Can I hook functions in linked libraries? - windows

Question

Using EasyHook I have successfully hooked both exported functions and known vtable functions for various C++ classes. In all these cases target programs have used DLLs.
Provided I know the address of a function's entry point, is it possible to do the same when a library has been linked into the target program as opposed to being a separate library?

Answer 1

It appears with EasyHook you can hook any subroutine whose address is calculable.
In my case hooking static linked SSL_read and SSL_write in OpenSSL was as simple as identifying the offsets with my favourite debugger and then installing the hooks.
// delegate for EasyHook:
[UnmanagedFunctionPointer(CallingConvention.Cdecl,
SetLastError = true, CharSet = CharSet.Ansi)]
delegate Int32 SLL_readDelegate(IntPtr SSL_ptr, IntPtr buffer, Int32 length);
// import SSL_read (I actually did it manually, but this will work in most cases)
/* proto from ssl_lib.c -> int SSL_read(SSL *s,void *buf,int num) */
[DllImport("ssleay32.dll", SetLastError = true)]
public static extern Int32 SSL_read(IntPtr ssl, IntPtr buffer, Int32 len);
// the new routine
static Int32 SSL_readCallback(IntPtr SSL_ptr, IntPtr buffer, Int32 length)
{
/* call the imported SSL_read */
int ret = SSL_read(SSL_ptr, buffer, length);
/* TODO: your code here, e.g:
* string log_me = Marshal.PtrToString(buffer, ret);
*/
return ret;
}
Now all that's left is to install the hook:
private LocalHook sslReadHook;
public void Run(RemoteHooking.IContext InContext, String InArg1)
{
// ... initialization code omitted for brevity
/* the value for ssl_read_addr is made up in this example
* you'll need to study your target and how it's loaded(?) to
* identify the addresses you want to hook
*/
int ssl_read_addr = 0x12345678; /* made up for examples sake */
sslReadHook = LocalHook.Create(new IntPtr(ssl_read_addr),
new SSL_readDelegate(SSL_readCallback), this);
// ...
}
I should mention that in this example you'll need libeay32.dll and ssleay32.dll as the latter depends on the former.
Happy hooking!