An authentication system includes an authentication module and a user history database storing order information that includes, for each of multiple logins of the first user to a web property, at least one of: an indication of an order of hypertext transfer protocol (HTTP) headers that were previously received at the authentication module during the login, and an indication of an order of navigator object properties that were previously returned to the authentication module during the login. The authentication module is configured to: receive, from a web browser of a first entity attempting to log in to the web property, credentials of the first user; determine order information of the first entity's web browser; perform a comparison operation based on the order information of the first user and that of the first entity, and determine whether to allow the first entity to log in based on the comparison operation.