API Hooking Revealed

Environment: VC6 SP4, NT4 SP4, Windows 2000, Windows 9x/Me


Intercepting Win32 API calls has always been a
challenging subject among most of the Windows developers and I have to admit,
it’s been one of my favorite topics. The term Hooking represents a fundamental
technique of getting control over a particular piece of code execution. It
provides an straightforward mechanism that can easily alter the operating
system’s behavior as well as 3rd party products, without having their source
code available.

Many modern systems draw the attention to their ability
to utilize existing Windows applications by employing spying techniques. A key
motivation for hooking, is not only to contribute to advanced functionalities,
but also to inject user-supplied code for debugging purposes.

Unlike some relatively “old” operating systems like DOS and
Windows 3.xx, the present Windows OS as NT/2K and 9x provide sophisticated
mechanisms to separate address spaces of each process. This architecture offers
a real memory protection, thus no application is able to corrupt the address
space of another process or in the worse case even to crash the operating system
itself. This fact makes a lot harder the development of system-aware hooks.

My motivation for writing this article was the need
for a really simple hooking framework, that will offer an easy to use interface
and ability to capture different APIs. It intends to reveal some of the tricks
that can help you to write your own spying system. It suggests a single solution
how to build a set for hooking Win32 API functions on NT/2K as well as
98/Me (shortly named in the article 9x) family Windows. For the sake of simplicity
I decided not to add a support for UNICODE. However, with some minor modifications
of the code you could easily accomplish this task.

Spying of applications provides many advantages:

  1. API function’s monitoring

    The ability to control API function calls is extremely
    helpful and enables developers to track down specific “invisible” actions that
    occur during the API call. It contributes to comprehensive validation of
    parameters as well as reports problems that usually remain overlooked behind
    the scene. For instance sometimes, it might be very helpful to monitor memory
    related API functions for catching resource leaks.

  2. Debugging and reverse engineering

    Besides the standard methods for debugging API hooking
    has a deserved reputation for being one of the most popular debugging
    mechanisms. Many developers employ the API hooking technique in order to
    identify different component implementations and their relationships. API
    interception is very powerful way of getting information about a binary

  3. Peering inside operating system

    Often developers are keen to know operating system in
    dept and are inspired by the role of being a “debugger”. Hooking is also quite
    useful technique for decoding undocumented or poorly documented APIs.

  4. Extending originally offered
    by embedding custom modules into external
    Windows applications Re-routing the normal code execution by injecting hooks
    can provide an easy way to change and extend existing module functionalities.
    For example many 3rd party products sometimes don’t meet specific security
    requirements and have to be adjusted to your specific needs. Spying of
    applications allows developers to add sophisticated pre- and post-processing
    around the original API functions. This ability is an extremely useful for
    altering the behavior of the already compiled code.

Functional requirements of a hooking system

There are few important decisions that have to be made,
before you start implementing any kind of API hooking system. First of all, you
should determine whether to hook a single application or to install a
system-aware engine. For instance if you would like to monitor just one
application, you don’t need to install a system-wide hook but if your job is to
track down all calls to TerminateProcess() or
WriteProcessMemory() the only way to do so is
to have a system-aware hook. What approach you will choose depends on the
particular situation and addresses specific problems.

General design of an API spying

Usually a Hook system is composed of at least two parts
a Hook Server and a Driver. The Hook Server is responsible for injecting the
Driver into targeted processes at the appropriate moment. It also administers
the driver and optionally can receive information from the Driver about its
activities whereas the Driver module that performs the actual

This design is rough and beyond doubt doesn’t cover all possible
implementations. However it outlines the boundaries of a hook framework.

Once you have the requirement specification of a hook
framework, there are few design points you should take into account:

  • What applications do you need to hook
  • How to inject the DLL into targeted processes or
    which implanting technique to follow
  • Which interception mechanism to use

I hope next the few sections will provide answers to
those issues.

Injecting techniques

  1. Registry
    In order to inject
    a DLL into processes that link with USER32.DLL, you simply can add the DLL
    name to the value of the following registry key:


    Its value contains a single DLL name or group of DLLs
    separated either by comma or spaces. According to MSDN documentation [7], all
    DLLs specified by the value of that key are loaded by each Windows-based
    application running within the current logon session. It is interesting that
    the actual loading of these DLLs occurs as a part of USER32’s initialization.
    USER32 reads the value of mentioned registry key and calls LoadLibrary() for these DLLs in its DllMain code. However this trick applies only to
    applications that use USER32.DLL. Another restriction is that this built-in
    mechanism is supported only by NT and 2K operating systems. Although it is a
    harmless way to inject a DLL into a Windows processes there are few

    • In order to activate/deactivate the injection
      process you have to reboot Windows.

    • The DLL you want to inject will be mapped only into
      these processes that use USER32.DLL, thus you cannot expect to get your hook
      injected into console applications, since they usually don’t import
      functions from USER32.DLL.

