Claude Code
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XSS Vulnerability in Template Output
User-provided content displayed in template without escaping. Attacker can inject malicious JavaScript that executes in other users' browsers. Form submissions, comments, or user profiles become attack vectors.
Template renders user input directly, trusting it's safe when it's not.
Error Messages You Might See
XSS vulnerability detected in template
Malicious script executed in user context
Script injection through unsanitized output
Common Causes
- Raw HTML output without escaping: {{ userInput }} instead of properly escaped
- dangerouslySetInnerHTML or equivalent used without validation
- Server trust user input without sanitizing
- Missing Content Security Policy headers
- Frontend sanitization only, no backend validation
How to Fix It
Escape all user input by default. Use framework features: React auto-escapes, use innerText not innerHTML. Sanitize HTML: DOMPurify library on frontend. Backend validation required too. Use Content Security Policy header to block inline scripts. Never use dangerouslySetInnerHTML unless absolutely necessary and content is verified safe.
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Get HelpFrequently Asked Questions
How to prevent XSS in React?
React auto-escapes by default. Avoid: dangerouslySetInnerHTML. For HTML content, use DOMPurify: purify.sanitize(html)
What should be escaped?
All user input, especially from forms, URLs, file uploads. Escape: <, >, &, ", ' to <, >, etc.
What's Content Security Policy?
HTTP header restricts script execution: Content-Security-Policy: script-src 'self'. Blocks inline scripts and external URLs.