I thought about something I found ages ago in PHP4 and it's been long enough now. This is also quite funny because my server is vulnerable to this. So what happens if you escape PHP_SELF with htmlentities($_SERVER['PHP_SELF'], ENT_QUOTES)? Safe from XSS? I hope so. Safe from everything? Well not really or at least it didn't used to be.

He gives a simple example of how the PHP_SELF issue can be used to change the form's target just by using a few well-placed slashes. Thankfully, this seems to be only back in the world of PHP4, so those working with PHP5 should be safe.

One of the great benefits of PHP is its ease of access to new-comers. Its entry level is minimal and so attracts those looking for simple scripts to their sites. It is this same ease of access that becomes a problem as the new-comers begin to deal with input from users. Failure to adequately validate and sanitize data is the leading cause of security problems when dealing with PHP.

He looks at a few different areas that developers need to focus on (and be sure to filter on) like PHP_SELF, protection from email header injections, file inclusion and the use of error reporting to make handling user-generated errors "more correct".

I thought it might be a good idea to gather a few test cases demonstrating the problem. Why PHP allows these URL's is beyond me and it wouldn't take much work to filter out these malicious URL's in the PHP code.

He provides four test cases to show how simple it is to abuse - one using a HTTP header, another pushing XSS through, the third mentions search pages and the fourth a direct code injection.

You can download the code here.