    • On the other hand you don’t have any control over
      the injection process. It means that it is implanted into every single GUI
      application, regardless you want it or not. It is a redundant overhead
      especially if you intend to hook few applications only. For more details see
      [2] “Injecting a DLL Using the Registry”

  2. System-wide Windows Hooks
    Certainly a very popular technique for injecting DLL into
    a targeted process relies on provided by Windows Hooks. As pointed out in MSDN
    a hook is a trap in the system message-handling mechanism. An application can
    install a custom filter function to monitor the message traffic in the system
    and process certain types of messages before they reach the target window
    A hook is normally implemented in a DLL in
    order to meet the basic requirement for system-wide hooks. The basic concept
    of that sort of hooks is that the hook callback procedure is executed in the
    address spaces of each hooked up process in the system. To install a hook you
    call SetWindowsHookEx() with the appropriate
    parameters. Once the application installs a system-wide hook, the operating
    system maps the DLL into the address space in each of its client processes.
    Therefore global variables within the DLL will be “per-process” and cannot be
    shared among the processes that have loaded the hook DLL. All variables that
    contain shared data must be placed in a shared data section. The diagram
    bellow shows an example of a hook registered by Hook Server and injected into
    the address spaces named “Application one” and “Application two”.

    Figure 1

    A system-wide hook is registered just ones when SetWindowsHookEx()
    is executed. If no error occurs a handle to the hook is returned. The
    returned value is required at the end of the custom hook function when a
    call to CallNextHookEx() has to be made.
    After a successful call to SetWindowsHookEx() , the operating system injects
    the DLL automatically (but not necessary immediately) into all processes that
    meet the requirements for this particular hook filter. Let’s have a closer
    look at the following dummy WH_GETMESSAGE filter function:

    // GetMsgProc
    // Filter function for the WH_GETMESSAGE - it's just a
    // dummy function
      int code,       // hook code
      WPARAM wParam,  // removal option
      LPARAM lParam   // message
      // We must pass the all messages on to CallNextHookEx.
      return ::CallNextHookEx(sg_hGetMsgHook, code, wParam, lParam);

    A system-wide hook is loaded by multiple processes
    that don’t share the same address space.
    For instance
    hook handle sg_hGetMsgHook, that is obtained
    by SetWindowsHookEx() and is used as
    parameter in CallNextHookEx() must be used
    virtually in all address spaces. It means that its value must be shared among
    hooked processes as well as the Hook Server application. In order to make this
    variable “visible” to all processes we should store it in the shared data
    Follows an example of employing #pragma data_seg(). Here I would like to mention
    that the data within the shared section must be initialized, otherwise the
    variables will be assigned to the default data segment and #pragma data_seg() will have no effect.

    // Shared by all processes variables
    #pragma data_seg(".HKT")
    HHOOK sg_hGetMsgHook       = NULL;
    BOOL  sg_bHookInstalled    = FALSE;
    // We get this from the application who
    // calls SetWindowsHookEx()'s wrapper
    HWND  sg_hwndServer        = NULL;
    #pragma data_seg()

    You should add a SECTIONS statement to the DLL’s DEF file as well

      .HKT   Read Write Shared

    or use

    #pragma comment(linker, "/section:.HKT, rws")

    Once a hook DLL is loaded into the address space of
    the targeted process, there is no way to unload it unless the Hook Server
    calls UnhookWindowsHookEx() or the hooked
    application shuts down. When the Hook Server calls UnhookWindowsHookEx() the operating system loops
    through an internal list with all processes which have been forced to load the
    hook DLL. The operating system decrements the DLL’s lock count and when it
    becomes 0, the DLL is automatically unmapped from the process’s address
    Here are some of the advantages of this

    • This mechanism is supported by NT/2K and 9x Windows
      family and hopefully will be maintained by future Windows versions as well.

    • Unlike the registry mechanism of injecting DLLs
      this method allows DLL to be unloaded when Hook Server decides that DLL is
      no longer needed and makes a call to UnhookWindowsHookEx()

    Although I consider Windows Hooks as very handy
    injection technique, it comes with its own disadvantages:

    • Windows Hooks can degrade significantly the entire
      performance of the system, because they increase the amount of processing
      the system must perform for each message.

    • It requires lot of efforts to debug system-wide
      Windows Hooks. However if you use more than one instance of VC++ running in
      the same time, it would simplify the debugging process for more complex

    • Last but not least, this kind of hooks affect the
      processing of the whole system and under certain circumstances (say a bug)
      you must reboot your machine in order to recover it.

  3. Injecting DLL by using CreateRemoteThread() API function
    Well, this is my favorite one. Unfortunately it is
    supported only by NT and Windows 2K operating systems. It is bizarre, that
     you are allowed to call (link with) this API on Win 9x as well, but
    it just returns NULL without doing anything.

