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4.3. An In-Depth Look at Win32 Viruses

The world of computer antivirus research has changed drastically since Windows 95 appeared on the market18. One reason this happened was that a certain number of DOS viruses became incompatible with Windows 95. In particular, the tricky viruses that used stealth techniques and undocumented DOS features failed to replicate under the new system. Many simple viruses remained compatible with Windows 95, such as Yankee Doodle, a very successful old Bulgarian virus. Regardless of this, virus writers felt that the new challenge was to investigate the new operating system, to create new DOS executable viruses and boot viruses with special attention to Windows 95 compatibility. Because most virus writers did not have enough in-depth knowledge of the internal mechanisms of Windows 95, they looked for shortcuts to enable them to write viruses for the new platform. They quickly found the first one: macro viruses, which are generally not dependent on the operating system or on hardware differences.

Some young virus writers are still happy with macro viruses and develop them endlessly. After writing a few successful macro viruses, however, most grow bored and stop developing them. You may think, fortunately, but the truth is otherwise. Virus writers are looking for other challenges, and they usually find new and different ways to infect systems.

The first Windows 95 virus, W95/Boza, appeared in the same year that Windows 95 was introduced. Boza was written by a member of the Australian VLAD virus-writing group. It took a long time for other virus writers to understand the workings of the system but, during 1997, new Windows 95 viruses appeared, some of them in the wild.

At the end of 1997, the first Win32, Windows NTcompatible virus, Cabanas, was written by the same young virus writer (Jacky Qwerty/29A) who wrote the infamous WM/Cap.A virus. Cabanas is compatible with Windows 9x, Windows NT, and Win32s. (It is also compatible with Windows 98 and Windows 2000, even though the virus code was never tested on these systems by the virus writer because these systems appeared later than the actual virus.) Cabanas turned Microsoft's Win32 compatibility dream into a nightmare.

Although it used to be difficult to write such viruses, we suspected that file-infecting DOS viruses from the early years of computer viruses would eventually be replaced by Win32 creations.

This transition in computer virus writing was completed by 2004. Even macro viruses are now very rare; virus writers currently focus on 32-bit and 64-bit Windows viruses.

4.3.1. The Win32 API and Platforms That Support It

In 1995, Windows 95 was introduced by Microsoft as a new major operating system platform. The Windows 95 system is strongly based on Windows 3.x and DOS technologies, but it gives real meaning to the term Win32.

What is Win32? Originally, programmers did not even understand the difference between Win32 and Windows NT. Win32 is the name of an APIno more, no less. The set of system functions available to be called from a 32-bit Windows application is contained in the Win32 API. The Win32 API is implemented on several platformsone of them being Windows NT, the most important Win32 platform. Besides DOS programs, Windows NT also is capable of executing 16-bit Windows programs, OS/2 1.x character applications (and, with some extensions, even Presentation Managerbased 1.3 programs with some limitations). In addition, Windows NT introduced the new portable executable (PE) file format (format very similar to, if not based on, the UNIX COFF format) that can run Win32 applications (which call functions in the Win32 API set). As the word portable indicates, this format is supposed to be an easily portable file format, which is actually the most common and important one to run on Windows NT.

Other platforms are also capable of running Win32 applications. In fact, one of them was shipped before Windows NT. This platform is called Win32s. Anyone who has ever tried to develop software for Win32s knows that it was a very unstable solution.

Because Windows NT is a robust system that needs strong hardware on which to run, Win32 technology did not take the market position Microsoft wanted quickly enough. That process ended up with the development of Windows 95, which supported the new PE format by default. Therefore, it supports a special set of Win32 APIs. Windows 95 is a much better implementation of the Win32 APIs than Win32s. However, Windows 95 does not contain the full implementation of the Win32 APIs found in Windows NT.

Until Windows NT gained more momentum, Windows 9x was Microsoft's Win32 platform. After Windows NT, Windows 2000 and Windows 98/Me gained popularity and were replaced by Windows XP and the more secure Windows 2003 server editions, which support the .NET extension by default. On the horizon, Microsoft is talking about the next new Windows release, codenamed Longhorn. All of these systems will support a form of Win32 API that, in most cases, provides binary compatibility among all of these systems.

Last but not least, the Win32 API and the PE format are supported by Windows CE (Windows Mobile edition), which is used primarily by handheld PCs. The main hardware requirement includes 486 and above Intel and AMD processors for a Windows CE platform. However, current implementations seem to use SH3, ARM, and Intel XScale processors.

Now we get to the issue of CPUs. Both Windows NT and Windows CE are capable of running on machines that have different CPUs. The same PE file format is used on the different machines, but the actual executed code contains the compiled binary for the actual processor, and the PE header contains information about the actual processor type needed to execute the image. All of these platforms contain different implementations of Win32 functions. Most functions are available in all implementations. Thus a program can call them regardless of the actual platform on which it is running. Most of the API differences are related to the actual operating system capabilities and available hardware resources. For instance, CreateThread() simply returns NULL when called under Win32s. The Windows CE API set consists of several hundred functions, but it does not support trivial functions such as GetWindowsDirectory() at all because the Windows CE KERNEL is designed to be placed in ROM of the handheld PC. Due to the hardware's severe restrictions (Windows CE must run on machines with 2 or 4MB of RAM without disk storage), Microsoft was forced to create a new operating system that had a smaller footprint than either Windows NT or Windows 95.

Although several manifestations of the Win32 API implement some of the Win32 APIs differently or not at all, in general it is feasible to write a single program that will work on any platform that supports Win32 APIs. Virus writers already understand this fact very well. Their first such virus creation attacked Windows 95 specifically, but virus writers slowly improved the infection methods to attack the PE file format in such a way that the actual infected program remains compatible and also executes correctly under Windows NT/2000/XP systems.

Most Windows 95 viruses depend on Windows 95 system behavior and functionality, such as features related to VxD (virtual device driver) and VMM (virtual machine manager), but some of them contain only a certain amount of bugs and need only slight fixes to be able to run under more than one Win32 platform, such as Windows 95/Windows NT.

Detection and disinfection of such viruses is not a trivial task. In particular, the disinfection can be difficult to implement. This is because, so far, the PE structure is much more complicated than any other executable file format used by DOS or Windows 3.x. However, it is also a fact that the PE format is a much nicer design than, for example, NE.

