Backend security encompasses the server-side measures and coding practices that protect application data, infrastructure, and business logic from unauthorized access and malicious exploitation. It sits at the critical junction where user input meets sensitive data stores, authentication systems, and core application workflows—making it the primary defense layer against injection attacks, privilege escalation, and data breaches. The OWASP API Security Top 10 provides the foundational framework, but modern backend security extends far beyond preventing SQL injection: it requires layered defenses spanning input validation, cryptographic implementation, secrets management, and continuous monitoring. One key insight often overlooked: security failures typically cluster around trust boundaries—wherever user input crosses into privileged operations, wherever external services interact with internal systems, and wherever authentication state transitions from unauthenticated to authenticated. Effective backend security means treating every boundary as hostile territory and never trusting data simply because it arrived from an "internal" source.