Nearly every web application starts with the login page. It is expected. For users, it is barely noticeable. If it functions perfectly, no one gives it a second thought. But threat actors do. To an attacker, authentication is anything but invisible. It is the front door, the entry point into your architecture.
According to Verizon's 2025 Data Breach Investigations Report, credential abuse accounts for 22% of all incidents, and the average cost of a breach now sits at $4.88 million. The issue is not that there are not enough security tools, but choosing the right ones and setting them up properly.
In this guide, we will walk you through the essential components of a secure authentication system, the mistakes that trip up even experienced teams, and how to keep your system visible and monitored once it is running.
What You Are Defending Against
Before writing a line of code, it helps to understand what you are actually up against. Whether you are handling this internally or working with a web application development company, knowing the attack patterns shapes every design decision that follows.
Today, most authentication system attacks fall into a few repeatable patterns.
- Credential stuffing is the most widespread. Attackers take usernames and passwords leaked in earlier breaches and try them on other sites. Since roughly 41% of users reuse compromised passwords, one exposed database can quickly lead to access elsewhere.
- Session hijacking exploits tokens stored in accessible locations like localStorage, or transmitted over non-HTTPS connections, allowing attackers to impersonate authenticated users without ever knowing their password.
- MFA fatigue attacks bombard users with push notification requests until frustration leads someone to hit "approve" just to make it stop.
- AI-assisted attacks make all of this easier to scale. Automated tools can move through login steps across many accounts, reducing the effort required to carry out these attacks at volume.
Each of these attacks comes down to decisions in how you design and configure your authentication system.
The Core Components of a Secure Authentication System
At its core, authentication is a combination of several moving parts. Each one supports the others, and weakness in one area can expose the whole system. These are the parts that deserve the most attention.
1. Password Storage
If your application stores passwords, they need to be hashed with a slow, adaptive algorithm. bcrypt has been the long-standing standard. Argon2id is increasingly recommended for new systems because it resists both GPU-based and side-channel attacks.
What to avoid: MD5, SHA-1, or plain SHA-256. These are fast by design, which is exactly what makes them dangerous for password storage. Speed is an advantage for attackers running cracking attempts.
Also worth adding: check user passwords against known compromised credentials using the Have I Been Pwned API. It's free, privacy-preserving through k-anonymity, and lets you block the most dangerous passwords before they ever get stored.
2. Multi-Factor Authentication
MFA is no longer optional for serious web applications. Even a basic TOTP implementation through an authenticator app reduces account takeover risk significantly.
Options range from TOTP codes (Google Authenticator, Authy) to SMS-based OTPs (convenient but vulnerable to SIM swapping) to hardware security keys via WebAuthn. For most applications, TOTP is a reasonable default. For high-value or administrative accounts, WebAuthn and passkeys are now the stronger choice.
Whatever method you choose, NIST's guidance in SP 800-63B is the clearest authoritative framework for getting these decisions right.
3. Session Management and Token Handling
This is where a lot of implementations fall apart. The two main approaches are session cookies and token-based authentication using JWTs, and each has failure modes worth knowing.
JWTs done wrong are common. Mistakes include using the none algorithm, storing tokens in localStorage where XSS can grab them, and setting expiry windows measured in days instead of minutes.
A better approach: short-lived access tokens (15 minutes is a reasonable default), refresh tokens stored in HTTP-only Secure cookies, and server-side revocation for logout and incident response. An access token sitting in JavaScript memory with a 15-minute window limits the damage from an XSS exploit considerably.
For token signing, use asymmetric keys like RS256 or ES256 rather than HS256, especially if your architecture involves multiple services or third-party token validation.
4. OAuth 2.0 and OpenID Connect
If your application allows third-party login or exposes an API to other services, you're dealing with delegated authorization. OAuth 2.0 is the standard protocol here. OpenID Connect extends it to handle authentication specifically.
For single-page applications and mobile clients, always use PKCE (Proof Key for Code Exchange). Without it, authorization codes can be intercepted by malicious apps on the same device. The OWASP Authentication Cheat Sheet covers this and other common OAuth mistakes in detail.
Role-Based Access Control: Auth Doesn't End at Login
Strong authentication is only part of the picture. Once a user is inside your system, the next question becomes just as important.
Authentication answers "who are you?" Authorization answers "what can you do?" Treating them as the same thing is a common source of vulnerabilities.
Broken access control is consistently one of OWASP's top risks for a reason. A typical mistake: checking permissions on the client side only, so hiding an admin button in the UI feels like security. It isn't. Server-side authorization checks are non-negotiable. Every API endpoint should verify the requesting user's role and permissions independently of what the frontend sends.
A basic RBAC model with roles like customer, moderator, and admin goes a long way. More granular systems use attribute-based access control (ABAC), but for most web applications, RBAC is sufficient to start.
Monitoring, Logging, and Rate Limiting
Strong authentication controls are important, but you also need to see how they are being used. Without visibility, attacks can run for days before anyone notices.
Authentication events are often the first sign that something is wrong. Every login attempt, whether successful or failed, should be logged with a timestamp, IP address, user agent, and outcome. That data helps you detect credential stuffing, brute force attempts, and account takeovers early.
Rate limiting on login endpoints should be standard practice. After a certain number of failed attempts, require additional verification, such as CAPTCHA or temporary lockouts. Make sure limits apply at both the IP level and the account level, since attackers often spread attempts across many addresses to avoid simple per-IP restrictions.
Putting It Together
Secure authentication is a set of interlocking decisions, not a single feature. Strong password hashing matters less without rate limiting. MFA loses value if session storage is vulnerable. Good token handling is undermined by client-side-only RBAC enforcement.
The standards are mature and well-documented. OWASP, NIST SP 800-63B, and OAuth 2.0 with PKCE give you a solid foundation to build from. The fundamentals aren't changing much. What does change is the sophistication of attacks and the growing surface area as applications add APIs, third-party integrations, and mobile clients.
Start with the basics. Implement them correctly. Then add more advanced controls as your risk level increases. Authentication may not be the most visible part of your system, but it plays a major role in keeping it secure over time.
