Sat, 10 Feb 2024

Reframing Multi-factor Authentication

In December, I was the victim of a domain hijacking attack in which Google Domains transferred control of all of my domains from my Google account to an attacker's Google account. These domains are used for my personal and business email, and all of my online accounts were associated with my email addresses at these domains. By gaining control of my domains, the attacker was able direct email sent to my domains to a mail server under his control so that he could intercept incoming email for my domains.

With control of incoming email, the attacker was able to gain control of some of the accounts registered using these email addresses. He was able to compromise those accounts that did not have multi-factor authentication enabled, requiring that, after password authentication, a one-time password sent by SMS or a generated time-based one-time password (TOTP) using a shared secret.

As a result of this attack, I have a new perspective on the value of multi-factor authentication. Re-framing how multi-factor authentication is described, in terms of possessions that one is unlikely to concurrently lose control over, makes its value clearer and provides a better way of thinking about the risks of an account being compromised.

A list of recommendations is provided at the end.

Traditional Framing of MFA

The traditional description of multi-factor authentication is a mechanism that requires two things in order to authenticate oneself:

The "something the user knows" is typically a password. The "something the user has" is often a mobile phone, which either can receive SMS messages or has an application installed such as Google Authenticator, which stores shared keys used to generate one-time codes.

Describing of MFA in these terms is misleading for a few reasons:

  1. the password is no longer something the user knows
  2. "something the user has" is too abstract
  3. suggests there are only two distinct things which the user must have

It is no longer common nor recommended for users to know the password used to access each online service. Most people use tens to hundreds of services. It's not feasible to remember separate passwords for each service. Since users do not have control over how services store passwords, it's also not recommended to use the same password for multiple services. There have been numerous cases of passwords being leaked due to security compromises.

Because it's impractical to know your passwords, a password manager must be used. To make use of a password manager, you will typically want it to be available on the computers you are using regularly. These are probably your laptop/desktop and your phone. Storing your web application passwords in your Google account or your iCloud keychain, for example, keep your passwords where you need to access them.

Similarly, when you need to generate TOTP codes, it makes to do so on your phone or on your computer using an application like 1Password.

Multi-factor authentication, using a password manager and a TOTP generator therefore is better described as "something the user has" (a password manager) and "something else the user has" (a TOTP generator). If both of these are available on the same device, an attacker gaining control of one will likely gain control of the other.

So prior to the attack I underestimated the value of multi-factor authentication because I thought of it primarily in terms of what would happen if an attacker gained control of my computer. If gaining control gave them access to all of the factors needed to access an account, multi-factor authentication wasn't better than one-factor authentication.

Better Framing As Possessions

A better way to think about multi-factor authentication is:

In the domain hijacking attack, my computer wasn't compromised. None of my account passwords were compromised. None of the keys used to generate TOTP codes were compromised. What was compromised was the possession that can be described as control over incoming email (COIE), and COIE is often equivalent to a password.

Some applications make the equivalence of a password and COIE explicit. Slack, for example, will email you a one-time code to log in by default. (Authenticating using a password is also possible.)

Most other applications will allow you to reset your password if you have COIE, implicitly making the password and COIE equivalent with respect to authentication.

Equivalent Possessions

When authenticating using a service that makes a password and COIE equivalent, one of the required factors is therefore one of two possessions. If your account also has MFA configured to require a TOTP code, you need these two possessions to authenticate:

Some services may allow email to be used to receive one-time codes after authenticating using a password. If the service also allows resetting the password through email verification, these two factors are effectively reduced to one.

Similarly, if a service allows you to reset your password by verifying control over incoming SMS and also allows you to receive one-time codes via SMS, two-factor authentication would effectively be reduced to a single factor.

Now consider a scenario in which your passwords are stored in Google Chrome on your laptop computer and one-time passwords are sent via SMS to your phone. This ostensibly requires two distinct possessions to authenticate, your laptop and your phone. But you can inadvertently reduce this to a single possession if your SMS messages are available on your computer through iMessage or Google's Messages web client (, for example. (Google has recently started trying mitigate this risk by displaying messages such as "Use the Google Messages app on your phone to chat with Microsoft" when viewing messages from Microsoft or Google on the Messages web client. The browser notifications still show the full messages so this is apparently still a work-in-progress.)

Additional Considerations

Account Access Chaining

Often, one service will grant access to possessions that can be used as one or more factors to authenticate to another service. For example, if you use Gmail for email and your Google account is compromised, that grants access to the COIE possession used for authentication for many other services.

Delegated Authentication

Some services allow using another service's account to authenticate. For example, a service may allow you to sign in with your Google or Facebook account. There is a trade-off here. If you are using your Facebook to authenticate to multiple other services, and your Facebook account is compromised, the attacker can use it to gain access to all of those other services.

On the other hand, providing robust multi-factor authentication for a service is non-trivial. Facebook certainly devotes more resources to account security than many other services so it may be easier to maintain control of one Facebook account than a multitude of services with their own authentication mechanisms.

Segregation of Possessions

You may also want to segregate your possessions such that you have multiple instances of the same type of possession which are used to access different accounts. For example, you can use one email address for your bank accounts, and a different email address that's used for correspondence and to register for social media accounts or the marketing emails from your e-commerce accounts.

You could use a separate phone to generate your TOTP codes. It can be an old phone that doesn't have service that is kept at your desk. This makes access to your desk, in effect, a required possession for authentication (assuming the recovery codes that are treated as equivalent possessions are printed out and kept at the desk as well). There is a trade-off in that you will only be able to log into these accounts while at your desk, but it may be worthwhile or even desirable for certain types of accounts.


You want to be notified when one of your factors is used unexpectedly. Even if the second factor fails, you want to be notified if someone enters your password unexpected from a new location, for example.

This is analogous to having a deadbolt lock on your front door, the key to which is factor 1, along with a safe in your home to protect valuables, the code to which is factor 2 (unless you have lots of safes, you probably can keep this password in your head). It's useful to have an alarm on the front door that goes off even if a thief can't successfully break into your safe on their first try.


When evaluating the security of existing online accounts and when setting up accounts with new services, I offer the following suggestions.

In addition to securing access to your accounts, you may want to take additional precautions in securing your financial accounts. In the US, you can place a fraud alert on your credit file at each of the three main credit reporting agencies. You can do this prior to being a victim of fraud. You have to renew it annually, but it notifies potential creditors to take extra precautions in verifying your identity. You can also put a freeze on your credit reports, which should prevent any new creditors from issuing new credit using your name and social security number until you lift the freeze.

To reduce the likelihood of credit card fraud, you might consider not storing your credit card details with e-commerce sites. Large sites like Amazon will require that credit cards be re-entered when shipping orders to a new address, but smaller retailers may not. As with delegated authentication, consider delegating paying by credit card to Google Pay, Apple Pay, or Amazon Pay when available.

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The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat