A web application sits between users, browsers, application code, databases, APIs, and other services. Security problems often appear where one component trusts information another component can influence. Understanding those trust boundaries is more useful than memorizing vulnerability names.
Authentication establishes identity
Authentication answers who a user is. Passwords, one-time codes, passkeys, recovery flows, and rate limits can all influence that decision. Secure design also considers how sessions are created and invalidated after authentication changes.
Authorization controls actions
Authorization answers what an authenticated identity may do. Hiding a button in a browser is not a security boundary if the server still accepts an unauthorized request. Access decisions should be enforced where the protected resource is controlled.
Sessions carry identity between requests
Web requests are independent, so applications use session mechanisms to remember identity. Session identifiers and cookies are therefore security-sensitive. They should be protected in transit, given appropriate lifetimes, and invalidated when account security changes.
External input crosses a boundary
Applications receive data from forms, URLs, headers, APIs, uploads, and connected services. Secure design treats external input as data to validate and safely handle rather than instructions to trust automatically.
Security is a system property
An application can have strong passwords and still expose data through weak authorization. It can validate one input correctly and still trust another unsafe source. Security comes from multiple controls working together.
Practice on intentional targets
Locinode web challenges use applications deliberately created for authorized training. Public guides explain the defensive concepts; hands-on experimentation stays inside the assigned isolated lab.