Unfortunately, over the period from 1995 to 2004, virus writers utilized these platforms aggressively, resulting in the appearance of more than 16,000 variants of 32-bit Windows viruses. However, the principles of these viruses have not changed much. In the next section, you will find details about infection techniques of the PE file format from the perspective of an attacker.

Note

Win64 is almost the same as Win32, but for 64-bit Windows architectures. There are a couple of minor modifications in Win64 to accommodate the platform differences.


4.3.2. Infection Techniques on 32-Bit Windows

This section describes the different ways in which a 32-bit Windows virus can infect different kinds of executable programs used by Windows 95/Windows NT. Because the most common file format is the PE format, most of the infection methods are related to that. The PE format makes it possible for viruses to jump easily from one 32-bit Windows platform to another. We shall concentrate on infection techniques that attack this particular format because these viruses have a strong chance of remaining relevant in the future.

Early Windows 95 viruses have a VxD part, which is dropped by other infected objects such as DOS, EXE, and COM executables or a PE application. Some of these infection methods are not related to Win32 platforms on the API level. For instance, VxDs are only supported by Windows 9x and Windows 3.x, not by Windows NT. VxDs have their own 32-bit, undocumented, linear executable (LE) file format. It is interesting to note that this format was 32-bit even at the time of 16-bit Windows. Microsoft could not drop the support of VxDs from Windows 95 because of the many third-party drivers developed to handle special hardware components. The LE file format remained undocumented by Microsoft, but there are already several viruses, such as Navrhar, that infect this format correctly. I will describe these infection techniques briefly to explain the evolution of Win32 viruses.

4.3.2.1 Introduction to the Portable Executable File Format

In the following section, I will provide an introductory tour of the PE file format that Microsoft designed for use on all its Win32 operating systems (Windows NT, Windows 95, Win32s, and Windows CE). There are several good descriptions of the format on the Microsoft Developer Network CD-ROM, as well as in many other Windows 95related books, so I'll describe the PE format from the point of view of known virus infection techniques. To understand how Win32 viruses work, you need to understand the PE format. It is that simple.

The PE file format will play a key role in all of Microsoft's operating systems for the foreseeable future. It is common knowledge that Windows NT has a VAX VMS and UNIX heritage. As I mentioned earlier, the PE format is very similar to COFF (common object file format), but it is an updated version. It is called portable because the same file format is used under various platforms.

The most important thing to know about PE files is that the executable code on disk is very similar to what the module looks like after Windows has loaded it for execution. This makes the system loader's job much simpler. In 16-bit Windows, the loader must spend a long time preparing the code for execution. This is because in 16-bit Windows applications, all the functions that call out to a DLL (dynamic loaded library) must be relocated. Some huge applications can have thousands of relocations for API calls, which have to be patched by the system loader while reading the file in portions and allocating memory for its structures one by one. PE applications do not need relocation for library calls anymore. Instead, a special area of the PE file, the import address table (IAT), is used for that functionality by the system loader. The IAT plays a key role in Win32 viruses, and I shall describe it later in detail.

For Win32, all the memory used by the module for code, data, resources, import tables, and export tables is in one continuous range of linear address space. The only thing that an application knows is the address where the loader mapped the executable file into memory. When the base address is known, the various pieces of the module can easily be found by following pointers stored as part of the image.

Another idea you should become familiar with is the relative virtual address, or RVA. Many fields in PE files are specified in terms of RVAs. An RVA is simply the offset of an item to where the file is mapped. For instance, the Windows loader might map a PE application into memory starting at address 0x400000 (the most common base address) in the virtual address space. If a certain item of the image starts at address 0x401234, then the item's RVA is 0x1234.

Another concept to be familiar with when investigating PE files and the viruses that infect them is the section. A section in a PE file is roughly equivalent to a segment in a 16-bit NE file. Sections contain either code or data (and occasionally a mixture of both). Some sections contain code or data declared by the actual application, whereas other data sections contain important information for the operating system. Before jumping into the important details of the PE file, examine Figure 4.28, which shows the overall structure of a PE file.

Figure 4.28. A high-level view of the PE file image.


4.3.2.1.1 The PE Header

The first important part of the PE format is the PE header. Just like all the other Microsoft executable file formats, the PE file has a header area with a collection of fields at an easy-to-find location. The PE header describes vital pieces of the portable executable image. It is not at the very beginning of the file; rather, the old DOS stub program is presented there.

The DOS stub is just a minimal DOS EXE program that displays an error message (usually "This program cannot be run in DOS mode"). Because this header is presented at the beginning of the file, some DOS viruses can infect PE images correctly at their DOS stub. However, Windows 95 and Windows NT's system loaders execute PE applications correctly as 32-bit images, and the DOS stub program remains as a compatibility issue with 16-bit Windows systems.

The loader picks up the PE header's file address from the DOS header lfanew field. The PE header starts with an important magic value of PE\0\0. After that is the image file header structure, followed by the image optional header.

From now on, I will describe only the important fields of the PE header that are involved with Windows 9x/Win32 viruses. The fields are in order, but I will concentrate on the most commonly used valuesso several will be missing from the list.

Figure 4.28 shows the high-level structure of a PE file image.

The following paragraphs list important fields of the image file header.

  • WORD Machine

    Indicates the CPU for which this file is intended. Many Windows 9x virus check this field by looking for the Intel i386 magic value before actual infection. However, some bogus viruses do not check the machine type and infect PE files for other platforms and cause such files to crash when the virus code is executed on the wrong platform. There is a certain risk that we will see viruses with multiprocessor support in the future. For example, the same viruses could target ARM as well as IA64 and regular X86 PE files.

  • WORD NumberOfSections

    The number of sections in the EXE (DLL). This field is used by viruses for many different reasons. For instance, the NumberOfSections field is incremented by viruses that add a new section to the PE image and place the virus body in that section. (When this field is changed by the virus code, the section table is patched at the same time.) Windows NTbased systems accept up to 96 sections in a PE file. Windows 95based system do not inspect the section number.

  • WORD Characteristics

    The flags with information about the file. Most viruses check these flags to be sure that the executable image is not a DLL but a program. (Some Windows 9x viruses infect KERNEL32.DLL. If so, the field is used to make sure that the executable is a DLL.) This field is not usually changed by viruses.

