How to securely implement authentication in Single Page Applications (SPAs) with a decoupled API

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I have been researching how best to implement authentication in a React/Next.js application - specifically, how to store authentication tokens in a manner which is practical, while maintaining a needful emphasis on security. There is some existing debate about this topic on SO but as far as I can see, none offer concrete solutions.

Having spent much of yesterday and today trawling the internet for answers, I came across the following:

  • Local Storage API. I found that some basic guides suggest the use of localStorage (though many rightfully advise against it). I do not like this approach because data stored in localStorage could be accessed in the event of an XSS attack.

  • Web Workers. If the token is stored in a web worker, the user will not be logged in if a new tab is opened. This makes for a substandard and confusing user experience.

  • Closures. Same as Web Workers - there is no persistence.

  • HttpOnly Cookies. On the one hand, such cookies cannot be read by JavaScript so it's not prone to XSS. However, on the other hand, we now have to deal with CSRF, which is a new debate altogether: how does one implement CSRF tokens with an SPA + REST API?

While the use of HttpOnly cookies seems most favourable, implementing CSRF in an SPA seems unconventional and perhaps experimental, contravening the maxim of not "rolling your own" when it comes to security. I'd rather use a proven method if one exists.

With SPAs being all the rage nowadays, I am surprised that it's proving so difficult to find a consensus (or even better, a library) for such a prevalent scenario.

How is everyone else doing it?

Update: After some thought, I wonder if localStorage is really that bad if there is a strong CORS policy? If there happens to be an XSS vulnerability, couldn't the attacker start sending requests from within the browsing context anyway, negating any perceived benefit of using cookies and CSRF protection?

single-page-application csrf xss rest security
2 Answers

I'm happy that you're asking this question. There are recurring memes regarding oauth2 on the frontend that are really polluting the debate, and finding factual information is difficult.

First, regarding some excluded options which I suggest reconsidering: if you need the same authentication on multiple tabs, you can still use any option that would store tokens in a window scope, but individually manage tokens and get a new one on page refresh (silent refresh, thus standard prompt=none flow). This opens some options: service worker, web workers, closures... True, some of this isn't meant for that originally, but it solves the problem nicely. This also solves a bunch of race conditions about refresh tokens (they can only be used once, so having one for each tab solves a bunch of problems).

That being said, here are the options:

  • Local storage: in case of successful XSS attacks, tokens can be stolen. XSS=game over anyway IMO (no hacker will care about your token in such a case, it's not needed). It can also be mitigated by having short-lived tokens in comparison with the typicial hours/days validity of cookies. In any case, short-lived tokens are recommended.

    Now, stolen tokens in case of XSS seem to be an important issue for some people, so let's look at the other options anyway.

  • Session storage: same downsides as local storage (XSS can lead to session leakage), introduces its own CSRF issues, but also solves some others (refresh...).

  • Web workers: this is actually a nice solution. In case of successful XSS in a random part of the application, it won't be able to steal tokens. In theory, if one could inject some script that would run at authentication (auth code or token exchange), it could be exploited too... but that's true for all flows, including cookies/sessions.

  • Closures: same as web workers. Less isolated (easier to replace by one that would steal your token).

  • Service worker: ideal solution in my opinion. Easy to implement (you can just intercept fetch requests and add your token in a few lines code). Can't be injected by XSS. You could even kind of argue that it's actually meant for that exact use case. It also solves the case of multiple applications on a page (they share 1 service worker which adds token when required), which none of the other options solves nicely. Only downside: browser can terminate it, you need to implement something to extend it's lifetime (but there is a documented, standard way).

  • HttpOnly Cookies: in short, you're then transitioning to a traditional server-side web application (at least for some parts), it's not an independent SPA with standard oidc or aouth2 anymore. It's a choice (it's not been mine for some years now), but it shouldn't be motivated by token storage, as there are options for that which are even secure and arguably better.

Conclusion: my recommendation is to just use local/session storage. Successful XSS will probably cost you your job or your customer anyway (hint: nobody is interested in your tokens when they can call the pay(5000000, lulzsecAccount) API).

If you're picky about token storage, service worker is the best choice IMO.

user_5460160

Your comparison is a bit confusing... When you talk about SPA authentication, you need a solution that alows you to store authentication tokens. The main purpose of Web Workers and Closures is to run code, not for to store authentication tokens. You can write it down as a hard coded variable but it's not its purpose and this is why a new tab won't identify it.

So we're left with Local Storage and Session Cookies. Before SPA and client side rendering, we used to only have server side rendering and cookies. This is when HTTPOnly was invented to make it harder to steal session IDs and users' identities. When client side rendering was invented, stateless APIs were invented. These APIs do not use session IDs but Tokens instead, such as JWT. It means that the server do not save any information about the user in a session that is identified by a session ID. Instead, JWT tokens contains the signed information within the token itself, and the server do not remember anything and reauthenticate the user in each request. There is also a hybrid approach of tokens that are saved in NoSQL DBs in the server side such as Redis, to make the authentication process faster.

But the bottom line is that HTTPOnly cookies are used with stateful APIs that use sessions, and LocalStorage are used with stateless APIs that use tokens. You can also use it the other way around (cookies with tokens, LocalStorage with sessions) but its "less natural".

As for XSS, HttpOnly mechanism in cookies makes attackers' life a bit harder, but even if it is used, attackers can still do a lot of damage with phishing for example. So its not critical as this is just a compensating control and not a main mitigation in any way, so you can safely use LocalStorage. Furthermore, cookies are prone to CSRF attacks where LocalStorage does not. So both options are valid.

You can read about it some more in here: What is the difference between localStorage, sessionStorage, session and cookies?

user_24649180
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