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

Question

I have been researching how best to store authentication tokens in a Single Page Application (SPA). 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, I read that this can protect from XSS. However, on the other hand, wouldn't this mean that we now have to deal with CSRF? Then it's a new debate altogether: how does one implement CSRF tokens with an SPA + REST API?

How is everyone else doing it?

Answer 1

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.

Answer 2

The solution is a HttpOnly SameSite Cookie.

In the question, you correctly note that HttpOnly protects from XSS. SameSite in turn protects from CSRF. Both options are supported by all modern browsers.

This is orders of magnitude simpler and safer than other solutions. It's easy to set up on the API and completely transparent to the SPA. Security is built-in.

Concrete solution: The actual authentication can be done by your API or by an external provider that redirects to your API. Then, when logged-in, your API creates a JWT token and stores it in a HttpOnly SameSite Cookie. You can see this at work with NestJS at nestjs-starter as explained in: OAuth2 in NestJS for Social Login (Google, Facebook, Twitter, etc).

One limitation is that API and SPA have to be on the same domains.

For rather storing the token client-side, this other article is very comprehensive.