    Injecting DLLs by
    remote threads is Jeffrey Ritcher’s idea and is well documented in his article [9] “Load
    Your 32-bit DLL into Another Process’s Address Space Using INJLIB”.
    The basic concept is quite simple, but very elegant. Any
    process can load a DLL dynamically using LoadLibrary() API. The issue is how do we force an external process to call LoadLibrary() on our behalf, if we don’t have any access to process’s threads? Well, there is a
    function, called CreateRemoteThread() that
    addresses creating a remote thread. Here comes the trick have a look at the
    signature of thread function, whose pointer is passed as parameter (i.e. LPTHREAD_START_ROUTINE) to the CreateRemoteThread():

    DWORD WINAPI ThreadProc(LPVOID lpParameter);

    And here is the prototype of LoadLibrary() API

    HMODULE LoadLibrary(LPCTSTR lpFileName);

    they do have “identical” pattern. They use the same calling convention WINAPI, they both accept one parameter and the
    size of returned value is the same. This match gives us a hint that we can use
    LoadLibrary() as thread function, which will
    be executed after the remote thread has been created. Let’s have a look at the
    following sample code:

    hThread = ::CreateRemoteThread(hProcessForHooking,

    By using GetProcAddress() API we get
    the address of the LoadLibrary() API. The
    dodgy thing here is that Kernel32.DLL is mapped always to the same address
    space of each process, thus the address of LoadLibrary() function has the same value in
    address space of any running process. This ensures that we pass a valid
    pointer (i.e. pfnLoadLibrary) as parameter
    of CreateRemoteThread().
    As parameter of the thread function we use the full path
    name of the DLL, casting it to LPVOID. When
    the remote thread is resumed, it passes the name of the DLL to the
    ThreadFunction (i.e. LoadLibrary). That’s
    the whole trick with regard to using remote threads for injection purposes.
    There is an important thing we should consider, if
    implanting through CreateRemoteThread() API.
    Every time before the injector application operate on the virtual memory of
    the targeted process and makes a call to CreateRemoteThread(), it first opens the process
    using OpenProcess() API and passes PROCESS_ALL_ACCESS flag as parameter. This flag is
    used when we want to get maximum access rights to this process. In this
    scenario OpenProcess() will return NULL for some of the processes with low ID number. This error
    (although we use a valid process ID) is caused by not running under security context
    that has enough permissions. If you think for a moment about it, you
    will realize that it makes perfect sense. All those restricted processes are
    part of the operating system and a normal application shouldn’t be allowed
    to operate on them. What would happen if some application has a bug and accidentally
    attempts to terminate an operating system’s process? To prevent the
    operating system from that kind of eventual crashes, it is required that
    a given application must have sufficient privileges to execute APIs that might
    alter operating system behavior. To get access to the system resources (e.g.
    smss.exe, winlogon.exe, services.exe, etc) through OpenProcess() invocation, you must be granted the
    debug privilege. This ability is extremely powerful and offers a way to access
    the system resources, that are normally restricted. Adjusting the process
    privileges is a trivial task and can be described with the following logical

    • Open the process token with permissions needed to
      adjust privileges

    • Given a privilege’s name “SeDebugPrivilege”, we should locate its local
      LUID mapping. The privileges are specified by name and can be found in
      Platform SDK file winnt.h

    • Adjust the token in order to enable the “SeDebugPrivilege” privilege by calling AdjustTokenPrivileges() API
    • Close obtained by OpenProcessToken() process token handle

    For more details about changing privileges see [10] “Using

  4. Implanting through BHO add-ins
    Sometimes you will need to inject a custom code inside
    Internet Explorer only. Fortunately Microsoft provides an easy and well
    documented way for this purpose Browser Helper Objects. A BHO is implemented
    as COM DLL and once it is properly registered, each time when IE is launched
    it loads all COM components that have implemented IObjectWithSite interface.

  5. MS Office add-ins
    to the BHOs, if you need to implant in MS Office applications code of your
    own, you can take the advantage of provided standard mechanism by implementing
    MS Office add-ins. There are many available samples that show how to implement
    this kind of add-ins.

Interception mechanisms

Injecting a DLL into the address space of an external
process is a key element of a spying system. It provides an excellent
opportunity to have a control over process’s thread activities. However it is
not sufficient to have the DLL injected if you want to intercept API function
calls within the process.
This part of the article
intends to make a brief review of several available real-world hooking aspects.
It focuses on the basic outline for each one of them, exposing their advantages
and disadvantages.
In terms of the level where the
hook is applied, there are two mechanisms for API spying Kernel level and
User level spying. To get better understanding of these two levels you must be
aware of the relationship between the Win32 subsystem API and the Native API.
Following figure demonstrates where the different hooks are set and illustrates
the module relationships and their dependencies on Windows 2K:

Figure 2

The major implementation difference between them is that
interceptor engine for kernel-level hooking is wrapped up as a kernel-mode
driver, whereas user-level hooking usually employs user-mode DLL.

  1. NT Kernel level

    There are several methods for achieving
    hooking of NT system services in kernel mode. The most popular interception
    mechanism was originally demonstrated by Mark Russinovich and Bryce Cogswell
    in their article [3] “Windows NT System-Call Hooking”. Their basic idea is to
    inject an interception mechanism for monitoring NT system calls just bellow
    the user mode. This technique is very powerful and provides an extremely
    flexible method for hooking the point that all user-mode threads pass through
    before they are serviced by the OS kernel.
    You can
    find an excellent design and implementation in “Undocumented Windows 2000 Secrets”
    as well. In his great book Sven Schreiber explains how to build a
    kernel-level hooking framework from scratch [5].
    Another comprehensive analysis and brilliant
    implementation has been provided by Prasad Dabak in his book “Undocumented
    Windows NT” [17].
    However, all these hooking
    strategies, remain out of the scope of this article.

