I'm writing a web application with spring boot and want that the user is able to tell me what his identity provider is. In a same way as I can do it on Stackoverflow.
How can I identify a user in a unique way? I already read that I should use the sub/Subject for distinguishing users. Is this unique when using multiple providers?
My fear is that a user provides a malicious identity provider which then tells my app he is a different user.
How can I identify a user in a unique way? I already read that I should use the sub/Subject for distinguishing users. Is this unique when using multiple providers?
You'd store the combination of (iss, sub) as an identifier that is globally unique. As Kavindu mentioned already, the sub claim by itself is only locally unique.
My fear is that a user provides a malicious identity provider which then tells my app he is a different user.
There are two ways of using "multiple providers" with your app, via:
If someone's real identity is important to you, then you can choose the providers you trust to provide someone's identity details. People then can only sign in via one of the providers in your list.
But if it doesn't matter that much (normal username/password registrations also don't provide any guarantees), then you could also choose to let people login with a provider of their choosing. The correct provider may then be discovered from the user's "handle" via OIDC Discovery.
Q : How can I identify a user in a unique way? Is this unique when using multiple providers?
According to OpenID Conenct specification, "sub" claim is locally unique. Following is the extraction from specification which highlight this (reference),
Subject Identifier. A locally unique and never reassigned identifier within the Issuer for the End-User
So when you are dealing with a single identity provider, "sub" claim is unique. But that does not hold for multiple providers.
Q : My fear is that a user provides a malicious identity provider which then tells my app he is a different user.
I doubt about this scenario. Does your application allow end users to register different identity providers as they want ?
In OpenID Connect, there's a application registration step. Your application need a client identifier. Also registration process involve redirect URL registration. All these are done in registration step. Without these, OpenID Connect will not function.
Adding to that, different providers behave differently. For example, though "sub" is the standard claim to communicate end user identity, a provider may use a custom claim to define a specific user identity. This is allowed by OpenID Connect specification. So your application must only support known, well established identity providers which you know at the application design time.
