What's the best way to secure dll files [closed]

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We have a large application build in C++ builder, that at startup looks at a folder and loads all the present dll files. I figured this might not be such a good thing and tried my thoughts by creating a dll that only fired up a web browser and opened a picture with the word owned :P

Anyway, it worked. So the system could be compromised in any way by a dll being switched out or a new one added. Remote admin access, format drives, you name it.

So my thoughts went toward trying to plug this hole. What's the best practice for this? I was thinking of some kind of signing of the dll's. Having the .exe holding a list of names of valid dll's doesn't actually solve it since a dll could be replaces. A list of names and checksums would work, but would be nice to keep the auto-loading feature but only load correctly signed dll's.

Edit: I'm looking into using signtool.exe which seems to do the job. But I can't find any good reference as how to actually check the signature of the dll in C++ afterwards. Anyone who has a link?

Answer 1

It sounds like you are vulnerable to the dll hijacking exploit which affected basically everything written for Windows (and yet another example of how Microsoft couldn't secure a brick.). Microsoft has recommendations for mitigating this flaw.

Answer 2

You should protect the folder containing the DLLs so that only those with Administrative access can write to it.

Beyond that, anyone who can write to such a folder can do just about anything else they want to, and there's not much you can do about it.