Authentication and authorization are foundational security concepts that control who can access systems and what they can do. Authentication verifies identity—proving you are who you claim to be. Authorization determines permissions—what you’re allowed to do once identified. Understanding these concepts and implementing them correctly is essential for building secure applications and protecting sensitive data.

Authentication: Proving Identity

Username and Password authentication is the most common method. Users provide credentials, which the system validates against stored hashes (never plaintext passwords). While familiar, password authentication has weaknesses: users choose weak passwords, reuse passwords across sites, and phishing attacks steal credentials.

Multi-Factor Authentication (MFA) adds security by requiring multiple proof factors: something you know (password), something you have (phone or hardware token), or something you are (biometric). Even if passwords are compromised, attackers need the second factor, significantly improving security.

Single Sign-On (SSO) allows users to authenticate once and access multiple applications without re-authenticating. SAML, OAuth, and OpenID Connect enable SSO, improving user experience while centralizing authentication management.

API Keys identify API clients. Applications include API keys in requests, and servers validate them. Keys should be treated as secrets, rotated regularly, and scoped to minimum necessary permissions.

Certificate-Based Authentication uses cryptographic certificates to prove identity. This is common in service-to-service authentication, particularly in zero-trust architectures where services must mutually authenticate.

Password Security

Hashing transforms passwords into fixed-size strings using one-way functions. Systems store password hashes, not plaintext. During authentication, the provided password is hashed and compared to the stored hash.

Salt adds random data to passwords before hashing, ensuring identical passwords produce different hashes. This prevents rainbow table attacks where pre-computed hashes are used to crack passwords.

Password Hashing Algorithms like bcrypt, scrypt, or Argon2 are designed to be computationally expensive, slowing brute-force attacks. Avoid fast hashes like MD5 or SHA-1 for passwords.

Password Policies enforce minimum complexity: length requirements, character diversity, prohibiting common passwords. However, overly strict policies lead to password reuse and written-down passwords. Balance security with usability.

Token-Based Authentication

JWT (JSON Web Tokens) encode user identity and claims in cryptographically signed tokens. Clients include JWTs in requests, and servers validate signatures and extract claims without database lookups.

Stateless Authentication with JWTs means servers don’t maintain session state. Tokens contain all necessary information, enabling horizontally scalable services. The tradeoff is difficulty revoking tokens before expiration.

Refresh Tokens enable long-term authentication without exposing access tokens. Short-lived access tokens (minutes to hours) provide temporary access, while long-lived refresh tokens (days to weeks) obtain new access tokens. If access tokens are compromised, exposure is limited to their lifetime.

Token Revocation addresses JWT’s revocation challenge. Maintain revocation lists (blacklists) of compromised tokens, implement token versioning (invalidating all tokens when user’s version increments), or use short-lived tokens requiring frequent refreshes.

Authorization: Controlling Access

Role-Based Access Control (RBAC) assigns permissions to roles and users to roles. Users with the “admin” role get admin permissions; users with “viewer” role get read-only access. RBAC simplifies management: change role permissions rather than updating thousands of users.

Attribute-Based Access Control (ABAC) makes decisions based on attributes: user attributes (department, clearance level), resource attributes (classification, owner), environment attributes (time, location). ABAC enables fine-grained, dynamic policies but increases complexity.

Access Control Lists (ACLs) specify which users or groups can access each resource. File systems use ACLs: this file is readable by Alice and Bob, writable by Alice. ACLs work for small-scale scenarios but become unwieldy for complex permissions across many resources.

OAuth 2.0 provides delegated authorization. Users grant third-party applications limited access to their data without sharing credentials. The OAuth flow involves users authorizing applications, servers issuing access tokens, and applications using tokens to access protected resources.

Principle of Least Privilege

Grant minimum permissions necessary for tasks. Users shouldn’t have admin rights if they only need to read data. Services shouldn’t have write permissions if they only perform queries. This limits damage from compromised accounts or software vulnerabilities.

Regularly audit permissions, removing unnecessary access. People change roles, projects end, and access requirements evolve. Periodic reviews ensure permissions remain appropriate.

Session Management

Session Tokens identify authenticated sessions. After successful authentication, servers generate session tokens that clients include in subsequent requests. Sessions typically have timeouts, requiring re-authentication after periods of inactivity or absolute timeouts.

Session Storage can be server-side (tokens map to server-stored session data) or client-side (encrypted, signed tokens containing session data). Server-side enables easy revocation but requires shared session storage in distributed systems. Client-side eliminates server state but complicates revocation.

Session Hijacking occurs when attackers steal session tokens and impersonate users. Mitigations include secure transmission (HTTPS), HttpOnly cookies (preventing JavaScript access), SameSite cookies (protecting against CSRF), and binding sessions to IP addresses or user agents.

Zero Trust Architecture

Traditional security uses perimeter defense: trusted internal networks and untrusted external networks. Zero trust assumes breach, requiring authentication and authorization for every request regardless of origin.

Never Trust, Always Verify: Don’t assume network location implies trustworthiness. Authenticate and authorize every request, even from internal networks.

Least Privilege Access: Grant minimum necessary permissions for each request. Default to deny unless explicitly allowed.

Continuous Verification: Don’t authenticate once and trust indefinitely. Continuously verify identity and authorization, possibly stepping up authentication requirements for sensitive operations.

Common Vulnerabilities

Credential Stuffing uses leaked credentials from one site on others, exploiting password reuse. Mitigations include breach monitoring, MFA, and CAPTCHA for login attempts.

Brute Force Attacks try many password combinations. Defenses include account lockout after failed attempts, rate limiting login attempts, and CAPTCHA after failures.

Session Fixation tricks users into using attacker-controlled session IDs. Always regenerate session IDs after authentication to prevent this.

Privilege Escalation exploits vulnerabilities to gain unauthorized elevated permissions. Mitigations include input validation, secure coding practices, and regular security audits.

Best Practices

Use HTTPS everywhere to protect credentials and tokens in transit. Never send sensitive data over unencrypted connections.

Store passwords using strong, salted hashing algorithms. Never store plaintext passwords or reversibly encrypted passwords.

Implement MFA for sensitive operations and accounts. The security benefit far outweighs the slight usability impact.

Apply principle of least privilege rigorously. Start with minimal permissions and add only what’s necessary.

Use established libraries and frameworks for authentication and authorization. Implementing cryptography or auth protocols from scratch invites vulnerabilities.

Log authentication and authorization events for audit trails. Monitor for suspicious patterns like failed login attempts, unusual access patterns, or privilege changes.

Regularly update dependencies and patch vulnerabilities. Many breaches exploit known vulnerabilities in outdated software.

Test security controls. Penetration testing, security audits, and vulnerability scanning identify weaknesses before attackers do.

Authentication and authorization are fundamental to application security, requiring thoughtful implementation and ongoing attention. Understanding authentication methods, authorization models, and common vulnerabilities enables building systems that effectively protect user data and system resources. Security isn’t a one-time implementation but an ongoing process of assessment, improvement, and response to evolving threats. The goal is layered defense where multiple security controls work together to protect against diverse attack vectors.