Well it depends severely on what the form is, whether it is a POST login form or if it is using XML files to determine it's login information. SQL attacks or the like, myysql_real_escape_string, a hundred other bits of information.
What restrictions did you put on the file?


start googling for "securty tips" just get used to validating input and using your langs built in functions to block inject etc also if you have root you can look at things like mod_security and IDS etc ..