  2. Win32 User level hooking
    1. Windows subclassing.
      method is suitable for situations where the application’s behavior might be
      changed by new implementation of the window procedure. To accomplish this
      task you simply call SetWindowLongPtr()
      with GWLP_WNDPROC parameter and pass the
      pointer to your own window procedure. Once you have the new subclass
      procedure set up, every time when Windows dispatches a message to a
      specified window, it looks for the address of the window’s procedure
      associated with the particular window and calls your procedure instead of
      the original one.
      The drawback of this mechanism is
      that subclassing is available only within the boundaries of a specific
      process. In other words an application should not subclass a window class
      created by another process.
      Usually this approach is
      applicable when you hook an application through add-in (i.e. DLL / In-Proc
      COM component) and you can obtain the handle to the window whose procedure
      you would like to replace.
      For example, some time
      ago I wrote a simple add-in for IE (Browser Helper Object) that replaces the
      original pop-up menu provided by IE using subclassing.

    2. Proxy DLL (Trojan DLL)
      easy way for hacking API is just to replace a DLL with one that has the same
      name and exports all the symbols of the original one. This technique can be
      effortlessly implemented using function forwarders. A function forwarder
      basically is an entry in the DLL’s export section that delegates a function
      call to another DLL’s function.
      You can accomplish
      this task by simply using #pragma

      comment(linker, "/export:DoSomething=DllImpl.ActuallyDoSomething")

      However, if you decide to employ this method, you
      should take the responsibility of providing compatibilities with newer
      versions of the original library.For more details see [13a] section “Export
      forwarding” and [2] “Function Forwarders”.

    3. Code overwriting
      There are
      several methods that are based on code overwriting. One of them changes the
      address of the function used by CALL instruction. This method is difficult,
      and error prone. The basic idea beneath is to track down all CALL
      instructions in the memory and replace the addresses of the original
      function with user supplied one.
      Another method of
      code overwriting requires a more complicated implementation. Briefly, the
      concept of this approach is to locate the address of the original API
      function and to change first few bytes of this function with a JMP
      instruction that redirects the call to the custom supplied API function.
      This method is extremely tricky and involves a sequence of restoring and
      hooking operations for each individual call. It’s important to point out
      that if the function is in unhooked mode and another call is made during
      that stage, the system won’t be able to capture that second call.
      The major problem is that it contradicts with the rules
      of a multithreaded environment.
      However, there is a
      smart solution that solves some of the issues and provides a sophisticated
      way for achieving most of the goals of an API interceptor. In case you are
      interested you might peek at [12] Detours implementation.

    4. Spying by a debugger
      alternative to hooking API functions is to place a debugging breakpoint into
      the target function. However there are several drawbacks for this method.
      The major issue with this approach is that debugging exceptions suspend all
      application threads. It requires also a debugger process that will handle
      this exception. Another problem is caused by the fact that when the debugger
      terminates, the debugee is automatically shut down by Windows.

    5. Spying by altering of the Import Address Table
      This technique was originally published by Matt Pietrek
      and than elaborated by Jeffrey Ritcher ([2] “API Hooking by Manipulating a
      Module’s Import Section”) and John Robbins ([4] “Hooking Imported
      Functions”). It is very robust, simple and quite easy to implement. It also
      meets most of the requirements of a hooking framework that targets Windows
      NT/2K and 9x operating systems. The concept of this technique relies on the
      elegant structure of the Portable Executable (PE) Windows file format. To
      understand how this method works, you should be familiar with some of the
      basics behind PE file format, which is an extension of Common Object File
      Format (COFF). Matt Pietrek reveals the PE format in details in his
      wonderful articles – [6] “Peering Inside the PE.”, and [13a/b] “An In-Depth
      Look into the Win32 PE file format”. I will give you a brief overview of the
      PE specification, just enough to get the idea of hooking by manipulation of
      the Import Address Table.
      In general an PE binary
      file is organized, so that it has all code and data sections in a layout
      that conform to the virtual memory representation of an executable. PE file
      format is composed of several logical sections. Each of them maintains
      specific type of data and addresses particular needs of the OS loader.
      The section .idata, I
      would like to focus your attention on, contains information about Import
      Address Table. This part of the PE structure is particularly very crucial
      for building a spy program based on altering IAT.
      Each executable that conforms with PE format has layout
      roughly described by the figure bellow.

      Figure 3

      The program loader is responsible for loading an application along with
      all its linked DLLs into the memory. Since the address where each DLL is
      loaded into, cannot be known in advance, the loader is not able
      to determine the actual address of each imported function. The loader must perform
      some extra work to ensure that the program will call
      successfully each imported function. But going through each executable image in the memory
      and fixing up the addresses of all imported functions one by one would
      take unreasonable amount of processing time and cause huge performance degradation.
      So, how does the loader resolves this challenge? The key point is that each
      call to an imported function must be dispatched to the same address,
      where the function code resides into the memory. Each call to an imported function
      is in fact an indirect call, routed through IAT by an indirect JMP instruction.
      The benefit of this design is that the loader doesn’t have to search through
      the whole image of the file. The solution appears to be quite simple
      – it just fixes-up the addresses of all imports inside the IAT. Here is an
      example of a snapshot PE File structure of a simple Win32 Application, taken
      with the help of the [8] PEView utility. As you can see TestApp import table
      contains two imported by GDI32.DLL function TextOutA() and GetStockObject().

