A Defensive Computing Checklist    by Michael Horowitz
NOTE: I gave a presentation on Defensive Computing at the HOPE conference in July 2022
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Using an ATM requires both a plastic card and a password. Two things. Two factors. In computing "two factors" refers to needing the usual userid/password and something else to gain access to a system. Thus, a stolen password becomes useless as its only half the story.

The robotic response from every computer nerd is to use Two Factor Authentication (2FA). But, it is not that simple. In the topic on SIM Swaps there are links to articles by people who became vulnerable only because they were using 2FA. These victims had their cellphone number stolen, so that it could be used to abuse 2FA text messages and change the passwords on many accounts. No 2FA text messages, no password changes.

A hidden problem with two factor authentication is the fallback scheme for when it breaks. For example, you can not get text messages when your cellphone is lost, stolen, broken or in the middle of nowhere without a cell signal. What then?

There are different types of 2FA and no one right answer for everyone.




The most secure option involves a physical thingy (often called a "key") you connect to a computer/tablet/phone that verifies your identity. No key no access. Here is a good introductory article: How Does a Hardware Security Key Like YubiKey Work by Leo A. Notenboom (July 13, 2023).

There are, of course, downsides to the use of security keys

  1. The thingies cost money, expect to pay about $50 US
  2. Different computing devices require different thingies (USB-A, USB-C, NFC, Lightning, etc)
  3. Not many systems support this type of 2FA. For example, Fidelity, which manages over 11 trillion dollars, does not support security keys. According to the Citibank Security Center they too, do not support security keys.
  4. The software on the security keys might be buggy.

Also, if the use of these security keys for a given account is optional, then your protection is only as good as the weakest link in the chain. If, for example, an account supports both text messages and security keys for 2FA, then bad guys can still exploit text messages by SIM swapping your phone number away from you.

Apple introduced support for security keys to safeguard an Apple account around March 2023 but they screwed up, big time. Anyone who steals an iPhone with the unlock code can just disable the security keys. For security keys to provide the maximum security, they must be required. If there is a back door way around them, then they are a waste of money.

Overkill: The Best Security Key for Multi-Factor Authentication by Yael Grauer and Thorin Klosowski for Wire Cutter (last updated July 27, 2022). In effect, what is described here is 3FA. it combines an authenticator app with a Security Key. Quoting:
"... the [Yubico YubiKey ] 5 Series can generate time-based one-time passcodes for up to 32 accounts, similar to how the Authy and Authenticator mobile apps work, but the credentials are stored on the key. This feature requires downloading the Yubico Authenticator app, and it works with services that support other authentication apps such as Authy. When you run into a site with software authentication but not key support, you can store those codes on the key. The Yubico app will then display those codes only if the key is connected, so even if someone managed to get your phone, they’d still need the key to access the authentication codes. None of the other keys we tested ... have this functionality. But using this feature puts the onus on you to save all the two-factor backup codes or to store credentials on a second key, so make sure you’re comfortable doing so."
The Yubico Authenticator app runs on Windows, macOS, Linux, iOS and Android.


More security means more hassle. Once you step up to 2FA, you should look to also create backup codes in case something goes wrong with the 2FA scheme. Not only should you create backup codes for every site using 2FA, you also need to save them (maybe print them out) where you can later find them. Some sites allow you to create a single backup code, others go up to 10. Google 2FA supports multiple backup codes. See How to retrieve your Google 2FA backup codes by Jack Wallen (Aug 2018)

To check if the companies you deal with offer 2FA, see 2fa.directory. It is pretty lame, but all we have.

In Alternative Ways to Protect Yourself from Being Spearfished (Jan 2020) Ivan Drucker relates his struggles trying to get non-techies to use an authenticator app. Then, he suggests using Google Voice as an alternative to both authenticator apps and your real cellphone number.

Background: Two-Factor Authentication Keeps the Hackers Out by Leo Notenboom (June 2016).

Background: Two-Factor Authentication: Who Has It and How to Set It Up by Eric Griffith (March 2019).

History: In the old days, there was a different type of two factor authentication. Citibank called theirs a security token. It was a physical device that created a second password, usually just a number. The Citibank model required you to press a button to generate the second password. Many other models would display the second password all the time, but change it every minute.

Making Authy more secure

  1. Google Fi hack victim had Coinbase, 2FA app hijacked by hackers by Lorenzo Franceschi-Bicchierai February 1, 2023. A Google Fi user got hit by a SIM swap attack that resulted in his Authy account getting hacked. Since the hacker had control of the phone number, the hacker was able to add a new device to the Authy account and from there take control of other accounts belonging to the victim.
  2. There is no defense mentioned in the above article, but Dan Goodin added some of his own (below) on Mastodon.
  3. Enroll at least one other device in your Authy account. This will ensure that you always have at least 2 devices enrolled, in case you lose one of them.
  4. Have your Authy account set to disable multi device. This will ensure that someone who gains control of your phone number will not be able to enroll new devices.


Getting Locked Out of Your Digital Life Is Bad. Here’s How to Avoid It. by Nicole Nguyen for the Wall Street Journal July 9, 2023. The focus here is on authenticator apps, warning that the bad ones store codes on your device (think phone). Lose the device and can not get into your accounts. Two such apps that backup to the cloud are Authy and Google Authenticator. The article discusses some other options for what to do after losing the device running your authenticator app. One option, is backup codes created for just this sort of thing. Gmail, for example, lets you generate and save a bunch of emergency codes (this is not mentioned in the article). Or, you may be able to login with just a password on another device, if you used the less secure option "Don’t ask again on this device". If you have another device logged in to Google or to Facebook, each company can use that device to verify your identity. The least likely option (my opinion) is a system that supports both an authenticator app and a security key (real physical key) where you use the security key to login if the authenticator app is not available.

A must-read. A Lost-Second-Factor Tale of Woe and How to Avoid Your Own by Leo A. Notenboom November 2022. Google Authenticator installs on one device. Lose that device, and you lose your authenticator, unless you plan ahead. When you associate Google Authenticator with an account, there is a QR code you need to scan with the Google Authenticator app. Take a screenshot of that QR code and save the image in a safe place. It can be used if you ever have to re-install Google Authenticator. There is also an option to save a string of text. Better yet, use Authy which can be installed on multiple devices that synch with each other.

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