Which type of authentication is considered the weakest?

Single-factor authentication is regarded as the weakest type of authentication because it relies on a single method to verify a user's identity. This typically involves something the user knows, such as a password or PIN. Since it relies solely on one factor, if that single element is compromised—whether through phishing, social engineering, or brute force attack—the entire authentication process is undermined.

In contrast, multi-factor authentication, two-factor authentication, and biometric authentication utilize multiple levels of security to enhance protection against unauthorized access. Multi-factor authentication employs two or more independent credentials from different categories—something you know, something you have, and something you are—making it significantly more challenging for attackers. Biometric authentication, on the other hand, relies on unique physical characteristics, adding another layer of complexity to the verification process. Two-factor authentication is a specific case of multi-factor authentication that uses two distinct factors, which also improves security over single-factor solutions.

Thus, the reliance on just one verification method in single-factor authentication renders it the least secure and most susceptible to breaches.