On the Smashing Magazine site today Phillip Tellis has a new article advocating a common thread in PHP application development (really, any application development) - sanitizing user input to help keep you and your application's user safe.

In this article, I'm joined by my colleague Peter (evilops) Ellehauge in looking at input filtering in more depth while picking on a few real examples that we've seen around the web. As you'll see from the examples below, insufficient input validation can result in various kinds of code injection including XSS, and in some cases can be used to phish user credentials or spread malware.

Several examples are included show things like unfiltered use of PHP's superglobals, incorrectly quoted HTML attributes and some commonly overlooked areas like title tag injection and javascript analytics handling. They also talk about the different contexts the data might need to be filtered on - HTML, attributes, URLs, javascript, CSS and others.