The power of using external authentication information in a web application

May 20, 2024

Recently, a colleague at work asked me if we were using the university's central authentication system to authenticate access to our Grafana server. I couldn't give them a direct answer because we use Apache HTTP Basic Authentication with a local password file, but I could give them a pointer. Grafana has the useful property that it can be configured to take authentication information from a reverse proxy through a HTTP header field, and you can set up Apache with Shibboleth authentication so that it uses the institutional authentication system (with appropriate work by several parties).

(Grafana also supports generic OAuth2 authentication, but I don't know if the university provides an OAuth2 (or OIDC) interface to our central authentication system.)

This approach of outsourcing all authentication decisions to some front-end system is quite useful to enable people to solve their specific authentication challenges for your software. The front-end doesn't particularly have to be Apache; as Grafana shows, you can have the front-end stuff some information in an additional HTTP header and rely on that. Apache is useful because people have written all sorts of authentication modules for it, but there are probably other web servers with similar capabilities.

You might think that your web application relying on OAuth2 would get you similar capabilities, but I believe there are two potential limitations. First, as the Grafana documentation on generic OAuth2 authentication demonstrates, this can make access control complicated. Either the system administrator has to stand up an OAuth2 provider that only authenticates some people and perhaps modifies the OAuth2 information returned from an upstream source, or your software needs to support enough features that you can exclude (or limit) certain people. Second, it doesn't necessarily enable a "single sign on" experience by default, because the OAuth2 provider may require people to approve passing information to your web application before it gives you the necessary information.

(Now that I look, I see that I can set my discount OIDC IdP to not require this approval, if I want to. In a production OIDC environment where the only applications are already trusted internal ones, we should set things this way, which I believe would give us a transparent SSO environment for purely OIDC/OAuth2 stuff.)

The drawback of entirely outsourcing authentication to a front-end system is that your web application doesn't provide busy system administrators with a standalone 'all in one' solution where they can plug in some configuration settings and be done; instead, they need a front-end as well and to configure authentication in it. I suspect that this is why Grafana supports other authentication methods (including OAuth2) along with delegating authentication to a front-end system. On the other hand, a web application that's purely for your own internal use could rely entirely on the front-end without worrying about authentication at all (this is effectively what we do with our use of Apache's HTTP Basic Authentication support).

My view is that your (general purpose) web application will never be able to support all of the unusual authentication systems that people will want; it's just too much code to write and maintain. Allowing a front-end to handle authentication (and with it possibly authorization, or at least primary authorization) makes that not your problem. You can implement a few common and highly requested stand alone authentication and authorization mechanisms (if you want to) and then push everything else away with 'do it in a front-end'.

PS: I'm not sure if what Grafana supports is actually OAuth2 or if it goes all the way to require OIDC, which I believe is effectively a superset of OAuth2; some of its documentation on this certainly seems to actually be working with OIDC providers. When past me wrote about mapping out my understanding of web based SSO systems, I neglected to work out and write down the practical differences between OAuth2 and OIDC.

Written on 20 May 2024.
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Last modified: Mon May 20 23:37:55 2024
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