      Figure 4

      Actually the hooking process of an imported function is not that complex as
      it looks at first sight. In a nutshell an interception system that uses
      IAT patching has to discover the location that holds the address of imported
      function and replace it with the address of an user supplied function by
      overwriting it. An important requirement is that the newly provided function
      must have exactly the same signature as the original one. Here are the logical
      steps of a replacing cycle:

      • Locate the import section from the IAT of each
        loaded by the process DLL module as well as the process itself

      • Find the IMAGE_IMPORT_DESCRIPTOR chunk of the DLL that
        exports that function. Practically speaking, usually we search this entry
        by the name of the DLL

      • Locate the IMAGE_THUNK_DATA which holds the original
        address of the imported function

      • Replace the function address with the user
        supplied one

      By changing the address of the
      imported function inside the IAT, we ensure that all calls to the hooked
      function will be re-routed to the function interceptor.
      Replacing the pointer inside the IAT
      is that .idata section doesn’t necessarily
      have to be a writable section. This requires that we must ensure that .idata section can be modified. This task can be
      accomplished by using VirtualProtect()
      Another issue that deserves attention is related
      to the GetProcAddress() API behavior on
      Windows 9x system. When an application calls this API outside the debugger
      it returns a pointer to the function. However if you call this function within
      from the debugger it actually returns different address than it would
      when the call is made outside the debugger. It is caused by the fact that that
      inside the debugger each call to GetProcAddress() returns a wrapper to the real
      pointer. Returned by GetProcAddress()
      value points to PUSH instruction followed by
      the actual address. This means that on Windows 9x when we loop through the
      thunks, we must check whether the address of examined function is a PUSH instruction (0x68 on x86 platforms) and
      accordingly get the proper value of the address function.
      Windows 9x doesn’t implement copy-on-write, thus
      operating system attempts to keep away the debuggers from stepping into
      functions above the 2-GB frontier. That is the reason why GetProcAddress() returns a debug thunk instead
      of the actual address. John Robbins discusses this problem in [4] “Hooking
      Imported Functions”.

Figuring out when to inject the
hook DLL

That section reveals some challenges that
are faced by developers when the selected injection mechanism is not part
of the operating system’s functionality. For example, performing the injection is not your concern
when you use built-in Windows Hooks in order to implant a DLL. It is an
OS’s responsibility to force each of those running processes that meet the
requirements for this particular hook, to load the DLL [18]. In fact Windows keeps track of all
newly launched processes and forces them to load the hook DLL. Managing
injection through registry is quite similar to Windows Hooks. The biggest advantage of
all those “built-in” methods is that they come as part of the OS.
Unlike the discussed above implanting techniques, to inject
by CreateRemoteThread()
requires maintenance of all currently running processes. If the injecting is made not on
time, this can cause the Hook System to miss some of the calls it claims as intercepted.
It is crucial that the Hook Server application implements a smart mechanism for
receiving notifications each time when a new process starts or shuts down.
One of the suggested methods in this case, is to intercept CreateProcess() API family functions and monitor all
their invocations. Thus when an user supplied function is called, it can call
the original CreateProcess() with dwCreationFlags OR-ed with CREATE_SUSPENDED flag. This means that the primary
thread of the targeted application will be in suspended state, and the Hook
Server will have the opportunity to inject the DLL by hand-coded machine
instructions and resume the application using ResumeThread() API. For more
details you might refer to [2] “Injecting Code with CreateProcess()".
The second method
of detecting process execution, is based on implementing a simple device driver.
It offers the greatest flexibility and deserves even more attention. Windows
NT/2K provides a special function PsSetCreateProcessNotifyRoutine() exported by
NTOSKRNL. This function allows adding a callback function, that is called
whenever a process is created or deleted. For more details see [11] and [15]
from the reference section.

Enumerating processes and

Sometimes we would prefer to use injecting of the DLL by
CreateRemoteThread() API, especially when
the system runs under NT/2K. In this case when the Hook Server is started it must
enumerate all active processes and inject the DLL into their address spaces.
Windows 9x and Windows 2K provide a built-in implementation (i.e. implemented by
Kernel32.dll) of Tool Help Library. On the other hand Windows NT uses for the
same purpose PSAPI library. We need a way to allow the Hook Server to run and
then to detect dynamically which process “helper” is available. Thus the system
can determine which the supported library is, and accordingly to use the
appropriate APIs.
I will present an object-oriented
architecture that implements a simple framework for retrieving processes and
modules under NT/2K and 9x [16]. The design of my classes allows extending the
framework according to your specific needs. The implementation itself is pretty
CTaskManager implements the system’s processor. It
is responsible for creating an instance of a specific library handler (i.e.
CPsapiHandler or CToolhelpHandler) that is able to employ the correct
process information provider library (i.e. PSAPI or ToolHelp32 respectively).
CTaskManager is in charge of creating and
marinating a container object that keeps a list with all currently active
processes. After instantiating of the CTaskManager object the application calls Populate() method. It forces enumerating of all
processes and DLL libraries and storing them into a hierarchy kept by CTaskManager‘s member m_pProcesses.
Following UML
diagram shows the class relationships of this subsystem:

Figure 5

It is important to highlight the fact that NT’s
Kernel32.dll doesn’t implement any of the ToolHelp32 functions. Therefore we
must link them explicitly, using runtime dynamic linking. If we use static
linking the code will fail to load on NT, regardless whether or not the
application has attempted to execute any of those functions. For more details
see my article “Single
interface for enumerating processes and modules under NT and Win9x/2K.”

Requirements of the Hook Tool

Now that I’ve made a brief introduction to the various concepts of the
hooking process it’s time to determine the basic requirements and explore the design of a
particular hooking system. These are some of the issues addressed by the Hook Tool System:

  • Provide a user-level hooking system for spying any
    Win32 API functions imported by name

  • Provide the abilities to inject hook driver into all
    running processes by Windows hooks as well as CreateRemoteThread() API. The framework should
    offer an ability to set this up by an INI file

  • Employ an interception mechanism based on the
    altering Import Address Table

  • Present an object-oriented reusable and extensible
    layered architecture

  • Offer an efficient and scalable mechanism for hooking
    API functions

  • Meet performance requirements
  • Provide a reliable communication mechanism for
    transferring data between the driver and the server

  • Implement custom supplied versions of TextOutA/W() and ExitProcess() API functions
  • Log events to a file
  • The system is implemented for x86 machines running
    Windows 9x, Me, NT or Windows 2K operating system

Design and

This part of the article discusses the key components of the
framework and how do they interact each other. This outfit is capable to capture
any kind of WINAPI imported by name
Before I outline the
system’s design, I would like to focus your attention on several methods for injecting
and hooking.
First and foremost, it is necessary to select
an implanting method that will meet the requirements for injecting the DLL
driver into all processes. So I designed an abstract approach with two
injecting techniques, each of them applied accordingly to the settings in
the INI file and the type of the operating system (i.e. NT/2K or 9x). They are
– System-wide Windows Hooks and CreateRemoteThread()

method. The sample framework offers the ability to inject the DLL on
NT/2K by Windows Hooks as well as to implant by CreateRemoteThread() means. This can be determined
by an option in the INI file that holds all settings of the system.
Another crucial moment is the
choice of the hooking mechanism. Not surprisingly, I decided to apply altering IAT as
an extremely robust method for Win32 API spying.
achieve desired goals I designed a simple framework composed of the following
components and files:

  • TestApp.exe a simple Win32 test application that
    just outputs a text using TextOut() API. The purpose of this app is to show
    how it gets hooked up.

  • HookSrv.exe – control program
  • HookTool .DLL – spy library implemented as Win32 DLL
  • HookTool.ini a configuration file
  • NTProcDrv.sys – a tiny Windows NT/2K kernel-mode
    driver for monitoring process creation and termination. This component is
    optional and addresses the problem with detection of process execution under
    NT based systems only.

HookSrv is a simple control program. Its main role is
to load the HookTool.DLL and then to activate the spying engine. After loading
the DLL, the Hook Server calls InstallHook()
function and passes a handle to a hidden windows where the DLL should post all
messages to.
HookTool.DLL is the hook driver and
the heart of presented spying system. It implements the actual interceptor
and provides three user supplied functions TextOutA/W() and ExitProcess() functions.
Although the article emphasizes on Windows internals and there
is no need for it to be object-oriented, I decided to encapsulate related
activities in reusable C++ classes. This approach provides more flexibility and
enables the system to be extended. It also benefits developers with the ability
to use individual classes outside this project.
Following UML class diagram illustrates the relationships
between set of classes used in HookTool.DLL’s implementation.

Figure 6

In this section of the article I would like to draw your
attention to the class design of the HookTool.DLL. Assigning responsibilities to
the classes is an important part of the development process. Each of the
presented classes wraps up a specific functionality and represents a particular
logical entity.
CModuleScope is the main doorway of the system. It
is implemented using Singleton pattern and works in a thread-safe manner. Its
constructor accepts 3 pointers to the data declared in the shared segment, that
will be used by all processes. By this means the values of those system-wide
variables can be maintained very easily inside the class, keeping the rule for
When an application loads the HookTool
library, the DLL creates one instance of CModuleScope on receiving DLL_PROCESS_ATTACH notification. This step just initializes the only instance of CModuleScope. An important piece of the CModuleScope object construction is the creation of an appropriate injector object. The decision which injector to use will be made after parsing the HookTool.ini file and determining the value of UseWindowsHook parameter under [Scope] section. In case that the system is running under Windows 9x, the value of this parameter won’t be examined by the system, because Window 9x doesn’t support injecting by remote threads.
After instantiating of the main processor object, a call to ManageModuleEnlistment() method will be made.
Here is a simplified version of its implementation:

// Called on DLL_PROCESS_ATTACH DLL notification
BOOL CModuleScope::ManageModuleEnlistment()
  BOOL bResult = FALSE;
  // Check if it is the hook server 
  if (FALSE == *m_pbHookInstalled)
    // Set the flag, thus we will know that the server
    // has been installed
    *m_pbHookInstalled = TRUE;
    // and return success error code
    bResult = TRUE;
  // and any other process should be examined whether
  // it should be hooked up by the DLL
    bResult = m_pInjector->IsProcessForHooking(m_szProcessName);
    if (bResult)
  return bResult;

