Which user authentication method is typically considered least secure?

The least secure user authentication method from the choices provided is username and password. This method relies solely on something the user knows, which can be vulnerable to a variety of attacks such as phishing, credential stuffing, and brute force attacks. If a user's password is weak or reused across multiple accounts, it becomes much easier for an attacker to gain unauthorized access.

Additionally, traditional username and password combinations do not require any additional verification to confirm the identity of the user, making them more susceptible to being compromised. In contrast, methods like biometric login, two-factor authentication, and single sign-on introduce additional layers of security. For instance, biometric login relies on unique physical characteristics, while two-factor authentication adds a second layer of verification, making unauthorized access much more difficult. Single sign-on simplifies user authentication but does so using trusted tokens that enhance overall security. Thus, the reliance purely on a username and a password makes this method the least secure among the options.