I'm working on a site where users can input URLs.
The URLs should point to sites where you can buy an item (for birthday/special event/...)
It also sends out mails to e-mail addresses the user provides.
Now, I would like to assess how unsafe this is and to mitigate the unsafeness if possible. (It's a hobby project, if it's too unsafe I'll abandon it).
I'm mostly concerned with the URLs.
I can imagine that it isn't impossible to find someplace in an online store where a redirect is issued (and where an attacker could inject a custom URL).