What are the security best practices for custom software development?

Security must be integrated throughout the development lifecycle (DevSecOps), not added as an afterthought. OWASP Top 10 mitigation: SQL injection (use parameterized queries exclusively, never string concatenation), XSS (sanitize output with context-aware encoding, implement Content-Security-Policy headers), CSRF (use anti-CSRF tokens for state-changing requests with cookie auth), broken authentication (implement rate limiting, account lockout, MFA, secure session management), and insecure deserialization (validate and sanitize all input, use safe serialization formats). Infrastructure security: HTTPS everywhere with HSTS headers, configure CORS restrictively (never use in production), implement security headers (X-Content-Type-Options, X-Frame-Options, Referrer-Policy), keep dependencies updated (use Dependabot, Snyk), and implement network segmentation. Data security: encrypt sensitive data at rest (AES-256), encrypt all data in transit (TLS 1.3), implement field-level encryption for PII, use database RLS for access control, and maintain data classification policies. Secret management: use vault services (HashiCorp Vault, AWS Secrets Manager), never commit secrets to version control (use.env files with.gitignore), rotate secrets regularly, and audit secret access. Code security: run SAST tools (SonarQube, Semgrep) in CI, conduct code reviews with security focus, perform penetration testing before major releases, and implement bug bounty programs for ongoing security testing. Incident response: maintain a security incident playbook, implement audit logging for security-relevant events, practice incident response regularly, and have a clear vulnerability disclosure process.