In this part of my “Security 101” series, I want to talk about different mechanisms for two factor authentication (2FA) as well as why we need it in the first place. Most of my considerations will be for the web and web applications, and I’m explicitly ignoring local login (e.g., device unlock) because the threat model is so different.

Why Two Factor At All

Username and password alone has proven itself inadequate time and time again. Have I Been Pwned reports 443 breached sites with more than 9 billion account credentials dumped. Users have a tendency to use predictable passwords, reuse passwords across sites, enter their credentials into phishing sites, and otherwise have poor habits with passwords. Two factor authentication helps with most of these concerns to varying degrees.

Potential Threats Against Passwords

Phishing is when a user inputs their username and password into a malicious website that has been setup to capture credentials for a service. Often this uses a clone of the legitimate website’s appearance, and may redirect the user to the legitimate login page after their submission (making users believe the first attempt failed and trying again). An attacker can then take the captured credentials and log into the users account.

Password reuse is when a user uses the same username and password on multiple sites. When one site is compromised, the username and password become available. Often, an attacker will use credential stuffing to then attempt to use these leaked credential lists on other sites to compromise accounts.

Keylogging is when the user enters their password into a computer or keyboard that logs keystrokes, exposing their specific credentials to an attacker. There is a broader variety of malware on the host machine (doing more than keylogging) that can capture clipboard input, HTML form fields, etc.

Weak passwords are a common problem for some users – using passwords based on names, dates of birth, or even just common passwords like “password123” or “zxcvbn”. These are relatively unlikely online attacks as most services will rate-limit or block the attacker trying a large number of passwords for a single user, but not all services will do this. In an enterprise, you can often find ancillary services that use the active directory/LDAP infrastructure for authentication but do not trigger failed login counter increments nor block login attempts.

Attacks on Two Factor Mechanisms

Once users started using two factors, attackers started attacking it. I’ll broadly discuss some of the threats two factor implementations face before getting into the implementations themselves.

Phishing remains a problem. Yes, it turns out that if you can convince someone to type in their username and password, you can also likely convince them to type in some form of second factor. Any form of two factor that requires the user to type in some input is at risk from phishing.

Malware on the user’s computer will get just about every form of authentication. Even if the two factor is protected in some way, an attacker can just steal the cookies or other session tokens to impersonate the user, inject javascript into the DOM of running sites, or tunnel traffic through the user’s computer (“man in the browser”). There’s very little that will save the day when you’re working on a pwned computer.

There are other attacks on specific mechanisms that I’ll discuss below.

Two Factor Mechanisms

Traditionally, factors have been grouped into categories, specifically:

  • Something you know (passwords)
  • Something you have (tokens, phone, device, etc.)
  • Something you are (biometrics)

Having “two factor” authentication meant having two of the 3 categories above. I’ll discuss how closely each of the 2FA mechanisms matches the taxonomy systems. I’m not aware of any implementations of biometrics for client/server use, so none of those come into play.

Security Questions are NOT Two Factor

Some web applications seem to treat security questions as a second factor. Firstly, these fit clearly in the “something you know” category (along with passwords), so you don’t have a second factor from that perspective. Even if you believe that a second set of information would help, if people are answering the questions correctly, these are almostly certainly going to be shared among multiple sites. If that wasn’t damning enough, a lot of these questions ask information that’s publicly available, making targeted attacks quite trivial. Please, please, don’t use security questions.

SMS (Text Message) Codes

So many sites have support for SMS-based two factor authentication. When you go to login, the site texts you a one time code that is valid for a limited lifetime, then you enter the code into the site to provide the second factor.

In theory, this is a “something you have” mechanism for validating that you have your cell phone. In practice, this is validating that someone has access to your phone number, whether that’s through an SMS to web gateway or other mechanisms. It’s very much vulnerable to phishing attacks, and there have been documented cases of individuals being social engineered to give up an SMS code by forwarding it to someone else.

In addition to these concerns, SMS messages are not encrypted end-to-end, meaning the cell carriers and governments may have access to it. While government access to the 2FA code might not be a concern in some countries, authoritarian regimes likely have used these to access private accounts. Additionally, the SS7 system used by mobile carriers to coordinate traffic is known to be insecure, and this weakness has been used to steal money even from accounts supposedly protected by two factor. Finally, attackers have been known to perform SIM Swapping where they convince your mobile carrier to port your number to their phone, so they begin receiving your SMS codes. NIST stopped recommending SMS-based two factor authentication in 2016 because of these issues.