The implementation of the method ManageModuleEnlistment() is straightforward and examines whether the call has been made by the Hook Server, inspecting the value m_pbHookInstalled points to. If an invocation has been initiated by the Hook Server, it just sets up indirectly the flag sg_bHookInstalled to TRUE. It tells that the Hook Server has been started.
The next action taken by the Hook Server is to activate the engine through a single call to InstallHook() DLL exported function. Actually its call is delegated to a method of CModuleScope InstallHookMethod(). The main purpose of this function is to force targeted for hooking processes to load or unload the HookTool.DLL.

// Activate/Deactivate hooking engine
BOOL CModuleScope::InstallHookMethod( BOOL bActivate,
                                      HWND hWndServer)
  BOOL bResult;
  if (bActivate)
    *m_phwndServer = hWndServer;
    bResult = m_pInjector->InjectModuleIntoAllProcesses();
    *m_phwndServer = NULL;
    bResult = TRUE;
  return bResult;

HookTool.DLL provides two mechanisms for self injecting into the address space of an external process one that uses Windows Hooks and another that employs injecting of DLL by CreateRemoteThread() API. The architecture of the system defines an abstract class CInjector that exposes pure virtual functions for injecting and ejecting DLL. The classes CWinHookInjector and CRemThreadInjector inherit from the same base – CInjector class. However they provide different realization of the pure virtual methods InjectModuleIntoAllProcesses() and EjectModuleFromAllProcesses(), defined in CInjector interface.
CWinHookInjector class implements Windows Hooks injecting mechanism. It installs a filter function by the following call

// Inject the DLL into all running processes
BOOL CWinHookInjector::InjectModuleIntoAllProcesses()
  *sm_pHook = ::SetWindowsHookEx(WH_GETMESSAGE,
                                 0 );
  return (NULL != *sm_pHook);

As you can see it makes a request to the system for registering WH_GETMESSAGE hook. The server executes this method only once. The last parameter of SetWindowsHookEx() is 0, because GetMsgProc() is designed to operate as a system-wide hook. The callback function will be invoked by the system each time when a window is about to process a particular message. It is interesting that we have to provide a nearly dummy implementation of the GetMsgProc() callback, since we don’t intend to monitor the message processing. We supply this implementation only in order to get free injection mechanism provided by the operating system.

After making the call to SetWindowsHookEx(), OS checks whether the DLL (i.e. HookTool.DLL) that exports GetMsgProc() has been already mapped in all GUI processes. If the DLL hasn’t been loaded yet, Windows forces those GUI processes to map it. An interesting fact is, that a system-wide hook DLL should not return FALSE in its DllMain(). That’s because the operating system validates DllMain()‘s return value and keeps trying to load this DLL until its DllMain() finally returns TRUE.

A quite different approach is demonstrated by the CRemThreadInjector class. Here the implementation is based on injecting the DLL using remote threads. CRemThreadInjector extends the maintenance of the Windows processes by providing means for receiving notifications of process creation and termination. It holds an instance of CNtInjectorThread class that observes the process execution. CNtInjectorThread object takes care for getting notifications from the kernel-mode driver. Thus each time when a process is created a call to CNtInjectorThread ::OnCreateProcess() is issued, accordingly when the process exits CNtInjectorThread ::OnTerminateProcess() is automatically called. Unlike the Windows Hooks, the method that relies on remote thread, requires manual injection each time when a new process is created. Monitoring process activities will provide us with a simple technique for alerting when a new process starts.

CNtDriverController class implements a wrapper around API functions for administering services and drivers. It is designed to handle the loading and unloading of the kernel-mode driver NTProcDrv.sys. Its implementation will be discussed later.

After a successful injection of HookTool.DLL into a particular process, a call to ManageModuleEnlistment() method is issued inside the DllMain(). Recall the method’s implementation that I described earlier. It examines the shared variable sg_bHookInstalled through the CModuleScope‘s member m_pbHookInstalled. Since the server’s initialization had already set the value of sg_bHookInstalled to TRUE, the system checks whether this application must be hooked up and if so, it actually activates the spy engine for this particular process.

Turning the hacking engine on, takes place in the CModuleScope::InitializeHookManagement()‘s implementation. The idea of this method is to install hooks for some vital functions as LoadLibrary() API family as well as GetProcAddress(). By this means we can monitor loading of DLLs after the initialization process. Each time when a new DLL is about to be mapped it is necessary to fix-up its import table, thus we ensure that the system won’t miss any call to the captured function.

At the end of the InitializeHookManagement() method we provide initializations for the function we actually want to spy on.

Since the sample code demonstrates capturing of more than one user supplied functions, we must provide a single implementation for each individual hooked function. This means that using this approach you cannot just change the addresses inside IAT of the different imported functions to point to a single generic interception function. The spying function needs to know which function this call comes to. It is also crucial that the signature of the interception routine must be exactly the same as the original WINAPI function prototype, otherwise the stack will be corrupted. For example CModuleScope implements three static methods MyTextOutA(),MyTextOutW() and MyExitProcess(). Once the HookTool.DLL is loaded into the address space of a process and the spying engine is activated, each time when a call to the original TextOutA() is issued, CModuleScope:: MyTextOutA() gets called instead.

