Check if path is child of directory

[Jeroen Bollen]

On Sunday, 9 February 2014 at 21:02:59 UTC, Jeroen Bollen wrote:
> I'm building a webserver using the Vibe.d library. Whenever the 
> user requests a page inside my /images/ folder; I want them to 
> output this file.
>
> Because there will be a lot of images present, and because 
> these are likely to change in the future, I would like to just 
> get the URL from the request, and automatically output the file.
>
> I am aware though, that users could perform tricks like 
> "images/../../../../sensitive_file_here". In order to prevent 
> that I would like a solid way of making sure the entered path 
> is actually inside the images directory.
>
> How do I do this?

I just figured out vibe.d handles this automatically, but I'd 
still like to know of a secure way to do this, for future 
reference.