Question

I’m putting together a site that will make itself available for user input. I was wondering if writing a function like:

if(getenv("HTTP_REFERER") != 'http://www.myURL.com/submitArea'){
        die('don\'t be an jerk, ruin your own site');   
    }else{
        // continue with form processing    
    }

is enough to prevent cross site form submissions.

EDIT: And if not, what is the best practice for preventing forms from being submitted from other hosts?

Answer 1

Actually yes, according to the OWASP CSRF Prevention Cheat Sheet in most cases checking the referer is enough to patch a CSRF vulnerability. Although it is trivial to spoof the referer on your OWN BROWSER it is impossible to spoof it on another browser (via CSRF) because it breaks the rules.

In fact checking the referer is very common to see on embedded network hardware where Memory is scarce. Motorola does this for their Surfboard Cable Modems. I know this first hand, because I hacked them with csrf and then they patched it using a referer check. This vulnerability received a severity metric of 13.5 and according to the Department of Homeland Security this is the most dangerous CSRF vulnerability ever discovered and in the top 1,000 most dangerous software flaws of all time.