Many two-factor authentication systems have some issues that prevent them becoming popular. For instance, a lot of the two-factor solutions only support one method of authentication, such as SMS messages, one-time passwords or hardware tokens. Duo Security, in contrast, is very flexible, as it supports automated phone calls, SMS messages, smartphone apps and hardware tokens. [...] Moreover, Duo Security can be linked to websites, Unix logins, VPN logins and so on. If you’re still using just a password to keep attackers out, Duo Security is a solution to seriously consider.