Cross-site scripting

Cross-site scripting attacks occur when user input is not properly sanitized and ends up in pages sent back to users. This makes it possible for an attacker to include malicious scripts in a page by providing them as input to the page. The scripts will be no different than scripts included in pages by the website creators, and will thus have all the privileges of an ordinary script within the page—such as the ability to read cookie data and session IDs. We already saw an example of cross-site scripting in the previous chapter on virtual patching, and in this chapter we will look in more detail on how to prevent such attacks.

The name "cross-site scripting" is actually rather poorly chosen—the name stems from the first such vulnerability that was discovered, which involved a malicious website using HTML framesets to load an external site inside a frame. The malicious site could then manipulate the loaded external site in various ways—for example, read form data, modify...