OWASP (Open Web Application Security Project) publishes the most dangerous web vulnerabilities list annually. 2021 Top 3: A01 Broken Access Control (user accessing unauthorized areas), A02 Cryptographic Failures (weak or missing encryption), A03 Injection (SQL, OS, LDAP injection).
Practical tips for developers: SQL injection protection — prepared statements and parameterised queries. XSS protection — output escaping, Content Security Policy header. CSRF — synchronizer token pattern. Broken Authentication — session management, brute force protection, MFA. Security is not a "feature," it's architecture — you must think about it from the beginning.
Our courses on this topic