Read
this.
CSRF
(wikipedia), a cross-site request forgery, is one of the most recent and
under-publicized attacks on the web. It boils down to a user browsing to a web site,
downloading a web page, and unknowingly, submitting requests to other web servers,
usually with malicious intent. The example linked above demonstrates a hack where a
specially crafted URL is formed that actually changes the settings on a common DSL
router used in Mexico (by relying on the fact the user name/password combination is
well known and unlikely to have changed).
If you allow user input in a web application our on a web site (even as part of a
forum or blog comments), you could be unwittingly providing hackers a friendly spot
in which they can inject these well-formed, but malicious URLs to be hosted.
Unfortunately, they’re very difficult to track down.
I moderate all comments on my web site so that no unwanted links become published.
But, if you write software — consider all avenues of data entry, etc. Trust NO
INPUT. Don’t trust the data that is in your database. Assume that it has been
compromised. Assume nothing.