API Hooking Methods 


Sometimes you need to hook a specific function or API. Unfortunately there's no official way to do such things, instead you'll find a lot of different hacks that were invented by a lot of different people. All those API hooking hacks have advantages and disadvantages. There's no perfect solution.

When analysing API hooking methods you should ask the following questions. Each question does have a sense, of course. The questions are meant to make existing differences between all the available API hooking methods more evident:

1. Does the method need to manipulate the to-be-hooked binary files on the harddisk or is it capable to do it's work by only patching memory? The latter solution should be prefered in most cases.
2. Can the method install a hook on functions, which are not exported? Sometimes it is useful to hook some Delphi/C++ RTL functions or to hook non-virtual methods or similar stuff.
3. Can the method safely install a hook on each and every API, regardless of how the binary code of the API is structured? Some API hooking methods get unstable, if the binary code of the to-be-hooked API is shorter than 5 or 6 bytes or if it is structured strangely.
4. If hooking a specific API would end up in instability, does the hooking method automatically detect this and does it consequently react to the situation by either refusing to install the hook or even by automatically switching to an appropriate alternative hooking method?
5. Once installed, does the hook catch those API calls, which were statically linked before the hook was installed?
6. Once installed, does the hook catch those API calls, which were statically linked after the hook was installed?
7. Once installed, does the hook catch those API calls, which were dynamically linked before the hook was installed?
8. Once installed, does the hook catch those API calls, which were dynamically linked after the hook was installed?
9. Can the method properly undo an API hook?
10. Can the original API be called directly without having to temporarily unhook the API?
11. Is a real hook chain built automatically? In other words: What happens if you hook an API twice and then unhook in the wrong order? If the hook method doesn't build a real hook queue, you'll leave the hooked API in a wild state, which will result in a crash as soon as the API is called the next time.
12. Is it possible to safely unload a hook DLL? If a hook DLL uninstalls its API hooks when being unloaded from a process, some hook callback functions might still be in use. In that case the process will later crash.

Generally I prefer solutions which doesn't touch the standard/system DLLs on harddisk, so I'll only talk about methods here, which work purely in memory. Here comes a description of the most important hooking methods:

Import Table Patching

The best known and most often used method is "Import Table Patching". Each win32 module (application/DLL) has a so-called "import table", which is basically a list of all APIs, which this module calls. Patching this import table is a quite easy job and works very nicely. Unfortunately only the statically linked API calls go through the import table. Dynamically linked API calls (GetProcAddress) are not caught by this method at all. That means: It's absolutely possible, even quite probable that you'll miss some API calls when using this method.

To reduce the danger of missing API calls, you should not only patch the import table of the application's module, but you should also patch the import tables of each and every DLL that is loaded by the application.

Extended Import Table Patching

There are some ways to make "Import Table Patching" catch more APIs. You can hook the API "LoadLibrary" to be notified about when new DLLs are loaded. Then you can right away patch those DLLs, too. If you don't do that, you won't catch API calls that are made from DLLs, which are loaded after you did install the hook.

Another extension is to hook the API "GetProcAddress" and then when called not return the real API address, but the address of your callback function instead. This way you would also get those API calls, that are dynamically linked after you've installed your hook. Unfortunately if you do this, you can't unhook such calls, anymore.

Export Table Patching

Each win32 module has an "export table", which lists all functions that this module exports. Patching this export table is not very difficult and brings some interesting results: All new linkings (both static and dynamic) to the patched API will automatically be bound to your callback function instead of to the original function. That's very nice! Well, but unfortunately patching an export table has absolutely NO effect on already existing API linkings (e.g. static bindings of already loaded DLLs). So using export table patching alone is only of quite restricted use. Besides, undoing the patch has again no effect on past bindings. So you can't really undo an API hook which was established by export table patching.

You should not patch the export table of DLLs which are loaded in the shared area of win9x. Also you should not patch the export table of DLLs, which are not loaded there, but have a shared export table nevertheless. If you do that, you risk system stability again.

