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Push notification two-factor auth considered harmful

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hero image evil-hacker-lain
Image generated by Waifu Diffusion v1.2 -- an evil hacker at a laptop hacking into the pentagon, anime style, hacker den, monitors everywhere, serial experiments lain, evangelion

Sooooo Uber got popped. The hacker-internal rumor mill is churning out tons of scuttlebutt, but something came up that I've seen before and has made me adopt a radical new stance. I hope to explain it in this post for you all to learn from. Come along and join Cendyne and I on this magical journey!

Numa is delet
<Numa> tl;dr: push notification two-factor authentication is a great way to produce alarm fatigue, especially since threat actors can just spam prompts over and over until the employee accepts to make it stop.

You should really use WebAuthn as your two-factor auth solution with dedicated security hardware like Yubikeys or the security chip burned into most corp owned devices. Finally there’s a good use for the manglement engine that Stallman said was going to literally destroy Linux!

Authentication is a hard problem for humans. It is even harder for computers. In general though, we have three methods or "factors" that we can use to do authentication:

  • Something you are, like a retina or fingerprint scan
  • Something you know, like a password
  • Something you have, like a hardware two-factor auth token or authentication app that supports push notification based two-factor authentication

Most of the time you don't see retina and fingerprint scans required to log into Facebook. For a lot of people it's usually just that single password between anyone and their entire reputation being spoiled. There are efforts to try and get hardware authentication tokens into the hands of as many people as possible, but those are honestly a bad UX (user experience) compared to the "simplicity" of using a password. I doubt I'd be able to explain how to use a yubikey to my grandmother. Much less how to use a TOTP code generator.

Cadey is coffee
<Cadey> I want to stress that I am not a security engineer, I am just some random person on the internet who is tired of everything going to shit because someone got had.

I think that issuing everyone in the company a Yubikey and making every internal system work with that would be a better option. I think this because of the core problem of phishing: it works best when you are less vigilant. Many two factor authentication mechanisms lend themselves to phishing because of how they work. Here are my cynical thoughts about some common ones.

Cendyne is snoot
<Cendyne> Hello there! I’ll be contributing to this article too. In fact, Google partnered with Yubikey to deploy security keys to all staff and contractors, they reduced security incidents ten fold by moving to security keys. You can find out more on Google Eliminates Account Takeover with YubiKey.

TOTP Codes

Many online services support TOTP code generation as a two-factor authentication mechanism. These work by sharing a secret value between the client and server. Both sides take this secret, put in the current time and generate a six digit code that way.

Cendyne is laptop-thinking
<Cendyne> When you sign up for a TOTP on a service, the server may change a few defaults when it generates the shared secret (see RFC6238) like what hash function to use and how long the code should be. These codes are often tolerant of time differences, so multiple codes are valid at the same point for a given shared secret.

However, six digits is a very small space to search through when you are a computer. The biggest problem is going to be getting lucky, it's quite literally a one-in-a-million shot. Turns out you can brute force a TOTP code in about 2 hours if you are careful and the remote service doesn't have throttling or rate limiting of authentication attempts.

Numa is delet
<Numa> Guess what most services don't have lol.

Cadey is facepalm
<Cadey> You've got to be kidding me. You honestly, seriously, have got to be kidding me.

Cendyne is panic2
<Cendyne> Account specific rate limits are not enough! An attacker could go after multiple accounts at once from multiple IP addresses and get in any one of them. Most of the time users enter a TOTP after a password. Consider forcing a password reset after five failed subsequent code attempts.

Oh not to mention, the way that TOTP codes work basically means that it's trivial for an attacker to create an identical flow to your authentication setup on a phishing site hosted on a bulletproof host and then sniff credentials that way. Once the phisher has your password and a TOTP code, they can just use it and redirect you to the actual login form, making you think it failed and you need to try again. The protocol practically enables phishing with relative ease. It's horrifying that this is the security best practice for Discord and Twitter users.

Cadey is coffee
<Cadey> Don't even get me started on how messed up things like SMS based two factor authentication are. They use OTP codes on the backend for them, but then send them over the one carrier channel that is the most likely to have support be phished in order to have an attacker intercept those codes. It's a mess. I'm amazed that it was allowed to happen in the first place. The early 2010’s were wild.