    Important fields of the image optional header follow.

  • WORD Magic

    The optional header starts with a "magic" field. The value of the field is checked by some viruses to make sure that the actual program is a normal executable and not a ROM image or something else.

  • DWORD SizeOfCode

    This field describes the rounded-up size of all executable sections. Usually viruses do not fix the value when adding a new code section to the host program. However, some future viruses might change this value.

  • DWORD AddressOfEntryPoint

    The address where the execution of the image begins. This value is an RVA that normally points to the .text (or CODE) section. This is a crucial field for most Windows 9x/Win32 viruses. The field is changed by most of the known virus infection types to point to the actual entry point of the virus code.

  • DWORD ImageBase

    When the linker creates a PE executable, it assumes that the image will be mapped to a specific memory location. That address is stored in this field. If the image can be loaded to the specified address (currently 0x400000 in Microsoft programs), then the image does not need relocation patches by the loader. This field is used by most viruses before infection to calculate the actual address of certain items, but it is not usually changed.

  • DWORD SectionAlignment

    When the executable is mapped into memory, each section must start at a virtual address that is a multiple of this value. This field minimum is 0x1000 (4096 bytes), but linkers from Borland use much bigger defaults, such as 0x10000 (64KB). Most Win32 viruses use this field to calculate the correct location for the virus body but do not change the field.

  • DWORD FileAlignment

    In the PE file, the raw data starts at a multiple of this value. Viruses do not change this value but use it in a similar way to SectionAlignment.

  • DWORD SizeOfImage

    When the linker creates the image, it calculates the total size of the portions of the image that the loader has to load. This includes the size of the region starting at the image base up through the end of the last section. The end of the last section is rounded up to the nearest multiple of section alignment. Almost every PE infection method uses and changes the SizeOfImage value of the PE header.

    Not surprisingly, many viruses calculate this field incorrectly, which makes image execution impossible under Windows NT. This is because the Windows 9x's loader does not bother to check this value when executing the image. Usually (and fortunately) virus writers do not test their creations for long, if at all. Most Windows 95 viruses contain this bug. Some antivirus software used to calculate this field incorrectly when disinfecting files. This causes a side effect: A Windows NTcompatible Win32 program will not be executed by Windows NT but only by Windows 9x, even when the application has been disinfected.

  • DWORD Checksum

    This is a checksum of the file. Most executables contain 0 in this field. All DLLs and drivers, however, must have a checksum. Windows 95's loader simply ignores the checking of this field before loading DLLs, which makes it possible for some Windows 95 viruses to infect KERNEL32.DLL very easily. This field is used by some viruses to represent an infection marker to avoid double infections. Another set of viruses recalculates it to hide an infection even better.

4.3.2.1.2 The Section Table and Commonly Encountered Sections

Between the PE header and the raw data for the image's sections lies the section table. The section table contains information about each section of the actual PE image. (See the following dumps that I made with the PEDUMP tool.)

Basically, sections are used to separate different functioning modules from each other, such as executable code, data, global data, debug information, relocation, and so on. The section table modification is important for viruses to specify their own code section or to patch an already existing section to fit actual virus code into it. Each section in the image has a section header in the section table. These headers describe the name of each section (.text, … .reloc) as well as its actual, virtual, and raw data locations and sizes. First-generation viruses, like Boza, patch a new section header into the section table. (Boza adds its own .vlad section, which describes the location and size of the virus section.)

Sometimes there is no place for a section header in the file, and the patch cannot take its place easily. Therefore, viruses today (such as W95/Anxiety19 variants) attack the last existing section header and modify its fields to fit the virus code in that section. This makes the virus code section less visible and the infection method less risky.

Listing 4.1 is the section table example of CALC.EXE (the Windows Calculator).

Listing 4.1. Looking at the Section Table of CALC.EXE with PEDUMP
01.text VirtSize: 000096B0 VirtAddr: 00001000
raw data offs: 00000400 raw data size: 00009800
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 60000020 CODE MEM_EXECUTE MEM_READ

02 .bss VirtSize: 0000094C VirtAddr: 0000B000
raw data offs: 00000000 raw data size: 00000000
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: C0000080 UNINITIALIZED_DATA MEM_READ MEM_WRITE

03 .data VirtSize: 00001700 VirtAddr: 0000C000
raw data offs: 00009C00 raw data size: 00001800
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: C0000040 INITIALIZED_DATA MEM_READ MEM_WRITE

04 .idata VirtSize: 00000B64 VirtAddr: 0000E000
raw data offs: 0000B400 raw data size: 00000C00
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 40000040 INITIALIZED_DATA MEM_READ

05 .rsrc VirtSize: 000015CC VirtAddr: 0000F000
raw data offs: 0000C000 raw data size: 00001600
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 40000040 INITIALIZED_DATA MEM_READ

06 .reloc VirtSize: 00001040 VirtAddr: 00011000
raw data offs: 0000D600 raw data size: 00001200
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 42000040 INITIALIZED_DATA MEM_DISCARDABLE MEM_READ

The name of the section can be anything. It could even contain just zeros; the loader does not seem to worry about the name. In general, however, the name field describes the actual functionality of the section.

There is a chance for confusion here because the actual code is placed into a .text section of the PE files. This is the traditional name, the same as in the old COFF format. The linker concentrates all the .text section of the various OBJ files to one big .text section and places this in the first position of the section table. As I will describe later, the .text section contains not only code, but an additional jump table for DLL library calls. The Borland linker calls the .text section CODE, which is not a traditional name (but not one beyond normal understanding).

Another common section name is .data, where the initialized data goes. The .bss section contains uninitialized static and global variables. The .rsrc contains and stores the resources for the application.

The .idata section contains the import tablea very important part of the PE format for viruses. (Note that sections are only used as logical separators in the file image. Because nothing is mandatory, the ".idata" section's content might be merged in any other sectionsor not presented at all.)

The .edata section is also very important for viruses because it lists all the APIs that the actual module exports for other executables.

The .reloc section stores the base relocation table. Some viruses take special care of relocation entries of the executables; however, this section seems to disappear from most Windows 98 executables from Microsoft. Somehow the .reloc section had an early PE format design problem. The actual program is loaded before its DLLs, and the application is executed in its own virtual address spacethere seems to be no real need for that.

