Multi-factor authentication

Multi-factor authentication,''' also known as two-factor authentication, is an electronic authentication method in which a user is granted access to a website or application only after successfully presenting two or more distinct types of evidence to an authentication mechanism. MFA protects personal data—which may include personal identification or financial assets—from being accessed by an unauthorized third party that may have been able to discover, for example, a single password.
Usage of MFA has increased in recent years. Security issues which can cause the bypass of MFA are [|fatigue attacks], phishing and SIM swapping.
Accounts with MFA enabled are significantly less likely to be compromised.

Authentication factors

Authentication takes place when someone tries to log into a computer resource. The resource requires the user to supply the identity by which the user is known to the resource, along with evidence of the authenticity of the user's claim to that identity. Simple authentication requires only one such piece of evidence, typically a password, or occasionally multiple pieces of evidence all of the same type, as with a credit card number and a card verification code. For additional security, the resource may require more than one factor—multi-factor authentication, or two-factor authentication in cases where exactly two types of evidence are to be supplied.
The use of multiple authentication factors to prove one's identity is based on the premise that an unauthorized actor is unlikely to be able to supply all of the factors required for access. If, in an authentication attempt, at least one of the components is missing or supplied incorrectly, the user's identity is not established with sufficient certainty and access to the asset being protected by multi-factor authentication then remains blocked. The authentication factors of a multi-factor authentication scheme may include:

Something the user has: Any physical object in the possession of the user, such as a security token, a bank card, a key, a phone that can be reached at a certain number, etc.
Something the user knows: Certain knowledge only known to the user, such as a password, PIN, PUK, etc.
Something the user is: Some physical characteristic of the user, such as a fingerprint, eye iris, voice, typing speed, pattern in key press intervals, etc.

An example of two-factor authentication is the withdrawing of money from an ATM; only the correct combination of a physically present bank card and a PIN allows the transaction to be carried out. Two other examples are to supplement a user-controlled password with a one-time password or code generated or received by an authenticator that only the user possesses.
An authenticator app enables two-factor authentication in a different way, by showing a randomly generated and constantly refreshing code, rather than sending an SMS or using another method. This code is a Time-based one-time password, and the authenticator app contains the key material that allows the generation of these codes.

Knowledge

Knowledge factors are a form of authentication. In this form, the user is required to prove knowledge of a secret in order to authenticate.
A password is a secret word or string of characters that is used for user authentication. This is the most commonly used mechanism of authentication. Many multi-factor authentication techniques rely on passwords as one factor of authentication. Variations include both longer ones formed from multiple words and the shorter, purely numeric, PIN commonly used for ATM access. Traditionally, passwords are expected to be memorized, but can also be written down on a hidden paper or text file.

Possession

Possession factors have been used for authentication for centuries, in the form of a key to a lock. The basic principle is that the key embodies a secret that is shared between the lock and the key, and the same principle underlies possession factor authentication in computer systems. A security token is an example of a possession factor.
Disconnected tokens have no connections to the client computer. They typically use a built-in screen to display the generated authentication data, which is manually typed in by the user. This type of token mostly uses a OTP that can only be used for that specific session.
Connected tokens are devices that are physically connected to the computer to be used. Those devices transmit data automatically. There are a number of different types, including USB tokens, smart cards and wireless tags. Increasingly, FIDO2 capable tokens, supported by the FIDO Alliance and the World Wide Web Consortium, have become popular with mainstream browser support beginning in 2015.
A software token is a type of two-factor authentication security device that may be used to authorize the use of computer services. Software tokens are stored on a general-purpose electronic device such as a desktop computer, laptop, PDA, or mobile phone and can be duplicated.. A soft token may not be a device the user interacts with. Typically an X.509v3 certificate is loaded onto the device and stored securely to serve this purpose.
Multi-factor authentication can also be applied in physical security systems. These physical security systems are known and commonly referred to as access control. Multi-factor authentication is typically deployed in access control systems through the use, firstly, of a physical possession which acts as the identification credential, and secondly, a validation of one's identity such as facial biometrics or retinal scan. This form of multi-factor authentication is commonly referred to as facial verification or facial authentication.

