Web Authentication Fundamentals

Authentication is the process of verifying identity—confirming that users are who they claim to be. Every web application that stores user-specific data or restricts access to certain features must implement authentication, and every authentication system presents potential security vulnerabilities.

The Authentication Problem

HTTP is fundamentally stateless. Each request is independent, with no inherent mechanism for connecting one request to the next. Yet web applications need to remember who you are across multiple requests. This disconnect between protocol design and application requirements created the need for session management.

When you log into a website, you provide credentials that the server verifies. But the server can't ask for your password with every single request—that would be both insecure (credentials repeatedly transmitted) and unusable. Instead, servers issue tokens that represent your authenticated session. These tokens travel with subsequent requests, proving that you've already authenticated.

Session tokens must be unpredictable. If an attacker can guess or calculate valid session tokens, they can impersonate other users without knowing their passwords. Good session tokens are randomly generated with sufficient entropy, making them impossible to predict or brute-force.

Common Authentication Mechanisms

Username and password authentication remains the most common approach despite its well-known weaknesses. Users choose predictable passwords, reuse credentials across sites, and fall for phishing attacks. Applications often compound these problems with poor implementation—storing passwords in plaintext, using weak hashing algorithms, or transmitting credentials over unencrypted connections.

Session cookies are the traditional mechanism for maintaining authenticated state. After successful login, the server sets a cookie containing a session identifier. The browser automatically includes this cookie with subsequent requests, and the server uses it to recognize the authenticated user.

Token-based authentication, particularly using JSON Web Tokens (JWT), has gained popularity with the rise of single-page applications and APIs. Instead of server-side session storage, the token itself contains encoded information about the user. The server cryptographically signs the token, allowing later verification without database lookups.

Multi-factor authentication adds additional verification requirements beyond passwords. Something you know (password), something you have (phone), and something you are (biometrics) can be combined to make authentication more resistant to credential theft.

Where Authentication Goes Wrong

Understanding common authentication vulnerabilities helps you identify them during testing. Weak password policies allow users to choose easily guessable credentials. Missing brute-force protection permits automated password guessing. Improper session management might expose session tokens through URLs, allow session fixation attacks, or fail to invalidate sessions properly at logout.

The authentication boundary itself deserves scrutiny. Some applications authenticate the main application but leave administrative interfaces, API endpoints, or static files unprotected. Others implement authentication inconsistently, protecting some features while leaving related functionality accessible to unauthenticated users.

Even correctly implemented authentication can be bypassed through other vulnerabilities. XSS can steal session cookies. SQL injection can extract credentials. CSRF can trigger authenticated actions without user consent. Defense requires putting up multiple layers, not just a strong front door.