Last but not least, there is a common section name, the .debug section, which holds the debug information of the executable (if there is any). This is not important for viruses, although they could take advantage of it for infections.

Because the name of the section can be specified by the programmer, some executables contain all kinds of special names by default.

Three of the section table header's fields are very important for most viruses: VirtualSize (which holds the virtual size of the section), SizeOfRawData (which holds the size of the section after it has been rounded up to the nearest file alignment), and the Characteristics field.

The Characteristics field holds a set of flags that indicate the section's attributes (code, data, readable, writable, executable, and so on). The code section has an executable flag but does not need writable attributes because the data are separated. This is not the same with appended virus code, which must keep its data area somewhere in its code. Therefore viruses must check for and change the Characteristics field of the section in which their code will be presented.

All of this indicates that the actual disinfection of a 32-bit virus can be more complicated than that of a normal DOS EXE virus. The infection itself is not trivial in most methods, but so many sources are available on various Internet locations that virus writers have all the necessary support to write new virus variants easily.

4.3.2.1.3 PE File Imports: How Are DLLs Linked to Executables?

Most of the Windows 9x and Windows NT viruses are based heavily on the understanding of the import table, which is a very important part of the PE structure. In Win32 environments, DLLs are linked through the PE file's import table to the application that uses them. The import table holds the names of the imported DLLs and also the names of the imported functions from those DLLs. Consider the following examples:

ADVAPI32.DLL
Ordn Name
285 RegCreateKeyW
279 RegCloseKey

KERNEL32.DLL
Ordn Name
292 GetProfileStringW
415 LocalSize
254 GetModuleHandleA
52 CreateFileW
278 GetProcAddress
171 GetCommandLineW
659 lstrcatW
126 FindClose
133 FindFirstFileW
470 ReadFile
635 WriteFile
24 CloseHandle
79 DeleteFileW

The executable code is located in the .text section of PE files (or in the CODE section, as the Borland linker calls it). When the application calls a function that is in a DLL, the actual CALL instruction does not call the DLL directly. Instead, it goes first to a jump (JMP DWORD PTR [XXXXXXXX]) instruction somewhere in the executable's .text section (or in the CODE section in the case of Borland linkers).

The address that the jump instruction looks up is stored in the .idata section (or sometimes in .text) and is called an entry within the IAT (Import Address Table). The jump instruction transfers control to that address pointed by the IAT entry, which is the intended target address. Thus, the DWORD in the .idata section contains the real address of the function entry point, as shown in the following dump. In Listing 4.2, an application calls FindFirstFileA() in KERNEL32.DLL.

Listing 4.2. Function Imports
.text (CODE)
0041008E E85A370000 CALL 004137ED ; KERNEL32!FindFirstFileA

004137E7 FF2568004300 JMP [KERNEL32!GetProcAddress] ; 00430068
004137ED FF256C004300 JMP [KERNEL32!FindFirstFileA] ; 0043006C
004137F3 FF2570004300 JMP [KERNEL32!ExitProcess] ; 00430070
004137F9 FF2574004300 JMP [KERNEL32!GetVersion] ; 00430074

.idata (00430000)
.
00430068 1E3CF177 ;-> 77F13C1E Entry of KERNEL32!GetProcAddress
0043006C DBC3F077 ;-> 77F0C3DB Entry of KERNEL32!FindFirstFileA
00430070 6995F177 ;-> 77F19569 Entry of KERNEL32!ExitProcess
00430074 9C3CF177 ;-> 77F13C9C Entry of KERNEL32!GetVersion

The calls are implemented in this way to make the loader's job easier and faster. By thunking all calls to a given DLL function through one location, there is no longer the need for the loader to patch every instruction that calls a DLL. All the PE loader has to do is patch the correct addresses into the list of DWORDs in the .idata section for each imported function.

The import table is very useful for modern 32-bit Windows viruses. Because the system loader has to patch the addresses of all the APIs that a Win32 program uses by importing, viruses can easily get the address of an API they need to call by looking into the host program's import table.

With traditional DOS viruses, this problem does not exist. When a DOS virus wants to access a system service function, it simply calls a particular interrupt with the corresponding function number. The actual address of the interrupt is placed in the interrupt vector table and is picked up automatically during the execution of the program. The interrupt vector table is not saved from the running programs; all applications can read and write into it because there are no privilege levels in DOS. The OS and all applications share the same available memory with equivalent rights. Therefore access to a particular system function does cause problems for a DOS virus. It has access to everything it needs by default, regardless of the infection method used.

A Windows 95 virus must call APIs or system services to operate correctly. Most 32-bit applications use the import table, which the linker prepares for them. However, there are a couple of ways to avoid imports. Avoiding imports is often necessary for compatibility reasons. When an application is linked to a DLL, the actual program cannot be executed if the system loader cannot load all the DLLs specified in the import table. Moreover, the system loader checks all the necessary API calls and patches their addresses into the import table. If the loader is unable to locate a particular API by its name or ordinal value, the application cannot be executed.

Some applications must overcome this problem. For instance, if a Win32 program wants to list by name all the running processes under both Windows 95 and NT, it must use system DLLs and API calls under Windows 95 that are different from those under Windows NT. In such a case, the application is not linked directly to all the DLLs it wants to access because the program could not be executed on any system. Instead, the LoadLibrary() function is used to load the necessary DLLs, and GetProcAddress() is used to get the API's address. The actual program can access the API address of LoadLibrary() and GetProcAddress() from its import table. This solves the chicken-and-egg problem of how to call an API without knowing its address if an API call is needed.

As we will see later, Boza solves the problem by using hard-coded API addresses. Modern Win32 viruses, however, are capable of searching the import table during infection time and saving pointers to the .idata section's important entries. Whenever the application has imports for a particular API, the attached virus will be able to call it.

Note

One of the important differences in 64-bit and 32-bit PE files is their handling of import and export entries. The IA64 PE files use a PLABEL_DESCRIPTOR structure in place of any IAT entries. (This structure is detailed in Chapter 12.)