Cendyne is thumbs-down
<Cendyne> It’s 2022 and this is still a thing. I think many users and developers are conditioned to think this is a safe implementation because everyone is still doing SMS OTPs.

Cendyne is stonks
<Cendyne> It took us years to move user expectations towards unique passwords, social login and single sign on. A few big companies led the industry by example after recognizing the risk shared passwords posed. The rest who resist hip and trendy changes respond to regulatory and industry standards like PCI-DSS. At least these industry standards include password management training (even if some of it is bad, like rotating passwords constantly).

Cendyne is gimme2
<Cendyne> We may see greater adoption of phishing resistant technology when it is cheap, widely available, and a part of industry standards or regulations. Most phones come with a trusted platform module (TPM) chip or secure enclave. Mac has had a secure enclave since 2020. Windows 11 now requires a TPM by default. So I believe that platform support will be considered available to the masses within a year or two.

Cadey is coffee
<Cadey> Until then, we continue to see more and more reports of stolen credentials causing untold amounts of damage to companies. Both reputationally and financially. It should not take a company almost getting hacked out of existence to make the security state of the world better, but sometimes that’s what it takes to take it seriously. I really hate that this is the case.

Yubikey Press

Yubikeys have multiple authentication methods built in. One of them is a legacy authentication method that makes the yubikey pretend to be a keyboard and type out a complicated long code based on a private key burned into the firmware of the device. This can also be phished. This was the main barrier between any employee and arbitrary user accounts at a past job. You got a corp issued yubikey and you had to use yubikey presses to get into secure areas of the admin panel.

This is also phishable, such as by shitposters on twitter:

Mara is wat
<Mara> Wait, people fell for this? Holy crap.

Cadey is coffee
<Cadey> Apparently people call this a "yubisneeze". It happens more often than you'd think. I really hope that shitpost didn't cause anyone's production environment to get breached. It'd be hilarious if this actually got added to any security training guides. You can disable the OTP interface by following this guide from Yubico.

Cendyne is heartburn
<Cendyne> Check out OTPs Explained from Yubico. A "yubisneeze" has several pieces of info in it, such as a counter to prevent replay. However, it does not contain the time. As a timeless token, it may be scraped and silently submitted elsewhere in the future. The only mitigation would be to immediately verify a newer token with Yubico. Yubikey OTPs are technically interesting but I am not convinced they raise the bar for security proportionate to their proprietary implementation.

Numa is delet
<Numa> vvccccfrjcfnncggttgrbgnevtrfcljdrrkntttcgrun!

So those are out of the question when it comes to protecting production access. All similar code based two-factor authentication methods suffer from the same phishablity problem.

Notification Fatigue As-A-Service

Another common way to do two factor authentication is to have a user sign into a mobile app. After that, when you sign in on your computer, it sends a push notification to your phone. Then you accept the authentication and you can go post your minecraft seeds to facebook marketplace or whatever.

Google and Facebook have these available as two-factor authentication methods. This means that a significant percentage of people on the planet likely use this authentication method. It is hilariously insecure in practice but makes people think that it is safe. I mean, you trust your phone, right?

Apple is another example, but instead of an app they hook into the desktop and mobile operating system with your iCloud account. Though, Apple requires a code to be manually relayed into the other device, which makes these a little harder to accidentally accept. Nothing a little phishing prep can’t get around.

Numa is delet
<Numa> Codes are lame, but it's the future! Everyone has phones now! People trust their phones. Can't we just have the computer spit out a notification to a person's phone so they use that for authentication instead of having to type in a code?

Mara is hmm
<Mara> Wait, isn't that whole "send a push notification" thing abusable? Couldn't a hacker send like a billionty push notifications over and over until the poor user hits accept to make it stop?

Allegedly that's what the Uber hacker did. They spammed two factor auth notifications over and over for an hour while the poor victim was trying to sleep. You know, when your guard is down by instinct and you are more likely to act on instinct to just make the noise box shut up. Honestly as an information security professional I have to almost give that attack method credit, especially the part where they sent the person a message on WhatsApp pretending to be IT saying they need to approve the request to secure access to their account. It's ingenious and I'd probably fall for it.

