Check your URL’s, make sure they are not open for hackers to abuse. A post on Google’s blog, Open redirect URLs: Is your site being abused?, talks about how hackers can use open redirect URLs to redirect links to their spammy or phishing pages. In case of a lawsuit the owner of the site can be responsible for the losses.

Some sites use URLs like the following to either redirect the page or call for a download file:

domain.com/go.php?url=
domain.com/some_program/download/?url=

If these URLs get discovered by spammers, they can easily redirect them to any page of their own. This will allow them to use your real, trustworthy URL as the starting point. They can easily insert your URL into an email they send or anywhere else, which will make the user think they are going to the legitimate website, but won’t realize how they are quietly redirected to a spam site.

If you have to use a redirect like this, try to use variables that are not as obvious as “url”, “redirect” or “go”. I really recommend to stay away from this sort of redirects though, there are pleanty of other ways to handle these situations.