Proposed design of the spying engine itself is quite efficient and offers great flexibility. However, it is suitable mostly for scenarios where the set of functions for interception is known in advance and their number is limited.

If you want to add new hooks to the system you simply declare and implement the interception function as I did with MyTextOutA/W() and MyExitProcess(). Then you have to register it in the way shown by InitializeHookManagement() implementation.

Intercepting and tracing process execution is a very useful mechanism for implementing systems that require manipulations of external processes. Notifying interested parties upon starting of a new processes is a classic problem of developing process monitoring systems and system-wide hooks. The Win32 API provides a set of great libraries (PSAPI and ToolHelp [16]) that allow you to enumerate processes currently running in the system. Although these APIs are extremely powerful they don’t permit you to get notifications when a new process starts or ends up. Luckily, NT/2K provides a set of APIs, documented in Windows DDK documentation as “Process Structure Routines” exported by NTOSKRNL. One of these APIs PsSetCreateProcessNotifyRoutine() offers the ability
to register system-wide callback function which is called by OS each time when a new process starts, exits or has been terminated. The mentioned API can be employed as a simple way to for tracking down processes simply by implementing a NT kernel-mode driver and a user mode Win32 control application. The role of the driver is to detect process execution and notify the control program about these events. The implementation of the Windows process’s observer NTProcDrv provides a minimal set of functionalities required for process monitoring under NT based systems. For more details see articles [11] and [15]. The code of the driver can be located in NTProcDrv.c file. Since the user mode implementation installs and uninstalls the driver dynamically the currently logged-on user must have administrator privileges. Otherwise you won’t be able to install the driver and it will disturb the process of monitoring. A way around is to manually install the driver as an administrator or run HookSrv.exe using offered by Windows 2K Run as different user” option.  

Last but not least, the provided tools can be administered by simply changing the settings of an INI file (i.e. HookTool.ini). This file determines whether to use Windows hooks (for 9x and NT/2K) or CreateRemoteThread() (only under NT/2K) for
injecting. It also offers a way to specify which process must be hooked up and
which shouldn’t be intercepted. If you would like to monitor the process there
is an option (Enabled) under section [Trace] that allows to log system
activities. This option allows you to report rich error information using the
methods exposed by CLogFile class. In fact ClogFile provides thread-safe
implementation and you don’t have to take care about synchronization issues
related to accessing shared system resources (i.e. the log file). For more
details see CLogFile and content of HookTool.ini file.

Sample code

The project compiles with VC6++ SP4 and requires Platform SDK. In a production Windows NT environment you need to provide PSAPI.DLL in order to use provided CTaskManager implementation. Before you run the sample code make sure that all the settings in HookTool.ini file have been set according to your specific needs.

For those that will like the lower-level stuff and are interested in further development of the kernel-mode driver NTProcDrv code, they must install Windows DDK.

Out of the scope

For the sake of simplicity these are some of the subjects I intentionally left out of the scope of this article:

  • Monitoring Native API calls
  • A driver for monitoring process execution on Windows
    9x systems.

  • UNICODE support, although you can still hook UNICODE imported APIs


This article by far doesn’t provide a complete guide for the unlimited API hooking subject and without any doubt it misses some details. However I tried to fit in this few pages just enough important information that might help those who are interested in user mode Win32 API spying.


“Windows 95 System Programming Secrets”, Matt Pietrek
[2] “Programming Application for
MS Windows”
, Jeffrey Richter

[3] “Windows NT System-Call
, Mark Russinovich and Bryce Cogswell, Dr.Dobb’s Journal January 1997
[4] “Debugging applications , John Robbins
[5] “Undocumented Windows 2000
, Sven Schreiber

[6] “Peering Inside the PE: A Tour of the Win32 Portable Executable File Format” by Matt Pietrek, March 1994
[7] MSDN Knowledge base Q197571
PEview Version 0.67
, Wayne J. Radburn
[9] “Load Your 32-bit DLL into Another Process’s Address Space Using INJLIB” MSJ May 1994
[10] “Programming Windows
, Keith Brown
[11] Detecting Windows NT/2K
process execution
Ivo Ivanov, 2002

[12] Detours” Galen Hunt and Doug Brubacher

[13a] “An In-Depth
Look into the Win32 PE file format”
, part 1, Matt Pietrek, MSJ February 2002
[13b] “An In-Depth
Look into the Win32 PE file format”
, part 2, Matt Pietrek, MSJ March 2002
[14] “Inside MS Windows 2000 Third
, David Solomon and Mark Russinovich
[15] “Nerditorium”,
James Finnegan, MSJ January 1999

[16] Single interface for
enumerating processes and modules under NT and Win9x/2K.

, Ivo Ivanov, 2001
[17] “Undocumented Windows NT”
, Prasad Dabak, Sandeep Phadke and Milind Borate
[18] Platform SDK: Windows User Interface, Hooks


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