HTML editors are great. However, care must be taken to avoid security abuses. An application that uses HTML editors, expects that the submitted HTML content comes correctly formatted and well-formed. That happens when real users use real browsers to edit the content.

However, an attacker may create a program that pretends to be a real browser and submit specially crafted HTML with Javascript that may open security holes.

Manuel talks a bit about what cross-site scripting means in this context and a simple (Javascript) example of how a user could abuse it. His solution? Parse the incoming data (filter it!) and look for potentially harmful tag types. To do this, he recommends the PHP Input Filter class. There's even a simple example of how to use it included in the post.