4.3.2.1.4 PE File Exports

The opposite of importing a function is exporting a function for use by EXEs or other DLLs. A PE file stores information about its exported functions in the .edata section. Consider the following dump, which lists a few exports of KERNEL32.DLL:

Entry Pt Ordn Name
000079CA 1 AddAtomA
.
0000EE2B 38 CopyFileA
.
0000C3DB 131 FindFirstFileA
.
00013C1E 279 GetProcAddress

KERNEL32.DLL's export table consists of an Image_Export_directory, which has pointers to three different lists: the function address table, the function name table, and the function ordinal table. Modern Windows 95/NT viruses search for the "GetProcAddress" string in the function name table to be able to retrieve the API function entry-point value.

When this value is added to the ImageBase, it gives back the 32-bit address of the API in the DLL. In fact, this is almost the same algorithm that the real GetProcAddress() from KERNEL32.DLL follows internally. This function is one of the most important for Windows 95 viruses that want to be compatible with more than one Win32-based system. When the address of GetProcAddress() is available, the virus can get all the API addresses it wants to use.

4.3.2.2 First-Generation Windows 95 Viruses

The first Windows 95 virus, known as W95/Boza.A, was introduced in the VLAD virus writer magazine. Boza's authors obviously wanted to be the first with their creation, and they had to find a Windows 95 beta version very quickly to do so. Pioneer viruses used to be very buggy, and Boza was no exception. Basically, the virus cannot work on more than two Windows 95 versions: a beta release and the final version. Even on those two Windows 95 releases, the virus causes many general protection faults during replication. Infected files are often badly corrupted.

Boza is a typical appending virus that infects PE applications. The virus body is placed in a new section called .vlad. First the .vlad section header is patched into the section table as the last entry, and the number of sections field is incremented in the PE header. The body of the virus is appended to the end of the original host program, and the PE header's entry point is modified to point to the new entry point in the virus section.

Boza uses hard-coded addresses for all the APIs it has to call. That approach is the easiest, but, fortunately, it is not very successful. The authors of the virus worked on a beta version of Windows 95 first and used addresses hard-coded for that particular implementation of KERNEL32.DLL. Later they noticed that the actual virus did not remain compatible with the final release of Windows 95. This happened because Microsoft did not have to provide the same ordinal values and addresses for all the APIs for every system DLL in all releases. This would be impossible. Different Windows 95 implementationsbetas, language versions, OSR2 releasesdo not share the same API addresses. For instance, the first API call in Boza happens to be GetCurrentDirectoryA(). Figure 4.29 shows that the ordinal values and entry points of GetCurrentDirectoryA are different in the English version of Windows 95 and in the Hungarian OSR2 Windows 95 release of KERNEL32.DLL.

Figure 4.29. The ordinal references on two releases of Windows 95.
Entry Pt Ordn
A. 00007744 304 GetCurrentDirectoryA (Windows 95 ENG)
B. 0000774C 307 GetCurrentDirectoryA (Windows 95 OSR2-HUN)

ImageBase is 0xBFF70000 in both KERNEL32.DLL releases, but the procedure address of GetCurrentDirectoryA() is 0xBFF77744 in the English release and 0xBFF7774C in the Hungarian OSR2 version. When Boza wants to replicate on the Hungarian version of Windows 95, it calls an incorrect address and, obviously, fails to replicate. Therefore, Boza cannot be called a real Windows 95compatible virus. It turns out that Boza is incompatible with most Windows 95 releases.

Regardless of these facts, many viruses try to operate with hard-coded API addresses. Most of these Windows 95 viruses cannot become in the wild. Virus writers seem to understand Win32 systems much better already, creating viruses that are compatible not only with all Windows 95 releases but also with Windows 98 and Windows NT versions.

4.3.2.2.1 Header Infection

This type of Windows 95 virus inserts itself between the end of the PE header (after the section table) and the beginning of the first section. It modifies the AddressOfEntryPoint field in the PE header to point to the entry point of the virus instead. The first known virus to use this technique was W95/Murkry.

The virus code must be very short in Windows 95 header infections. Because sections must start at an offset that is a multiple of the FileAlignment, the maximum available place to overwrite cannot reach much more than the FileAlignment value. When the application contains too many sections and the FileAlignment is 512 bytes, there is no place for the virus code. The AddressOfEntryPoint field is an RVA; however, the virus code is not placed in any of the sections and, therefore, the actual RVA is the real physical offset in the file that the virus must place in the header. It is interesting to note that the entry point does not point into any code section but, regardless of that fact, Windows 95's loader happily executes the infected program.

There is a chance that a scanner will fail to detect the second generation of such viruses. This happens when the scanner is only tested on first-generation samples. In first-generation samples, the AddressOfEntryPoint points to a valid section. When the scanner looks for the entry point of the program, it must check all the section headers and whether the AddressOfEntryPoint points to any of them. There is a chance that this function is not implemented to handle those cases in which the entry point does not point to any of the sections. Some scanners may skip the file instead of scanning it from the real entry point, thereby failing to detect the infection in second-generation samples.

4.3.2.2.2 Prepending Viruses

The easiest way to infect PE files is to overwrite their beginning. Some DOS viruses infect PE files this way, but none of the known Windows 95 viruses use this infection method. Of course, the application will not work correctly after the infection. Such viruses are discovered almost immediately for this reason, which is why viruses that do not want to handle the complicated file format of PE files use the prepending method. Such viruses are usually written in a high-level language (HLL) such as C or even Delphi. This method consists of prepending the virus code to the PE file. The infected program starts with the EXE header of the virus. When the virus wants to transfer control to the original program code, it has to extract it to a temporary file and execute it from there.

Disinfection of such viruses is easy. The original header information is available at the very end of the infected program in a nonencrypted format. Virus writers will recognize that and will encrypt the original header information later on. This will make disinfection more complicated.

4.3.2.3 Appending Viruses That Do Not Add a New Section Header

A more advanced appending method is used by the W95/Anxiety virus. Anxiety is very similar to Boza in its infection mechanism, but its code is more related to the somewhat bogus W95/Harry virus.

The Anxiety virus does not add a new section header at the end of the section table. Rather, it patches the last section's section header to fit into that section. In this way, the virus can infect all PE EXE files easily. There is no need to worry that the actual section header does not fit into the section table.

By modifying the VirtualSize and SizeOfRawData fields, the virus code can be placed at the end of the executable. In this way, the NumberOfSection field of the PE header should not need to be modified. The AddressOfEntryPoint field is changed to point to the virus body, and the SizeOfImage is recalculated to represent the new size of the program. Listing 4.3 is the last section of CALC.EXE before and after the W95/Anxiety.1358 infection.

