Encoding for XSS Prevention: HTML, JS, CSS, and URL Contexts
XSS prevention requires encoding user data for the output context: HTML entities for HTML, \uXXXX for JS, %XX for URLs, and CSS escaping for style attributes.
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Tags: security, xss, encoding
Encoding for XSS Prevention: HTML, JS, CSS, and URL Contexts Cross-Site Scripting (XSS) is not primarily an input validation problem. It is an output encoding problem. The root cause of XSS is placing untrusted data into an HTML document without correctly encoding it for the specific context it occupies. The OWASP XSS Prevention Cheat Sheet identifies six distinct output contexts, each requiring different encoding. Using the wrong encoding for a context — or skipping encoding entirely — creates an XSS vulnerability. Using the right encoding makes injection structurally impossible. This article covers each context with concrete examples and code. --- Why Context Determines Encoding Consider a web application that reflects a user-supplied parameter in its response. The name appears in…
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