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17. Allow read only external User Management

  • Status: proposed
  • Deciders: @butonic, @micbar, @rhafer
  • Date: 2022-02-08

Context and Problem Statement

oCIS needs to be integrated with various external Authentication and Identity Management Systems. Usually oCIS will have no administrative access to such a system and we will not be allowed to reconfigure it to suit our needs (e.g. we will not be able to enhance the schema of an already existing LDAP Directory). In most of the cases our access will be read-only.

Sidenote: There is a difference between users, identities and accounts: A user may have multiple identities which he can authenticate with, e.g. his facebook, twitter, microsoft or google identity. Multiple identities can be linked to an account in ocis, allowing to fall back to another identity provider should one of them shut down. This also allows migrating from one identity provider to another.

There are different cases where oCIS requires access to users:

  1. While we settled on using OpenID Connect (OIDC) as the authentication protocol for oCIS, we need to build a user object during authentication with at least an account UUID (to identify the account) and the email or a name (for display purposes).
  2. When searching for share recipients we need to be able to query existing users in the external identity management system
  3. When listing files we need to be able to look up a users display properties (username, email, avatar…) based on the account UUID

oCIS internally relies on a stable and persistent identifier (e.g. a UUID) for accounts in order to implement permissions and sharing. Unfortunately, some deployments are unable to deliver this kind of stable identifier for users:

  • In OIDC itself the only stable identifier that is guaranteed to be provided by the IDP is combination of the sub and iss claims. IDPs can optionally return other claims, but we cannot rely on a specific claim being present.
  • When no other services (LDAP, SCIM, …) are available that could be used look up a user UUID

Decision Drivers

  • oCIS should be a single binary that can run out of the box without external dependencies like an LDAP server.
  • Time: we want to build a release candidate asap.
  • oCIS should be easy to integrate with standard external identity management systems

Considered Options

There are two case to consider:

  • External identity management system provides an OIDC IdP and an interface to query users
  • External identity management system provides just an OIDC IdP with no possibility to query users

Decision Outcome

It’s not really possible single out any of the options for this ADR. In the end we will likely need to support both scenarios.

Positive Consequences:

  • Very flexible integration with a wide range of external systems

Negative consequences:

  • configuration complexity, high support efforts
  • Increasingly complex code in oCIS

Pros and Cons of the Options

External identity management system is read only and provides an interface to query users (e.g. Corporate Active Directory)

IdP sends sub & iss and mail or username claims, Identity Management System provides APIs (e.g. LDAP, SCIM, REST …) to lookup additional user information. All oCIS services use the CS3 API to look up the account for the given email or username, where CS3 then uses a backend that relies on the APIs provided by the IdM.

  • Good, because we can rely on the external identity management
  • Good, because ocis services only need to know about the CS3 user provider API, which acts as an abstraction layer for different identity management systems
  • Good, because there is only a single source of truth (the external IdM) and we don’t need to implement a synchronization mechanism to maintain an internal user database (we will likely need some form of caching though, see below)
  • Bad, because the identity management needs to provide a stable, persistent, non-reassignable user identifier for an account, e.g. owncloudUUID or ms-DS-ConsistencyGuid
  • Bad, because we need to implement tools that can change the account id when it did change anyway
  • Bad, because without caching we will hammer the identity management system with lookup requests

External identity management system is read only and does NOT provide an API to query users

Idp sends sub & iss and mail or username claims. We need to provision an internal account mapping, creating a unique ID, upon the first login of a user to be able to look up user properties by account id.

  • Good, because this has very little external requirements
  • Good, because we have accounts fully under our control
  • Bad, because we have to provide the user lookup APIs
  • Bad, because users will only a visible after the first login
  • Bad, because our internal account mapping might get out of date when user attribute (e.g. name or mail) change. At least until the next time that user logs in