Listing 4.3. The Section Modification of W95/Anxiety.1358
06 .reloc VirtSize: 00001040 VirtAddr: 00011000
raw data offs: 0000D600 raw data size: 00001200
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 42000040 INITIALIZED_DATA MEM_DISCARDABLE MEM_READ
06 .reloc VirtSize: 00002040 VirtAddr: 00011000
raw data offs: 0000D600 raw data size: 00001640
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: E0000040 INITIALIZED_DATA MEM_EXECUTE MEM_READ
MEM_WRITE

The Characteristics field of the last section header is changed to have writable/executable attributes. The writable characteristic is enough in itself to execute self-modifying code from any section, but many virus writers initially did not realize that.

Viruses like W32/Zelly use two or more infection strategies. In basic infection mode Zelly adds two sections to the host program. In advanced infection mode, it merges all sections of the host into a single section, and appends the virus to the end of the image. This integrates the virus body tighter into the host program.

4.3.2.4 Appending Viruses That Do Not Modify the Entry Point

Some Windows 95 and Win32 viruses do not modify the AddressOfEntryPoint field of the infected program. The virus appends its code to the PE file, but it gets control in a more sophisticated way. It calculates where the original AddressOfEntryPoint points to and places a JMP instruction there that points to the virus body. Fortunately, it is very difficult to write such viruses.

This is because the virus must take care of the relocation entries that point to the overwritten part of the code. The W32/Cabanas virus masks out the relocation entries that point to that area. W95/Marburg does not place a JMP instruction at the entry point if it finds relocations for that area; instead, it modifies the AddressOfEntryPoint field. The JMP instruction should not be the first instruction in the program. W95/Marburg shows this by placing the JMP instruction after a random garbage block of code when no relocations are present in the first 256 bytes of entry-point code. In this way, it is not obvious to scanners and integrity checkers how to figure out the entry point of the virus code.

4.3.2.5 KERNEL32.DLL Infection

Most Windows 95 viruses attack the PE format, but some of them also infect DOS COM, EXE programs, VxDs, Word documents, and 16-bit Windows new executables (NE). Others may infect DLLs accidentally because these are linked in PE (or NE) formats, but the infection is not able to spread further because the standard entry point of the DLLs is not called by the system loader. Instead, the DLL's execution normally starts at its specified DLL entry point.

KERNEL32.DLL infectors do not attack the entry point. Instead, this type of virus must gain control differently. PE files have many other entry points that are useful for viruses, especially DLLs, which are export APIs (their entry points) by nature. Therefore, the easiest way to attack KERNEL32.DLLs is to patch the export RVA of one of the APIs (for instance, GetFileAttributesA) to point to the virus code at the end of the DLL image. W95/Lorez20 uses this approach. Viruses like this are able to go "resident" easily. The system loads the infected DLL during the system initialization period. After that, every program that has KERNEL32.DLL imports will be attached to this infected DLL. Whenever the application has a call to the API in which the virus code has been attached, the virus code gets control.

All the system DLLs contain a precalculated checksum in their PE header, placed there by the linker. Unlike Windows 95, Windows NT recalculates this checksum before it loads the DLL. If the calculated checksum is not the same as in the header of the DLL, the system loader stops with an error message during the blue screen boot-up period. However, this does not mean that such a virus cannot be implemented for Windows NTit just makes implementation a bit more complicated. Although the checksum algorithm is not documented by Microsoft, there are APIs available in IMAGEHLP.DLL for these purposeslike CheckSumMappedFile()which are efficient enough to calculate a new, correct checksum after the actual infection is done. This is not enough, however, for Windows NT's loader. There are several other steps to take, but there is no doubt that virus writers will be able to solve these questions soon. There is a need for virus scanners to check the consistency of a KERNEL32.DLL by recalculating the PE header checksum, especially if the scanner is a Win32 application itself and is attached to an infected KERNEL32.DLL.

4.3.2.6 Companion Infection

Companion viruses are not very common. Nevertheless, some virus writers do develop Windows 95 companion viruses. A path companion virus depends on the fact that the operating system always executes files with a COM extension first in preference to an EXE extension, if the names of two files in the same directory differ only in their extensions. These viruses simply look for a PE application with an EXE extension and then copy themselves with the same name into the same directory (or somewhere on the path) with a COM extension, using the host's name. W95/Spawn.4096 uses this technique. This functionality is implemented by using FindFirstFileA(), FindNextFileA() APIs for search, CopyFileA() to copy the virus code, and CreateProcessA() to execute the original host program.

4.3.2.7 Fractionated Cavity Infection

I originally predicted this infection technique as one that would possibly be developed in the future. However, the W95/CIH virus had already introduced this technique before my first lecture on Win32 viruses.

There is slack space between most sections, which is usually filled with zeros (or 0xCC) by the linker. This is because the sections have to start at the file alignment, as described in the PE header's FileAlignment field. The actual virtual size each section uses is usually different from the raw data representation. Usually, the virtual size is a smaller value. In most cases, Microsoft's Link program generates PE files like that. The difference between the raw data size of the section and the virtual size is the actual alignment area, which is filled by zeros and not loaded when the program is mapped into its own address space.

Because the default value of FileAlignment is 512 bytes (usual sector size), the usual slack area size is smaller than 512 bytes. When I first considered this kind of infection method, I thought that no such viruses would be developed because less than 512 bytes is not big enough for an average PE infector virus of that kind. However, two minutes later I had to recognize that this simple problem would not stop virus writers from developing such viruses. The only thing that has to be done by the virus is to split its virus body into several parts and then into as many section alignments as are available. The loader code for these blocks can be very short, first moving each separated code block to an allocated memory area, one by one. This code itself fits into a big enough section alignment area.

This is the precise method used by the W95/CIH virus. This makes the job of the scanner and the disinfector much harder. The virus changes the virtual size of the section to be the same as the raw data size in each section header, into which it injects a part of its virus body. The exact identification of such viruses is more difficult than for normal viruses because the virus body must be fetched from different areas of the PE image first.

