Over on the Fediverse, Simon Tatham had a comment about (using) cloud identity providers that's sparked some discussion. Yesterday I wrote about the facets of identity providers. Today I'm sort of writing about why you might not want to run your own identity provider, despite the hazards of depending on the security of some outside third party. I'll do this by talking about what I see as being involved in the whole thing.

The hardcore option is to rely on no outside services at all, not even for multi-factor authentication. This pretty much reduces your choices for MFA down to TOTP and perhaps WebAuthn, either with devices or with hardware keys. And of course you're going to have to manage all aspects of your MFA yourself. I'm not sure if there's capable open source software here that will let people enroll multiple second factors, handle invalidating one, and so on.

One facet of being an identity provider is managing identities. There's a wide variety of ways to do this; there's Unix accounts, LDAP databases, and so on. But you need a central system for it, one that's flexible enough to cope with with real world, and that system is load bearing and security sensitive. You will need to keep it secure and you'll want to keep logs and audit records, and also backups so you can restore things if it explodes (or go all the way to redundant systems for this). If the identity service holds what's considered 'personal information' in various jurisdictions, you'll need to worry about an attacker being able to bulk-extract that information, and you'll need to build enough audit trails so you can tell to what extent that happened. Your identity system will need to be connected to other systems in your organization so it knows when people appear and disappear and can react appropriately; this can be complex and may require downstream integrations with other systems (either yours or third parties) to push updates to them.

Obviously you have to handle primary authentication yourself (usually through passwords). This requires you to build and operate a secure password store as well as a way of using it for authentication, either through existing technology like LDAP or something else (this may or may not be integrated with your identity service software, as passwords are often considered part of the identity). Like the identity service but more so, this system will need logs and audit trails so you can find out when and how people authenticated to it. The log and audit information emitted by open source software may not always meet your needs, in which case you may wind up doing some hacks. Depending on how exposed this primary authentication service is, it may need its own ratelimiting and alerting on signs of potential compromised accounts or (brute force) attacks. You will also definitely want to consider reacting in some way to accounts that pass primary authentication but then fail second-factor authentication.

Finally, you will need to operate the 'identity provider' portion of things, which will probably do either or both of OIDC and SAML (but maybe you (also) need Kerberos, or Active Directory, or other things). You will have to obtain the software for this, keep it up to date, worry about its security and the security of the system or systems it runs on, make sure it has logs and audit trails that you capture, and ideally make sure it has ratelimits and other things that monitor for and react to signs of attacks, because it's likely to be a fairly exposed system.

If you're a sufficiently big organization, some or all of these services probably need to be redundant, running on multiple servers (perhaps in multiple locations) so the failure of a single server doesn't lock you out of everything. In general, all of these expose you to all of the complexities of running your own servers and services, and each and all of them are load bearing and highly security sensitive, which probably means that you should be actively paying attention to them more or less all of the time.

If you're lucky you can find suitable all-in-one software that will handle all the facets you need (identity, primary authentication, OIDC/SAML/etc IdP, and perhaps MFA authentication) in a way that works for you and your organization. If not, you're going to have to integrate various different pieces of software, possibly leaving you with quite a custom tangle (this is our situation). The all in one software generally seems to have a reputation of being pretty complex to set up and operate, which is not surprising given how much ground it needs to cover (and how many protocols it may need to support to interoperate with other systems that want to either push data to it or pull data and authentication from it). As an all-consuming owner of identity and authentication, my impression is that such software is also something that's hard to add to an existing environment after the fact and hard to swap out for anything else.

(So when you pick an all in one open source software for this, you really have to hope that it stays good, reliable software for many years to come. This may mean you need to build up a lot of expertise before you commit so that you really understand your choices, and perhaps even do pilot projects to 'kick the tires' on candidate software. The modular DIY approach is more work but it's potentially easier to swap out the pieces as you learn more and your needs change.)

The obvious advantage of a good cloud identity provider is that they've already built all of these systems and they have the expertise and infrastructure to operate them well. Much like other cloud services, you can treat them as a (reliable) black box that just works. Because the cloud identity provider works at a much bigger scale than you do, they can also afford to invest a lot more into security and monitoring, and they have a lot more visibility into how attackers work and so on. In many organizations, especially smaller ones, looking after your own identity provider is a part time job for a small handful of technical people. In a cloud identity provider, it is the full time job of a bunch of developers, operations, and security specialists.

(This is much like the situation with email (also). The scale at which cloud providers operates dwarfs what you can manage. However, your identity provider is probably more security sensitive and the quality difference between doing it yourself and using a cloud identity provider may not be as large as it is with email.)