Inherent

Inherent factors, are factors associated with the user, and are usually biometric methods, including fingerprint, face, voice, or iris recognition. Behavioral biometrics such as keystroke dynamics can also be used.

Location

Increasingly, a fourth factor is coming into play involving the physical location of the user. While hard wired to the corporate network, a user could be allowed to login using only a pin code. Whereas if the user was off the network or working remotely, a more secure MFA method such as entering a code from a soft token as well could be required. Adapting the type of MFA method and frequency to a users' location will enable you to avoid risks common to remote working.
Systems for network admission control work in similar ways where the level of network access can be contingent on the specific network a device is connected to, such as Wi-Fi vs wired connectivity. This also allows a user to move between offices and dynamically receive in each.

Mobile phone-based authentication

Two-factor authentication over text message was developed as early as 1996, when AT&T described a system for authorizing transactions based on an exchange of codes over two-way pagers.
Many multi-factor authentication vendors offer mobile phone-based authentication. Some methods include push-based authentication, QR code-based authentication, one-time password authentication, and SMS-based verification. SMS-based verification suffers from some security concerns. Phones can be cloned, apps can run on several phones and cell-phone maintenance personnel can read SMS texts. Not least, cell phones can be compromised in general, meaning the phone is no longer something only the user has.
The major drawback of authentication including something the user possesses is that the user must carry around the physical token, practically at all times. Loss and theft are risks. Many organizations forbid carrying USB and electronic devices in or out of premises owing to malware and data theft risks, and most important machines do not have USB ports for the same reason. Physical tokens usually do not scale, typically requiring a new token for each new account and system. Procuring and subsequently replacing tokens of this kind involves costs. In addition, there are inherent conflicts and unavoidable trade-offs between usability and security.
Two-step authentication involving mobile phones and smartphones provides an alternative to dedicated physical devices. To authenticate, people can use their personal access codes to the device plus a one-time-valid, dynamic passcode, typically consisting of 4 to 6 digits. The passcode can be sent to their mobile device by SMS or can be generated by a one-time passcode-generator app. In both cases, the advantage of using a mobile phone is that there is no need for an additional dedicated token, as users tend to carry their mobile devices around at all times.
Notwithstanding the popularity of SMS verification, security advocates have publicly criticized SMS verification, and in July 2016, a United States NIST draft guideline proposed deprecating it as a form of authentication. A year later NIST reinstated SMS verification as a valid authentication channel in the finalized guideline.
As early as 2011, Duo Security was offering push notifications for MFA via a mobile app. In 2016 and 2017 respectively, both Google and Apple started offering user two-step authentication with push notifications as an alternative method.
Security of mobile-delivered security tokens fully depends on the mobile operator's operational security and can be easily breached by wiretapping or SIM cloning by national security agencies.
Advantages:

No additional tokens are necessary because it uses mobile devices that are carried all the time.
As they are constantly changed, dynamically generated passcodes are safer to use than fixed log-in information.
Depending on the solution, passcodes that have been used are automatically replaced in order to ensure that a valid code is always available, transmission/reception problems do not, therefore, prevent logins.

Disadvantages:

Users may still be susceptible to phishing attacks. An attacker can send a text message that links to a spoofed website that looks identical to the actual website. The attacker can then get the authentication code, user name and password.
A mobile phone is not always available—it can be lost, stolen, have a dead battery, or otherwise not work.
Despite their growing popularity, some users may not even own a mobile device, and take umbrage at being required to own one as a condition of using some service on their home PC.
Mobile phone reception is not always available—large areas, particularly outside of towns, lack coverage.
SIM cloning gives hackers access to mobile phone connections. Social-engineering attacks against mobile-operator companies have resulted in the handing over of duplicate SIM cards to criminals.
Text messages to mobile phones using SMS are insecure and can be intercepted by IMSI-catchers. Thus third parties can steal and use the token.
Account recovery typically bypasses mobile-phone two-factor authentication.
Modern smartphones are used both for receiving email and SMS. So if the phone is lost or stolen and is not protected by a password or biometric, all accounts for which the email is the key can be hacked as the phone can receive the second factor.
Mobile carriers may charge the user messaging fees.