W95/CIH uses the header infection method at the same time and infects Microsoft Linkercreated images without any problem. The fragmented cavity infection technique has a very important advantage from the virus's point of view. The infected file does not get bigger after the infection; its size remains the same. This makes noticing the virus much harder. The identification must be done very carefully because a virus like that may split its body at any offset, which might also separate the actual search string into several parts. This fact shows that it is very important to analyze new Windows 95 viruses with extreme care. Otherwise, the scanner might not find all generations of the same virus code.

4.3.2.8 Modification of the lfanew Field in an Old EXE Header

This is the second infection method that I originally intended to describe as one that has not yet been developed. However, as with the fragmented cavity infection method (discussed in the previous section), this technique appeared in a virus during the time I was writing about it. This infection method is one of the simplest to implement and therefore is used in many viruses. The first known virus to use this method was W95/Cerebrus. The method itself works on Windows NT, but there is a trivial bug in the virus that makes this impossible. Basically, this infection method is an appending typethe virus body is attached to the very end of the original program.

The important difference is that the virus code itself contains its own PE header. When the virus infects a PE application, it modifies the lfanew field (at 0x3c address) in the old EXE header. As described earlier, the lfanew field holds the file address of the PE header. Because this field points to a new PE header, the program is executed as if it contains only the virus code. The virus functions like a normal Win32 application. It has its own imports and can easily access any APIs it wants to call. When the replication is done, the virus creates a temporary file with a copy of the infected program. In this file, the lfanew field will point correctly to the original PE header. Thus, the original program is functional again when the virus executes the temporary file.

4.3.2.9 VxD-Based Windows 95 Viruses

Most Windows 95 viruses are direct-action infectors. Virus writers recognized the importance of fast infection and tried to look for solutions to implement Windows 95 resident viruses. Though not the easiest, the evident solution was to write a VxD virus. One of the first VxD-based viruses was W95/Memorial. It infects DOS, COM, EXE, and PE applications. The virus does not replicate without Windows 95. The infected programs use a dropping mechanism to extract the real virus codea VxD into the root directory of drive C: as CLINT.VXD.

When the VxD is loaded, the virus code is executed on ring 0, thus the virus can do anything it wants. VxDs can hook the file system easily, and that is exactly what most VxD viruses want to do. They simply hook the installable file system (IFS) with one simple VxD service routine. After that, the virus can monitor access to all files. The VxD code has to be extracted, and the dropper code needs different implementation for each and every format that the virus wants to infect. This makes the virus code very complicated and relatively big (12,413 bytes). Therefore, it is very unlikely that many viruses like this will be developed in the future.

4.3.2.10 PE Viruses That Operate as VxDs

A much easier solution has been introduced by the W95/Harry and W95/Anxiety viruses. These viruses can overcome complications by patching their code into the VMM (virtual machine manager) of Windows 95.

When an infected PE program is executed, the virus code takes control. Programs are executed on the application level, which is why they cannot call system-level functions (VxD calls) normally. These viruses bypass the system by installing their code into the VMM, which runs on ring 0. The installation routine of such a virus searches for a big enough hole in the VMM's code area after the 0C0001000h address.

If a large enough area, consisting of only 0FFh bytes, is detected, the virus looks for the VMM header at 0x0C000157Fh and checks this area by comparing it to VMM. If this is detected, the virus picks up the Schedule_VM_Event system function's address from the VMM and saves it for later use. Then it copies its code into the VMM by overwriting the previously located hole and changes the original Schedule_VM_Event's address to point to a new function. Finally, it executes the original host program by jumping to the original entry point. This all is possible because Microsoft is unable to protect that area from changes to keep backward compatibility with old Windows 3.x VxDs. The full VMM area is available for read and write access for application-level programs.

Before the host program can be executed, the VMM will call Schedule_VM_Event, which is now replaced by the initialization routine of the virus. This code is executed on ring 0 already, which enables it to call VxD functions. Anxiety hooks the IFS by calling IFSMgr_InstallFileSystemApiHook from there. This installs the new hook API of the virus.

The virus replication code needs special care. When VxD code is executed, VxD calls are patched by the VMM. The VMM turns the 0CDh, 20h, DWORD function ID (INT 20H, DWORD ID)21 to FAR CALLS. Some of the VxD functions consist of a single instruction. In this case, the VMM patches the six bytes with this single instruction, which fits there. The VMM does this dynamically with all the executed VxDs to speed up their execution.

When the virus code is executed, the VxD functions in the virus body are patched by the VMM, and the virus therefore cannot copy this image immediately to files again because the virus code would not work in a different Windows 95 environment. These viruses contain a function that patches all their VxD functions back to their normal format first and only after that replicates the code into the host program. Even if this technique looks very complicated, it is not very difficult for virus writers. W95/Anxiety variants used to be in the wild in many countries.

There is no doubt that several viruses will try to overcome the ring 3 to ring 0 problem using similar methods even on Windows NTbased systems. W95/CIH uses instructions that are available only from Intel 386 processors and above. It is interesting to note that the interrupt descriptor table is available to write under Windows 95 (because it is part of the VMM). W95/CIH uses the SIDT (store IDT) instruction to get a pointer to the IDT (this technique is detailed in Chapter 6). In this way, the virus can modify the gate descriptor of INT 3 (debug interrupt) in the IDT and allocate memory by using VxD services. The INT 3 routine will be executed as a ring 0 interrupt from its PE virus body. This trick shows how easy it is for virus writers to overcome the ring 3, ring 0 problem. Similar methods will be discovered by Windows 95 virus writers in the near future, resulting in an even simpler method.

4.3.2.11 VxD Infection

A few viruses, such as Navrhar, infect Windows Virtual Device Drivers (VxDs). Navrhar also infects Word documents that are in the OLE2 format and some standard system VxDs. The virus does not infect unknown VxDs, but only known system VxDs that are listed in its PE dropper. When an infected Word document is opened, the virus extracts its PE dropper, which is attached to the very end of the document. Therefore, the only way to access this code is to use Win32 APIs, which is why the virus imports KERNEL32.DLL APIs in its macro code. When the dropper's code is extracted from the document, the dropper is executed, checking for the listed VxDs and infecting them one by one. When the system is rebooted, one of the infected VxDs will be loaded by Windows 95. The virus takes control from the infected VxD, hooks the file system, and checks for Word document access.