Time Based Codes (e.g., Google Authenticator1)

This is a fairly common approach to two factor and most in the security world have seen it at least once. You set it up using a mobile app where you scan a QR code, and then you get a new code every 30 seconds. When you want to login, you input this code from your app. Most often, this is an implementation of Time-Based One Time Passwords (TOTP) as specified in RFC 6238. This uses a hash of the current time and a seed that was provided by the site via the QR code. Both sides have the same seed value and so can compute the same OTP code.

Common implementations include:

Unfortunately, like all other factors requiring the user to input a value into the website by hand, this is a phishable mechanism. Additionally, malware on the mobile device, such as Cerberus, is now known to steal codes directly from the authenticator apps.

Mobile App Confirmation (Out of Band Confirmation)

A few services have implemented a mechanism using their mobile applications to validate new sign-ins. These are tricky because you need to already be signed in on mobile, but basically they’ll just prompt on your mobile device to confirm that it’s you signing in.

Unlike SMS-based 2FA, this is usually carried over TLS, so not vulnerable to those attacks. Likewise, because the user input is just an acknowledgement, the credential itself cannot be phished, however users can still be social engineered into acknowledging it, especially if the attacker is proxying credentials to the legitimate service immediately from their phishing page.

Physical Token (e.g., RSA SecurID2)

These were quite popular for a while, and still seem to be in some heavily regulated (i.e., slow to change) industries. For these, you have a dedicated hardware token that displays a periodically-changing code on a screen (some may have a code that changes with the press of a button instead). Some implementations may just be hardware implementations of TOTP or the related Hash-Based One Time Passwords (HOTP), but there are proprietary implementations, such as RSA’s.

These usually keep the seed for the 2nd factor with the manufacturer, which can lead to problems, as in the 2011 breach of RSA that gave access to the seeds for its tokens. An attacker could then impersonate these tokens even without physical posession.

Once again, because of the user input step, we’re vulnerable to phishing as well.

Universal 2nd Factor/Web Authentication (U2F/FIDO/WebAuthn)

Web Authentication (WebAuthn), formerly known as Universal 2nd Factor (U2F), and developed by the FIDO Alliance (yeah, it’s had a few names) is a mechanism that uses a cryptographic signature to prove the authenticity of a device. U2F specifically is just a second factor to be used with a username and password, but WebAuthn even allows for the token to be the only authentication mechanism.

This is often implemented by a physical token, such as the YubiKey 53 or the Open-Source Hardware Solo. These store the secret key for the U2F/WebAuthn keypair in a hardware security element or a microcontroller on board. In either case, remotely extracting (e.g., via malware on the device) the two factor key is not possible. They also require a “user presence” test (e.g., tapping the device or pushing a button).

It can also be implemented on a phone by storing the key in a secure element or using ARM TrustZone, or (for example) using the fingerprint reader in the Apple touchbar. These depend on the security of the device to protect the keys involved.

In all cases, this is not vulnerable to phishing – the user is not inputting anything, and in fact, the origin of the site they’re logging into is part of the challenge and response flow. The flip side is that it does require support from the applications involved – for example, your browser must have support for WebAuthn to authenticate to the site. The browser speaks to the hardware/software token on your behalf, and attests to the origin being authenticated and a “key handle” that ensures that each site only gets back its own data.

There’s a lot more to it, but U2F/FIDO have come up with a very strong mechanism for two factor authentication. Properly implemented, it’s resistant to device cloning, phishing, and even active MITM. (Note that an active MITM in your TLS session can still steal your session cookies, but still won’t be able to impersonate you for future login attempts.)

Comparison Table

Mechanism Credential Stuffing Phishing Cloning
SMS Codes
Out of Band 4
Physical Token (Code) 5


I hope you find this overview of two factor mechanisms useful. There’s much more to it if you’re implementing an authentication scheme, but I wanted to provide an overview if you’re choosing an existing implementation to use, or you’re performing a penetration test or audit of an existing system. If you want to dive into much more detail about authentication systems, then NIST SP800-63-3 is worth a read, though I’d grab a coffee, Red Bull, or Club Mate first. As always, please let me know if you have any feedback.

(Also, notice how many times I had to mention something being vulnerable to phishing – gotta love that humans are the perpetually unpatchable vulnerability.)

  1. Google Authenticator is a Trademark of Google LLC. 

  2. SecurID is a Trademark of RSA Security LLC. 

  3. YubiKey is a Trademark of Yubico, Inc. 

  4. Some app implementations are more difficult to phish, such as instructing the user to select a particular code on their screen. 

  5. Cloning resistant so long as vendor not compromised.