SAML: group memberships

Question

I was told that it is possible to give information about group membership during a SAML authentication request. We have to connect to an application that does use SAML (we are at the end that is creating the SAML answer XML). Authenticating a user works fine but I can't find a way to specify a "member of" or similar attribute.

Can you explain to me how to pass group memberships in SAML during authentication or have an example ?

I know there is a possibility to take care of authorization in SAML at a socalled Policy Decision Point. But this would mean that a SAML flow would happen for each or some (if batched) entities we want to check authorization for. Let me give you an example what we are trying to do. This example is made up but shows the problem we want to solve. Let's assume you have a hard drive with lots of directories and files on it. We use SAML to authenticate the person that tries to access that drive. Members of the group "admin" are allowed to read and write and members of group "user" are only given read permission. Because of this we want to send the group memberships of a user when he or she authenticates. Because otherwise it would mean that the application has to check for every file if the user is in the necessary group. If it was clear from the beginning (after authentication took place) that someone is a member of a specific group the application can cache that in memory.

Badtempered

I found something that seems to do exactly what i was searching for: <Assertion Version="2.0" ID="..." IssueInstant="..." xmlns="urn:oasis:names:tc:SAML:2.0:assertion"> ... <AttributeStatement> <Attribute Name="member­of"> <AttributeValue>group1</AttributeValue> <AttributeValue>group2</AttributeValue> <AttributeValue>group3</AttributeValue> </Attribute> </AttributeStatement> </Assertion> – Badtempered

Answer 1

As with most things in SAML, this depends on the identity provider as well as the application.

Many identity providers have access control to allow or deny specific users or groups to access an application. Sign-on is then denied by the IdP after authentication based on the ID of the application (from the SAML request) and access control configured in the IdP. If you just want to control who has access to the application, then this is all you need, and most, if not all, identity providers should have you covered.

Sometimes you also want different users or groups to have different permissions inside the application, or you want memberships in certain groups on the IdP to be mapped to membership in groups inside the app's own user and group database (or other application-specific group-like concepts). There is no standard for this, but some identity providers allow the definition of attribute mapping rules based on group membership. For example, in ADFS, you can create a claim rule that sets a specific SAML attribute to a specific value if and only if the user is a member of a specific group in AD. You would then set up such a rule and set a SAML attribute that your application understands to a value it understands.

Alternatively, if an application supports configuration of multiple IdPs, then you can configure one app / IdP mapping per group, and for each group add an attribute mapping rule with a hard-coded group name in an attribute that your application understands.

Barbrabarbuda