Many websites offer a “Log in” capability where they don’t manage the user’s account; instead, they offer visitors the ability to “Login with <identity provider>.”
When the user clicks the Login button on the original relying party (RP) website, they are navigated to a login page at the identity provider (IP) (e.g. login.microsoft.com) and then redirected back to the RP. That original site then gets some amount of the user’s identity info (e.g. their Name & a unique identifier) but it never sees the user’s password.
Such Federated Identity schemes have benefits for both the user and the RP site– the user doesn’t need to set up yet another password and the site doesn’t have to worry about the complexity of safely storing the user’s password, managing forgotten passwords, etc.
In theory, everything works great. The IP subframe in the RP page knows who the user is (by looking at its own cookies or HTML5 localStorage or indexedDB data) and can release to the RP caller whatever identity information is appropriate.
Crucially, however, notice that this login flow is entirely dependent upon the assumption that the IP subframe is accessing the same set of cookies, HTML5 storage, and/or indexedDB data as the top-level IP page. If the IP subframe doesn’t have access to the same storage, then it won’t recognize the user as logged in.
Unfortunately, this assumption has been problematic for many years, and it’s becoming even more dangerous over time as browsers ramp up their security and privacy features.
The root of the problem is that the IP subframe is considered a third-party resource, because it comes from a different domain (identity.example) than the page (news.example) into which it is embedded.
For privacy and security reasons, browsers might treat third-party resources differently than first-party resources. Examples include:
- The Block 3rd Party cookies option in most browsers
- The SameSite Cookie attribute
- P3P cookie blocking in Internet Explorer2
- Zone Partitioning in Internet Explorer and Edge Spartan3
- Safari’s Intelligent Tracking Protection
- Firefox Content Blocking
- Microsoft Edge Tracking Prevention
When a browser restricts access to storage for a 3rd party context, our theoretically simple login process falls apart. The IP subframe on the relying party doesn’t see the user’s login information because it is loaded in a 3rd party context. The authentication library is likely to conclude that the user is not logged in, and redirect them back to the login page. A frustrating and baffling infinite loop may result as the user is bounced between the RP and IP.
The worst part of all of this is that a site’s login process might usually work, but fail depending on the user’s browser choice, browser configuration, browser patch level, security zone assignments, or security/privacy extensions. As a result, a site owner might not even notice that some fraction of their users are unable to log in.
So, what’s a web developer to do?
The first task is awareness: Understand how your federated login library works — is it using cookies? Does it use subframes? Is the IP site likely to be considered a “Tracker” by popular privacy lists?
The second task is to build designs that are more resilient to 3rd-party storage restrictions:
- Be sure to convey the expected state from the Identity Provider’s login page back to the Relying Party. E.g. if your site automatically redirects from news.example to identity.example/login back to news.example/?loggedin=1, the RP page should take note of that URL parameter. If the authentication library still reports “Not signed in”, avoid an infinite loop and do not redirect back to the Identity Provider automatically.
- Authentication libraries should consider conveying identity information back to the RP directly, which will then save that information in a first party context.For instance, the IP could send the identity data to the RP via a HTTP POST, and the RP could then store that data using its own first party cookies.
- For browsers that support it, the Storage Access API may be used to allow access to storage that would otherwise be unavailable in a 3rd-party context. Note that this API might require action on the part of the user (e.g. a frame click and a permission prompt).
The final task is verification: Ensure that you’re testing your site in modern browsers, with and without the privacy settings ratcheted up.
 The call back to the IP might not use an IFRAME; it could also use a SCRIPT tag to retrieve JSONP, or issue a fetch/XHR call, etc. The basic principles are the same.
 P3P was removed from IE11 on Windows 10.
 In Windows 10 RS2, Edge 15 “Spartan” started sharing cookies across Security Zones, but HTML5 Storage and indexedDB remain partitioned.