In our security audit practice at Brainstormer AI Labs, we have reviewed over 30 startup codebases. The patterns of vulnerability are alarmingly consistent. Here are the five most dangerous mistakes we see repeatedly. Mistake 1: Hardcoded API keys and secrets. We find credentials committed to Git repositories in nearly 40% of audits. The fix is simple: use environment variables, implement secret rotation, and add .env files to .gitignore. Tools like GitGuardian can scan your repos automatically. Mistake 2: No input validation. SQL injection and XSS attacks remain the most common vulnerabilities. Every user input — forms, URL parameters, API payloads — must be validated and sanitized. Use parameterized queries and established sanitization libraries. Mistake 3: Weak authentication. We still find startups using MD5 for password hashing or storing passwords in plain text. Use bcrypt or Argon2 for hashing, implement rate limiting on login endpoints, and require strong passwords. Better yet, implement OAuth or passkey authentication. Mistake 4: Missing HTTPS and security headers. In 2026, there is zero excuse for not using HTTPS everywhere. Beyond SSL, implement security headers: Content-Security-Policy, X-Frame-Options, Strict-Transport-Security, and X-Content-Type-Options. Mistake 5: No monitoring or logging. Many startups have no idea when they are being attacked. Implement centralized logging, set up alerts for suspicious activities, and regularly review access logs. A breach you detect in 5 minutes costs far less than one discovered after 5 months. The cost of fixing security issues increases exponentially over time. A vulnerability that costs $100 to fix during development can cost $10,000 to fix in production and $1,000,000 after a breach. Invest in security from day one. At Brainstormer AI Labs, every project we build undergoes a security review before launch. We also offer standalone security audits for existing applications. Prevention is always cheaper than cure.