Maybe we should consider this entire two factor authentication mechanism to be harmful towards reaching security goals. It looks impressive to executives, who are the ones that are usually making the decisions about the security products for some reason.

Cendyne is take-my-money
<Cendyne> There is a lot of money sloshing around security vendors who fail to provide secure software in the most basic sense. For example, CVE-2022-1388 allowed anyone to run shell commands on a public firewall product. The real money seems to be in selling "security" rather than providing security.

Numa is delet
<Numa> You could probably make a killing on selling literal snake oil in the security scene! So much seems to add theatre and inconvenience in the name of being secure". It’s laughable but it’s the same kind of thing that leads to alarm fatigue as a service to begin with.

Cendyne is wheeze
<Cendyne> You got me there. The most efficient code is code that doesn’t exist after all.

So you may be wondering something like:

Mara is hmm
<Mara> What can we even do to make authentication safer? Push notifications make people suffer from alarm fatigue. Code-based solutions can be phished. What can we use to help solve this today?

I'm not really sure what the best solution is here, but I can suggest something that I think would help reduce harm: Webauthn.

Cadey is coffee
<Cadey> Please note that we are intentionally focusing on what we can do today, because today is what we have. Tomorrow doesn’t exist yet.

Cendyne is whoa
<Cendyne> "Tomorrow" we are likely to see passkeys displace yubikey and other roaming hardware tokens. Apple Passkeys and Google FIDO authentication with passkeys are coming soon to let your phones authenticate your computer as a second factor. If you’d like to hear more, check out this podcast Passkeys featuring Adam Langley.

Cendyne is surprised-pikachu
<Cendyne> But uh… Good luck buying any hardware tokens in bulk right now. After the Uber incident, many are emptying Yubico's stock.


WebAuthn is a protocol for using hardware devices in order to authenticate users by proof of ownership. The basic idea is that you have a hardware security module of some kind (such as an iPhone’s Secure Enclave or a Yubikey) that contains a private key, and then the server validates signatures against the public key of the device to authenticate sessions. It is also set up so that phishing attacks are impossible to pull off, each WebAuthn registration is bound to the domain it was set up with. A keypair for cannot be used to authenticate with

The user experience is fantastic. A website makes a request to the authentication API and then the browser spawns an authentication window in such a way that cannot be replicated with web technologies. The browser itself will ask you what authenticator you want to use (usually this lets you pick between an embedded hardware security module or a USB security key) and then proceed from there. It is impossible to phish this. Even if the visual styles were copied, the authenticator will do nothing to authenticate the browser!

hero image blog/webauthn-test

Since the browser may not know which authenticator the user intends, it will prompt the user for the platform authenticator or for a roaming authenticator like a Yubikey. Platform authenticators combine biometrics like Face ID, Touch ID, or Windows Hello with the trusted platform module or secure enclave to authenticate. While roaming authenticators may require a presence test and optionally a PIN. This extra step also protects the user’s privacy from any drive-by client calls. When a biometric or PIN is verified, the server receives a flag that the user has been verified. Biometric and PIN data is not sent to the server. A standard yubikey without a PIN will appear as unverified.

Mara is happy
<Mara> To find out more about WebAuthn, I’d suggest checking out the BlackHat talk "Demystifying WebAuthn" (slide deck). It will help you get an understanding of how it works.

The work continues on making WebAuthn easier and better to use. Apple recently released Passkeys with iOS 16 that will allow you to use your iPhone as a hardware authenticator for any other device, including iPads and Windows machines. This effectively turns your iPhone’s Secure Enclave into a roaming security key that you use on other machines, giving you the best of both worlds. You benefit from Apple’s industry-leading on-device security processors and also have the ability to use that on your other machines too. All the existing guarantees of WebAuthn are carried over, including the fact that each WebAuthn credential is bound to a single website. Today, you can choose "Add a new Android phone” in Chrome and scan it with your iOS device. Safari has not caught up yet, we expect this later.

Cendyne is hooray
<Cendyne> WebAuthn’s strength against phishing is built on origin binding, that is whatever the user’s browser sees as the website is what the authenticator sees. Another website? Another key! One site’s WebAuthn authentication cannot be used on another site, since the origin (the site domain) is bound to the key that the authenticator uses! Not only that, but the user too!