Simple Code Overwriting

Instead of changing any tables, you can also directly manipulate the API's binary code in memory. The most often used method is to overwrite the first 5 bytes of the API code with a JMP instruction, which then jumps to your callback function.

This method is quite effective. It catches really 100% of all API calls. But it has also some big disadvantages. The biggest is that you can't call the original API in your callback function anymore, because when you would do that, the JMP instruction would again jump to your callback function, which would result in an endless loop.

Of course you could temporarily undo the hook in order to first restore and then call the original API, but that's a bad idea because of two reasons: (1) Steadily hooking and unhooking consumes precious time. (2) While you've temporarily unhooked the API, you're in big danger to miss a lot of API calls.

Finally the code of the to-be-hooked API has to be at least 5 bytes long. Otherwise you would overwrite more than the original API code, which would most probably result in a crash sooner or later. Furthermore there must not be any jumps later in the API code, which jump into the bytes 2-5 of the API code. In such a case overwriting the first 5 bytes would also probably result in a crash sooner or later.

Extended Code Overwriting

The biggest disadvantage of the "Simple Code Overwriting" method was that you can't call the original API, at least as long as the API is hooked. Now clever people thought about it and found a way to erase this problem completely.

The solution sounds simple: We need to overwrite 5 bytes of the API's code. So we simply copy these bytes to another location and call it there, whenever we want to call the original API. Unfortunately it's not as easy as it sounds. You need to know that every binary code consists of single instructions. Each instruction can have a different length. So if we copy 5 bytes from the API, that can be exactly one instruction, or one and a half, or two and a half or ... We don't know that for sure. But we know for sure that when executing half copied instructions we will make our program crash. That means we need a real little disassembler, which can tell us how many bytes we really have to copy, so that we copy complete instructions instead of half instructions.

The "Extended Code Overwriting" method is quite difficult to implement, but once you have it ready, it is a very good method. The only disadvantage is that the to-be-hooked API's code may be structured in a way, which doesn't allow you to use the "Code Overwriting" method without risking crashes.

Madshi's Code Overwriting

The method used by my "madCodeHook" package is a polished version of the "Extended Code Overwriting" method. When multiple madCodeHook hook dlls try to hook the same API, madCodeHook builds up an automatic hook queue, which allows you to unhook the APIs in any order without getting memory leaks or stability issues. Furthermore, DLL uninjection is automatically delayed, as long as any hook callback functions are still "in use". The only remaining problem now is that the code of some APIs is too short (or strangely structured) to be hookable by any code overwriting method.

madCodeHook's disassembler examines the whole to-be-hooked API before hooking it and then decides whether code overwriting can be safely used. If not, madCodeHook automatically switches to:

Madshi's Mixture Mode

Now what can we do with APIs, whose code is too short to be hooked by code overwriting? Well, we can simply enlarge the code! That is quite easily done in 2 steps: (1) We build an API header, which does nothing but jump to the original API. The header itself is long enough to be hooked by code overwriting. (2) Now we patch the export table and all import tables so that everything points to our newly allocated API header. From now on it looks as if the original API has always begun at our API header. And it will remain this way until the process exits. The result of the whole operation is simply, that now such a manipulated API can be hooked by code overwriting.

Because this solution combines import table patching, export table patching and code overwriting, I've called it the "Mixture Mode". It has one disadvantage: API calls that were linked dynamically before the API was hooked the first time, will not be caught, because they still jump directly to the original API instead of to our enlarged API header.


Now let's compare the best known API hooking methods/packages in a table. Each column is one API hooking method/package. Each row answers one of the 14 questions asked on the beginning of this page:

Ext. Code
1. only patches memory
2. hook non exported funcs
3. hook every API
4. detect non hookable APIs
5. catch old static links
6. catch new static links
7. catch old dynamic links
8. catch new dynamic links
9. can call original API
10. proper unhook possible
11. automatic hook queue
12. safe DLL uninjecting