What is Out-of-Band Authentication?
Out-of-band authentication, sometimes called out-of-band verification or two-factor authentication, is a method of verifying a user's identity for login or other access using a separate communication channel from the primary one. This separate channel is considered more secure than the primary one because it differs in technology, geography or ownership. Some common examples of out-of-band channels include SMS text messages, authentication apps, phone calls, security keys and hardware tokens.
How Does Out-of-Band Authentication Work?
Out-of-Band Authentication process, after a user enters their username and password on the primary channel like a website, the authentication system will trigger the secondary "out-of-band" channel to send a one-time verification code. This code must then be entered on the original device or login page to fully authenticate the user. If the code matches what was sent out-of-band, the login is confirmed as valid.
If an attacker has compromised only the primary channel credentials like username and password, they will not have access to the secondary out-of-band channel on the user's phone, security key or other device. This makes it much more difficult to hijack the authentication process without being detected. The secondary verification acts as an extra layer of protection beyond just passwords.
Text Messages as an Out-of-Band Channel
One common way sites implement out-of-band authentication is through SMS text messages. Upon login, the user is prompted to enter a verification code that is simultaneously sent as a text to their registered phone number. Attackers would need access to both the compromised account credentials and the user's actual mobile device in order to bypass this two-factor mechanism.
That said, SMS has some downsides as well. Phone numbers could become targets for SIM swapping attacks where the carrier's authentication is hacked. Text messages can also be intercepted by third parties in limited cases. For an extra layer of security, it's preferable to use an authentication app or hard token that doesn't rely on cellular networks when possible.
Authentication Apps
Free authentication or second factor apps provide an alternative out-of-band channel for user verification. Popular options include Google Authenticator, Microsoft Authenticator and OTP Authenticator. During setup, these apps generate time-based one-time passwords (TOTP) that are synced to the associated account but not stored on the web servers.
When logging in, rather than receiving a text, the user will open the authentication app to view the current TOTP code. This code is then entered on the website to complete the login. Since the codes are generated onboarding the user's own device versus being transmitted, apps avoid the pitfalls of SMS interception or SIM hijacking that could potentially occur.
Emerging Methods - Security Keys
Physical security keys like YubiKey provide one of the most secure forms of out-of-band authentication available today when supported by a service. During account registration, public keys on the security key are associated with the user profile in lieu of a traditional password.
To log in, the key must be present and activated which sends a challenge response from the key without revealing internal credentials. Even if an account was compromised, the lack of the physical key prevents the actual user from being impersonated remotely. Security keys significantly improve upon traditional approaches and their use will likely expand as more websites integrate support for the Universal 2nd Factor (U2F) and FIDO2 standards.
Beyond Login - Two-Factor Everywhere
Out-of-band authentication should not only be used for initial logins but also required for any sensitive or high-risk account actions. Some examples include changing payment details, resetting passwords, enabling new sign-in methods and making account profile modifications. Just as logging in with nothing more than a password leaves an account vulnerable, so too do these types of changes if not strongly verified through two-factor protection.
Stripe for example enforces mandatory two-factor authentication for all account sign-ups as well as any action that could potentially introduce risk like updating bank information or identity documents. By taking a "two-factor everywhere" approach, the risk of compromise is significantly minimized compared to only using multifactor login alone without extension to other vulnerable account events.
Choosing the Right Out-of-Band Authentication Method
The needs and capabilities of individual organizations and users will determine which authentication solutions make the most sense. However, in general security keys should be considered the gold standard when supported as they provide the strongest defense against even sophisticated attacks. Authentication apps are also quite secure and low cost to implement via common mobile devices. SMS remains handy for some services due to its near-universal reach but has limitations to consider as well. No matter the approach, multifactor is strongly recommended wherever user verification takes place.
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About Author:
Vaagisha brings over three years of expertise as a content editor in the market research domain. Originally a creative writer, she discovered her passion for editing, combining her flair for writing with a meticulous eye for detail. Her ability to craft and refine compelling content makes her an invaluable asset in delivering polished and engaging write-ups.
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