_mostafa21-Oct-08 1:07
memberIkram_mostafa21-Oct-08 1:07 
QuestionPlease explain the logic of the DynLoader code Pin
CodeBuddy2611-Jul-08 8:00
memberCodeBuddy2611-Jul-08 8:00 
AnswerRe: Please explain the logic of the DynLoader code Pin
klay.orkut27-May-09 9:29
memberklay.orkut27-May-09 9:29 
AnswerRe: Please explain the logic of the DynLoader code Pin
Member 462691819-Nov-10 11:15
memberMember 462691819-Nov-10 11:15 
GeneralRe: Please explain the logic of the DynLoader code Pin
CodeBuddy2619-Nov-10 13:54
memberCodeBuddy2619-Nov-10 13:54 
GeneralHexEdit has PE template [modified] Pin
Andrew Phillips12-May-08 23:33
memberAndrew Phillips12-May-08 23:33 
GeneralRe: HexEdit has PE template Pin
skinui24-Jun-08 17:36
memberskinui24-Jun-08 17:36 
GeneralGreate!! Pin
xx0boy6-Mar-08 22:42
memberxx0boy6-Mar-08 22:42 
Generalresize the code section Pin
zon_cpp4-Feb-08 22:03
memberzon_cpp4-Feb-08 22:03 
Questionhow to run exe in a panel or groupbox control in Pin
kk_upadhyay2-Feb-08 1:10
memberkk_upadhyay2-Feb-08 1:10 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

Permalink | Advertise | Privacy | Terms of Use | Mobile
Web01 | 2.8.170424.1 | Last Updated 27 Dec 2005
Article Copyright 2005 by Ashkbiz Danehkar
Everything else Copyright © CodeProject, 1999-2017
Layout: fixed | fluid