Cendyne is guess-i-will-die
<Cendyne> Though if the site does not bind usernames (or uses a dummy username) when authenticating, you will have a collision between users.

In conclusion, push notifications for authentication should be considered harmful. You should not use them and you should prioritize moving towards hardware authentication tokens such as Yubikeys. It is worth the hardware and training cost to do this.

Mara is hacker
<Mara> This part of the article is optional.

As a bonus, here's one of the ways that the web3 people get this kind of thing more right than wrong. They use the Ethereum cryptosystem as an authentication factor.

EIP-4361: Sign-In with Ethereum

WebAuthn is a protocol where you get a hardware element to sign a message to prove that you own the keypair in question, and that allows authentication to happen via the "something you own" flow. In Ethereum and other blockchain ecosystems, everyone has a keypair that signs messages for instructions like "transfer an NFT to another address" or "send this address some money". This is enough to let you construct an authentication factor. The strength of this authentication factor is...questionable, but by conforming to EIP-4361, you can turn an Ethereum keypair into an authentication factor for web applications. This will hook into a hardware wallet with WalletConnect or a software wallet with a browser extension like MetaMask. This works in a very similar way to how WebAuthn works, but with these core principles:

  • Authentication is done by confirming that the user has access to a private key to sign a message that is validated with the public key, much like WebAuthn.
  • Wallet apps show you the URL of the site you are trying to authenticate with and there is no way to easily forge that.
  • Generating a new Ethereum keypair is free and anyone can do it without having to purchase hardware to act as key escrow.

Mara is hacker
<Mara> The last point is significant because export restrictions on cryptography can make it very difficult for people in countries like Russia to purchase FIDO2 keys. Cryptography isn’t treated as a munition legally anymore, but it is something powerful enough to give government people pause. For example, OpenBSD is based out of Canada just in case export laws about cryptography in the US become relevant again.

But due to the implementation of all of this, it has the following weaknesses:

  • There’s no protocol level way to tell what kind of secure element the user is using, if any. There are several different types of hardware "cold wallets" and one of the most commonly used strategies is to make a "hot wallet" that’s just a keypair managed by a browser extension such as MetaMask.
  • In many web3 applications, the Ethereum keypair is the only authentication factor. This means if someone manages to get access to your keypair or recovery phrase somehow, they can steal your apes and you can’t get them back without trying to negotiate with the threat actor.
  • Extracting the private key of an Ethereum address is considered a security feature and people are encouraged to write their private keys down on paper and store it somewhere safe. The first-time user experience of many Ethereum space things will force you to write down the recovery phrase by quizzing you on what it is and I can only imagine how many people are doing that with an actually secure mechanism. There are some interesting products available for doing things like punching your seed phrase into metal so you can put that metal object in a safety deposit box.
  • This is implemented with browser extensions instead of properly embedded into the OS itself, meaning that it is theoretically possible to phish, but it looks like that is very difficult in practice.

Mara is hacker
<Mara> It’s worth noting that there’s such a thing as a multi-signature contract wallet where the wallet itself is a smart contract in the blockchain. An example of this is Safe, which I am told is the most widely used implementation. This lets you use additional means to ensure that m-of-n private keys sign a message instead of just putting all of your eggs in one basket with one key. Doing this does mean that you have to pay a gas fee for every time that you sign in, executing a smart contract isn’t free.

Cadey is coffee
<Cadey> Honestly I’m not sure if all of this is worth it, but at the very least it is something and it is embedded deep enough into the ecosystem that it’s done decently across multiple websites. There’s a lot of UX warts involved, but a lot of that is kind of endemic to grafting something onto browsers after the fact. However it is something and it is more than just a TOTP code.

Numa is happy
<Numa> You can use one of those fancy hardware bitcoin wallets as a WebAuthn credential though!

This article was posted on M09 17 2022. Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Tags: security infosec webauthn web3 collab

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The art for Mara was drawn by Selicre.

The art for Cadey was drawn by ArtZora Studios.

Some of the art for Aoi was drawn by @Sandra_Thomas01.