3rd-party cookies in practice



Recent days I was building a cross-site communication tool. So I was looking into CORS, window.postMessage and other relevant stuff.

While these mechanisms seem to be well-documented and have simple APIs, handling 3rd-party cookies is a little bit vague. In particular, I was interested in What exactly does it mean when user disables 3rd-party cookies?.

It turned out not so complicated, but you don’t know this until you actually have to use it, so hopefully this will help someone in future.

Questions

First of all, what is a 3rd-party cookie anyway? Common sense tells us this must be cookies in scope of domains other than the current page. But this is quite vague. Does it mean we only cannot set new cookies, should we be able to read the old ones? How should browser handle cookies set from Javascript? Does this rule affect requests triggered by images and other resources? How does it affect CORS requests? What about localStorage and sessionStorage? And how do we detect that user has disabled 3rd-party cookies?

Answers

I’ve done a quick testing trying to answer those questions and here is what I found. Everything was checked on Firefox 41.0.2, Google Chrome 46.0.2490.71, and Safari 9.0 under Mac OS El Capitan.

  • First of all, when 3rd-party cookies are disabled, browsers do not save, nor send cookies for domains other than the top-level window (current page). This affects all cross-domain requests including resources (e.g. <img> tags), iframes, and XMLHttpRequests (including CORS). This is something that you would expect, actually.
  • If there is a cookie that was set previously as a 1st-party, it won’t be used.
  • Javascript in cross-origin iframe cannot set cookies as well. No exceptions will be thrown, but document.cookie will always return an empty string, even if you set it to something.
  • No cookies in CORS requests too, even if you use .withCredentials parameter. Cookie header from server is just ignored.

localStorage/sessionStorage is a bit more tricky:

  • in Google Chrome any attempt to access localStorage will result in DOMException
  • in Safari, localStorage.getItem() and localStorage itself don’t raise errors, but if you try to use localStorage.setItem(), it will give you a QuotaExceededError
  • in Firefox, localStorage and sessionStorage are still working inside iframes. So you can save stuff in localStorage, and it will stay there. However, this will change in upcoming Firefox 43.

Safari pitfall

The term “3rd-party cookie” (actually “3rd-party origin”), seems to have different meanings across browsers. For example, Safari by default does send cookies to third-party domains, if it was visited before. There is a very nice story about that by Alex Volkov. Worth reading.

Detection

If you need to detect if 3rd-party cookies are enabled, one option is to take advantage of always-empty document.cookie. Just create an iframe pointing to a page on another domain like this:

<html>
<head>
<script>
var randStr = function(){
  return Math.random().toString(36).substring(7);
};

var thirdPartyCookiesEnabled;
var k = randStr(),
    v = randStr();
document.cookie = k + '=' + v;
if(document.cookie.indexOf(k + '=' + v) == -1) {
  thirdPartyCookiesEnabled = false;
} else {
  thirdPartyCookiesEnabled = true;
}
alert(thirdPartyCookiesEnabled);
</script>
</head>
<body></body>
</html>

Workaround(s)

If you really need to access cookies on another domain, there are some ways to do so.

The most bullet-proof one is just make sure that cookies are 1st-party. Essentially, it means redirects. So you can redirect user to a page on another domain, set the cookie there, and then immediately redirect him back. This is not the best user experience, but it works.

In some cases it is possible to use a subdomain for setting cookies. If you have access to example.com DNS, you can create a CNAME record foo.example.com, pointing to a server that needs to set cookies. Provided that cookies are not host-only (that is, they are set with domain attribute specified), you can share cookies between those domains.

Conclusion

What I have described above, is just a top of the iceberg. It is sufficient to understand the immediate side-effects of blocked cookies, but if you are really going to address this issue, you will most likely come across many caveats. And I believe there is no ideal solution to this problem.

On the other hand, the 3rd-party cookies problem has been there for a while, and now you can find lots of nice articles and posts with all kinds of reusable ideas. Check out this post by Alex Volkov for example.