The Paragon Initiative has posted a new tutorial that wants to provide you with everything you need to know about preventing cross-site scripting in PHP applications.

Cross-Site Scripting (abbreviated as XSS) is a class of security vulnerability whereby an attacker manages to use a website to deliver a potentially malicious JavaScript payload to an end user. XSS vulnerabilities are very common in web applications. They're a special case of code injection attack; except where SQL injection, local/remote file inclusion, and OS command injection target the server, XSS exclusively targets the users of a website.

[...] Cross-Site Scripting represents an asymmetric in the security landscape. They're incredibly easy for attackers to exploit, but XSS mitigation can become a rabbit hole of complexity depending on your project's requirements.

He introduces the concept of cross-site scripting (XSS) for those new to the term and provides a brief "mitigation guide" for those wanting to jump to the end. He then gets into some examples of what a XSS vulnerability could look like, both stored and reflected and provides the "quick and dirty" method for preventing them. He also mentions some tips in implementing your solution including avoiding HTML in your data if at all possible. He goes on to talk about the use of HTMLPurifier to prevent attacks, context-sensitive escaping (HTML vs JS vs CSS) and some of the browser-level features that help prevent XSS for the user.