[jakelazaroff]

If you use cookies as a storage mechanism and ignore the cookie header on your backend, you close the door to CSRF attacks.

Here's one glaring problem with local storage: literally any script on your page can access it (for example, vendor scripts). Cookies can only be accessed by scripts from the same domain from which they're created.

[metafunctor]

That's true, but if you run untrusted scripts on your site it's pretty much game over, anyway.

Why should those scripts limit themselves to stealing tokens when they can send authenticated requests from the browser? To put it another way, why would you care about knowing the root password when you have a way to run a root shell at will?

[filleduchaos]

It's interesting that every time this comes up people talk as though the only vector for a malicious script running on your site is you serving it yourself. A reminder that browsers have a ridiculously lax permissions/security model for extensions which extension developers have been shown again and again to abuse (see the Stylish incident for instance).

[jakelazaroff]

How can they send authenticated requests if they can't access your cookies and your backend ignores the cookie header?

[metafunctor]

The scenario is XSS, where the attacker manages to run their JS code on your page, and get all the same privileges as your own code on the page. Whatever mechanism your own JS code uses to perform authenticated requests, the attacker can do the same.

[jakelazaroff]

That is not the scenario you described (running untrusted scripts on your site). Cookies are not protected from XSS, but they are protected against malicious or compromised vendor/CDN scripts and browser extensions. Local storage, however, is vulnerable to all of the above.

[metafunctor]

It seems that you are saying that cookies are more protected from third party code than your own code. That is incorrect.

Let's get specific: let's say you have a page on mysite.com. When a user signs in, the server sets a HttpOnly session cookie to authenticate later requests from the user.

Now let's assume your page loads evilsite.com/tracker.js. The code in tracker.js can now send requests to mysite.com, and your HttpOnly session cookie will be sent. There is no extra protection for cookies that would check if the JS code doing the sending came from mysite.com.

Obviously tracker.js cannot read the value of your session cookie (and, indeed, neither can your own code), but mysite.com is more or less totally compromised.

[jakelazaroff]

You're describing CSRF, but again: this vulnerability doesn't exist in the scenario I'm describing.

If you don't set HttpOnly on your cookies and ignore the cookie header on your backend (i.e. only use cookies for storage, not for transport), cookies are strictly better than local storage, since the only difference between the two is now local storage's lax access policy.

The scenario you're describing can also be solved by using a CSRF token retrieved from the backend. Meanwhile, there is literally no way to secure secrets kept in local storage from third party scripts.