Authentication is the process of verifying the identity of users or systems before granting access to a web application. It is a critical component of web application testing and audit because weak authentication mechanisms can expose sensitive data and system functionality to unauthorized users. One of the most common methods is password-based authentication, where users provide a username and password combination. This method is simple to implement but requires secure storage techniques such as hashing and salting of passwords, enforcement of strong password policies, and protection against brute force attacks through account lockouts and rate limiting. Testing auditors often evaluate password complexity rules, encryption practices, and susceptibility to credential stuffing. Multi-factor authentication enhances security by requiring users to provide two or more forms of verification. These factors generally include something the user knows, something the user has, and something the user is. Common examples include one-time passwords sent through SMS or email, authenticator apps generating time-based codes, and hardware tokens. Web application auditors analyze the resilience of MFA mechanisms against phishing attacks, bypass techniques, and session hijacking risks. Proper implementation significantly reduces unauthorized access even if passwords are compromised. Biometric authentication uses unique