Claude Code ui

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
XSS vulnerability detected in templateMalicious script executed in user contextScript injection through unsanitized output

Common Causes

  1. Raw HTML output without escaping: {{ userInput }} instead of properly escaped
  2. dangerouslySetInnerHTML or equivalent used without validation
  3. Server trust user input without sanitizing
  4. Missing Content Security Policy headers
  5. 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.

Real developers can help you.

Matthew Jordan Matthew Jordan I've been working at a large software company named Kainos for 2 years, and mainly specialise in Platform Engineering. I regularly enjoy working on software products outside of work, and I'm a huge fan of game development using Unity. I personally enjoy Python & C# in my spare time, but I also specialise in multiple different platform-related technologies from my day job. Basel Issmail Basel Issmail ’m a Senior Full-Stack Developer and Tech Lead with experience designing and building scalable web platforms. I work across the full development lifecycle, from translating business requirements into technical architecture to delivering reliable production systems. My work focuses on modern web technologies, including TypeScript, Angular, Node.js, and cloud-based architectures. I enjoy solving complex technical problems and helping teams turn product ideas and prototypes into working platforms that can grow and scale. In addition to development, I often collaborate closely with product managers, business analysts, designers, and QA teams to ensure that solutions align with both technical and business goals. I enjoy working with startups and product teams where I can contribute both as a hands-on engineer and as a technical partner in designing and delivering impactful software. Jacek Rozanski Jacek Rozanski Senior PHP/Symfony developer and DevOps engineer with 20+ years of professional experience, running opcode.pl (web development agency, est. 2004). Day job: I'm the sole backend developer at merketing company where I own and maintain 11 PHP/Symfony microservices on AWS (ECS Fargate, RDS, S3, CloudFront), handle the full CI/CD pipeline (Bitbucket Pipelines, Docker), and manage monitoring with Sentry and CloudWatch. These services handle high request volumes in production every month. What I bring to AI-built apps: - I audit and fix security issues (OWASP methodology), performance bottlenecks, and architectural problems in codebases generated by Cursor, Claude Code, Lovable, Bolt, and v0 - I refactor AI-generated prototypes into production-grade applications with proper error handling, testing, and clean architecture (SOLID, DDD, hexagonal architecture) - I set up the infrastructure AI tools don't touch: AWS hosting, CI/CD pipelines, automated deployments, database optimization, monitoring, and alerting - I integrate external services: payment providers, email systems, partner APIs, SSO/auth Tech stack: PHP 8.x, Symfony, React, Next.js, PostgreSQL, MySQL, Docker, AWS (ECS, RDS, S3, SQS/SNS, CloudFront), Terraform, Supabase. I also use AI tools daily (Claude Code, Cursor) in my own workflow, so I understand both the strengths and the gaps in AI-generated code. Based in Poland (CET timezone). Available for async work and calls during EU/US business hours. Luca Liberati Luca Liberati I work on monoliths and microservices, backends and frontends, manage K8s clusters and love to design apps architecture Milan Surelia Milan Surelia Milan Surelia is a Mobile App Developer with 5+ years of experience crafting scalable, cross-platform apps at 7Span and Meticha. At 7Span, he engineers feature-rich Flutter apps with smooth performance and modern UI. As the Co-Founder of Meticha, he builds open-source tools and developer-focused products that solve real-world problems. Expertise: šŸ’” Developing cross-platform apps using Flutter, Dart, and Jetpack Compose for Android, iOS, and Web. šŸ–‹ļø Sharing insights through technical writing, blogging, and open-source contributions. šŸ¤ Collaborating closely with designers, PMs, and developers to build seamless mobile experiences. Notable Achievements: šŸŽÆ Revamped the Vepaar app into Vepaar Store & CRM with a 2x performance boost and smoother UX. šŸš€ Launched Compose101 — a Jetpack Compose starter kit to speed up Android development. 🌟 Open source contributions on Github & StackOverflow for Flutter & Dart šŸŽ–ļø Worked on improving app performance and user experience with smart solutions. Milan is always happy to connect, work on new ideas, and explore the latest in technology. Nam Tran Nam Tran 10 years as fullstack developer Yovel Cohen Yovel Cohen I got a lot of experience in building Long-horizon AI Agents in production, Backend apps that scale to millions of users and frontend knowledge as well. Prakash Prajapati Prakash Prajapati I’m a Senior Python Developer specializing in building secure, scalable, and highly available systems. I work primarily with Python, Django, FastAPI, Docker, PostgreSQL, and modern AI tooling such as PydanticAI, focusing on clean architecture, strong design principles, and reliable DevOps practices. I enjoy solving complex engineering problems and designing systems that are maintainable, resilient, and built to scale. David Olverson David Olverson Solo dev shipping production apps with AI-assisted development. I specialize in rescuing broken Lovable/Bolt/Cursor builds and taking them to production. 10+ apps shipped including SaaS CRMs, gaming platforms, real estate tools, and Discord bots. Stack: Next.js 16, TypeScript, Tailwind CSS, FastAPI, PostgreSQL, Prisma. I use Claude Code with 50+ custom skills for rapid delivery. Average turnaround: 2-4 weeks from broken prototype to production. Krishna Sai Kuncha Krishna Sai Kuncha Experienced Professional Full stack Developer with 8+ years of experience across react, python, js, ts, golang and react-native. Developed inhouse websearch tooling for AI before websearch was solved : )

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Frequently 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 &lt;, &gt;, etc.

What's Content Security Policy?

HTTP header restricts script execution: Content-Security-Policy: script-src 'self'. Blocks inline scripts and external URLs.

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