Navrhar illustrates that, unlike DOC files, PE applications are not so commonly exchanged by usersnot to mention VxDs, which are not normally exchanged at all. This is why modern Win32 viruses use some form of worm propagation mechanism instead (see Chapters 9 and 10).

4.3.2.12 DLL Load Insertion Technique

This particular infection technique is based on manipulation of PE files in such a way that when the host application is loaded, it will load an extra DLL, which is the virus code.

For example, W32/Initx loads a DLL with the name INITX.DAT via a single LoadLibrary() call inserted into the host program. This extra code is inserted into a slack space of the code section of the host, and the entry point of the host is modified to point to the inserted code. On execution of the host program and whenever the INITX.DAT file is available, the virus code is launched before the host program's code. After this, control is given to the original host entry-point code.

4.3.3. Win32 and Win64 Viruses: Designed for Microsoft Windows?

Microsoft's strategy is clear. The Designed for Microsoft Windows logo program's important requirement is that every application in your product must be a Microsoft Win32 program compiled with a 32-bit compiler that generates an executable file of the PE format. Not surprisingly, the number of Win32 programs developed by third parties has grown intensively during the last few years. People exchange and download more PE programs.

The main reason that Windows 95 and Win32 viruses did not cause big problems for a long time was that virus writers had to learn a lot to "support" the new systems. Young virus writers understand Microsoft's message: "Windows everywhere!" Their answer seems to be "Windows viruses everywhere!" These young guys will not waste their time with DOS viruses anymore but will continuously explore Win32 and Win64 platforms instead.

There is no longer any point in attackers' writing DOS viruses. Virus scanners are much weaker in handling Windows viruses generically and heuristicallydetection and disinfection are not that easy. Vendors must learn and understand the new 64-bit file formats and spend a reasonable amount of time researching and designing new scanning technology.

Because Windows 95 and Windows NT are more complicated systems, it is natural that the first period of such viruses took more time than DOS viruses. However, the number of Win32 viruses surpassed 10,000 in 2004. It took about 10 years for DOS viruses to reach 10,000 known variants, but only 9 years for Win32 threats. This indicates that, although virus writing slows down as new platforms appear (replacing older ones), eventually the growth ratio of any virus type will be exponential.

In the following section, I will describe some important issues that make a Windows 95 virus incompatible with Windows NT. This specifies the differences between the Windows 95 and Win32 prefixes that scanners use to identify 32-bit Windows viruses.

4.3.3.1 Important Windows 95 and NT System Loader Differences

Before I understood W32/Cabanas, I had a different picture of Windows NT from the security point of view because I had incorrect conclusions about the level of system security when the first Windows 95 virus, Boza, appeared. Most antivirus researchers immediately performed some tests with Boza on Windows NT. The result looked reassuring: Windows NT did not even try to execute the infected image as shown on Figure 4.30.

Figure 4.30. An error message is displayed when executing W95/Boza on Windows NT.


What is good for Window 95's loader is not good for Windows NT. Why? I answered this question myself by patching PE files.

The PE file format was designed by Microsoft for use by all its Win32 operating systems (Windows NT/2000/XP/2003, Windows 95/98/Me, Win32s, and Windows CE). (Later, the PE file format was extended to PE+ to accommodate the needs of 64-bit platforms.) That is why all the system loaders in Win32 systems have to understand this executable structure. However, the implementation of the loader is different from one system to another. Windows NT's loader simply checks more things in the PE file before it executes the image than Windows 95's loader does. Thus Windows NT finds the Boza-infected file suspicious. This happens because one field in the .vlad section header (which is patched into the section table of the host program) is not precisely calculated by the virus. As a result, correctly calculated sections and section headers can be added to a PE file without any problem. Thus the Windows NT's loader does not have any superior virus detection, as some may assume.

If this problem were fixed in Boza, the virus would be capable of starting the host program even on a Windows NT platform. However, the virus would still not be able to replicate. This is because of another incompatibility problem, from which all the initial Windows 95 viruses have suffered. Every Windows 95 virus must overcome a specific problem: It must be able to call two Win32 KERNEL APIs: GetModuleHandle() and GetProcAddress(). Because those APIs are in KERNEL32.DLL, Windows 95 viruses could access those functions from KERNEL32.DLL directly with a hack. Most Windows 95 viruses have hard-coded pointers to GetModuleHandle() and GetProcAddress() KERNEL APIs. By using GetProcAddress(), the virus can access all the APIs it wants to call. (Alternatively, some viruses use LoadLibrary() to get a module handle to KERNEL32.DLL, but this method is less common. This is because most applications already map the KERNEL32 API in their process address space.)

When the linker creates an executable, it assumes that the file will be mapped to a specific location in memory. In the PE file header, there is a field called ImageBase holding this address. For executables, this address is usually 0x400000 by default. In the case of Windows 95, the KERNEL32.DLL's ImageBase address is 0xBFF70000. Thus, the address of GetModuleHandle() and GetProcAddress() will be at a certain fixed location in the same release of KERNEL32.DLL. However, this address can be different in a new release, which makes Windows 95 viruses incompatible even with other Windows 95 releases. This ImageBase address is 0x77F00000 in Windows NT as the default. Thus Windows 95 viruses that operate with a Windows 95specific base address cannot work on Windows NT. (Interestingly enough, first-generation exploit code often suffers from similar problems and is only able to work on a single platform.)

The third reason for incompatibility is obvious: Windows NT does not support VxDs. Viruses such as Memorial cannot operate on Windows NT because such viruses are VxD-based. They should have included different infection algorithms at the driver level for Windows NT and Windows 95 to operate on both systems, which would make them complicated.

If a Windows 95 virus can overcome the preceding incompatibility and implementation problems, it will eventually work on Windows NT/2000/XP/2003 as well. Such viruses might have Unicode support, but it is not mandatory. W32/Cabanas supports all of these features, being able to trespass the OS barrier imposed by early Windows 95 creations.

Both Boza and Cabanas are 32-bit Win32 programs. Cabanas infects files under Windows 95/98/Me (and any other localized versions) and under all major Windows NTbased systems releases, such as 3.51, 4.0, 5.0 (Windows 2000), and 5.1 (Windows XP). Boza replicates only under the English Windows 95 release. Therefore, the prefix part of the virus name is Win32 for Cabanas and Win95 for Boza.

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