[stevekemp]

I can give you an example of somethign that is relatively common and "bad".

Imagine a website that will take an URL and extract the text from it - removing markup - or another service that will scan a URL for meta-tags, etc. All the kind of things that you can find easily.

Now imagine what happens if a user passes input such as "file:////etc/passwd". There are a whole bunch of services which will spit out the contents of the file, because they use some URL-fetching library and don't limit the protocols to http or https.

I wrote a blog-post about that, which was featured here a while back:

https://news.ycombinator.com/item?id=12478538