I am using C++ with Qt and I am struggling to find the way to achieve something I never did before. Here is what I want to achieve :
I have a client (let's call it Client.exe) which I don't have access to the source and a launcher (let's call it... Launcher.exe) which I have access to the source.
Cient.exe needs a password and a username, supposed to come from Launcher.exe.
If I had only one couple password/username, I know I could make a .dll and inject it, but since I can have a lot of combinaisons, that is impossible.
So here is my question, what is the way to make a link allowing me to send password and username from Launcher.exe to Client.exe ?
Second question would be : is there a way to use VirtualProtect and this kind of stuff (in order to modify some instructions in memory), with an executable, meaning without any injection ? (I guess the answer is no, but I want to be sure)
Your "Launcher.exe" and your DLL injected into "Client.exe" can communicate with each other via interprocess communication, for example through file mapping. This could be used for "Launcher.exe" to pass any desired username and password to "Client.exe".
However, the main problem I see is how to get "Client.exe" to use this data, if you do not have access to the source code and if it also does not provide an API for this.
If you want to trick "Client.exe" into using the data provided by you (or by your injected DLL) instead of the intended data, then you must reverse engineer the program and change the appropriate instructions so that they load your data instead of the original data. Since you do not have access to the C/C++ source code, you will have to understand the assembly language instructions to accomplish this.
In order to find out which instructions to change, you will likely need a debugging tool such as x64dbg, which is designed to debug applications that you haven't written yourself (and have no source code for) and possibly also a static analysis tool, such as IDA or Ghidra. Furthermore, if the program deliberately protects itself from reverse-engineering, you will have to learn how to overcome this (which can be very hard).
You could also accomplish this without injecting a DLL, by using WriteProcessMemory. You may need to also use VirtualAllocEx if you need extra memory inside the target process, for example for injecting instructions or data.
In any case, before tampering with another process's instructions or data, it may be advisable to suspend all of its threads using SuspendThread, and then resume all threads afterwards with ResumeThread. Otherwise, the program may run while its instructions or data are in an inconsistent state, which may cause the program to crash.