Modeling Authorization for Slack with OpenFGA

This tutorial explains how to model permissions for a communication platform like Slack using OpenFGA.

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Before you start

In order to understand this guide correctly you must be familiar with some OpenFGA concepts and know how to develop the things that we will list below. It would be helpful to have an understanding of some concepts of OpenFGA before you start.

OpenFGA concepts

Direct access

You need to know how to create an authorization model and create a relationship tuple to grant a user access to an object. Learn more →

Modeling concentric relationships

You need to know how to update the authorization model to allow having nested relations such as all writers are readers. Learn more →

Concepts & configuration language

• Some OpenFGA Concepts
• Configuration Language

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What you will be modeling

Slack is a messaging app for businesses that connects people to the information they need. By bringing people together to work as one unified team, Slack transforms the way organizations communicate. (Source: What is Slack?) In this tutorial, you will build a subset of the Slack permission model (detailed below) in OpenFGA, using some scenarios to validate the model.

As reference, you can refer to Slack’s publicly available docs:

• Role Management at Slack
• Types of Roles in Slack
• Permissions by Role in Slack
• Manage a Workspace on Enterprise Grid
• Manage channel posting permissions

Note: For brevity, this tutorial will not model all of Slack’s permissions. Instead, it will focus on modeling the scenarios outlined below.

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Requirements

This tutorial will focus on the following sections (this is a partial list of Slack’s roles): Workspace Roles:

• Guest: This type of user is limited in their ability to use Slack, and is only permitted to see one or multiple delegated channels.
• Member: This is the base type of user that does not have any particular administrative abilities, but has basic access to the organization’s Slack workspaces. When an administrative change needs to be made, these users need the support of admins and owners to make the changes.
• Legacy Admin: This type of user is the basic administrator of any organization, and can make a wide variety of administrative changes across Slack, such as renaming channels, archiving channels, setting up preferences and policies, inviting new users, and installing applications. Users with this role perform the majority of administrative tasks across a team.

System Roles:

• Channels Admin: This type of user has the permission to archive channels, rename channels, create private channels, and convert public channels into private channels.

Channel Settings:

• Visibility:
  • Public: Visible to all members and open to join
  • Private: Visible to admins and invited members
• Posting Permissions:
  • Open: Anyone can post
  • Limited: Only allowed members can post

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Defined scenarios

Use the following scenarios to be able to validate whether the model of the requirements is correct. There will be the following users:

• Amy
• Bob
• Catherine
• David
• Emily

These users will interact in the following scenarios:

• You will assume there is a Slack workspace called Sandcastle
• Amy is a legacy admin of the Sandcastle workspace
• Bob is a member of the Sandcastle workspace with a channels admin role (Read more about system roles at Slack here)
• Catherine and Emily are normal members of the Sandcastle workspace, they can view all public channels, as well as channels they have been invited to
• David is a guest user with only view and write access to #proj-marketing-campaign, one of the public channels in the Sandcastle workspace
• Bob and Emily are in a private channel #marketing-internal in the Sandcastle workspace which only they can view and post to
• All members of the Sandcastle workspace can view the general channel, but only Amy and Emily can post to it

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Modeling workspaces & channels

The goal by the end of this post is to ask OpenFGA: Does person X have permission to perform action Y on channel Z? In response, you want to either get a confirmation that person X can indeed do that, or a rejection that they cannot. E.g. does David have access to view #general? The OpenFGA is based on Zanzibar, a Relation Based Access Control system. This means it relies on objects and user relations to perform authorization checks. Setting aside the permissions, you will start with the roles and learn how to express the requirements in terms of relations you can feed into OpenFGA. The requirements stated:

• Amy is a legacy admin of the Sandcastle workspace
• Bob is a channels admin of the Sandcastle workspace
• Catherine and Emily are a normal members of the Sandcastle workspace
• David is a guest user

Here is how you would express than in OpenFGA’s authorization model: You have a type called “workspace”, and users can be related to it as a legacy_admin, channels_admin, member and guest

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01. Individual permissions

To keep things simple and focus on OpenFGA rather than Slack complexity, we will model only four roles (legacy_admin, channels_admin, member, guest). At the end of this section we want to have the following permissions represented To represent permissions in OpenFGA we use relations. For workspace permissions we need to create the following authorization model: The OpenFGA service determines if a user has access to an object by checking if the user has a relation to that object. Let us examine one of those relations in detail: If we want to say amy is a legacy_admin of workspace:sandcastle we create this relationship tuple We can now ask OpenFGA “is amy a legacy_admin of workspace:sandcastle?” We can also say that catherine is a member of workspace:sandcastle: And verify by asking OpenFGA Catherine, on the other hand, is not a legacy_admin of workspace:sandcastle. Repeat this process for the other relationships

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Verification

To verify, we can issue check request to verify it is working as expected. Let’s try to verify the followings:

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02. Updating The workspace Authorization Model With Implied Relations

Some of the queries that you ran earlier, while returning the correct response, do not match reality. One of which is: As you saw before, running this query will return amy is not a member of workspace:sandcastle, which is correct based on the data you have given OpenFGA so far. But in reality, Amy, who is a legacy_admin already has an implied channels_admin and member relations. In fact anyone (other than a guest) is a member of the workspace. To change this behavior, we will update our system with a concentric relationship model. With the following updated authorization model, you are informing OpenFGA that any user who is related to a workspace as legacy_admin, is also related as a channels_admin and a member . We can then verify amy is a member of workspace:sandcastle. We can check for other users and relationships.

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03. Updating the authorization model to include channels

So far, you have modeled the users’ relations to the workspace itself. In this task you will expand the model to include the relations concerning the channels. By the end of it, you will run some queries to check whether a user can view or write to a certain channel. Queries such as:

• is david related to channel:general as viewer? (expected answer: No relation, as David is a guest user with only a relation to #proj-marketing-campaign)
• is david related to channel:proj_marketing_campaign as viewer? (expected answer: There is a relation, as there is a relation between David and #proj-marketing-campaign as a writer)
• is bob related to channel:general as viewer? (expected answer: There is a relation, as Bob is a member of the Sandcastle workspace, and all members of the workspace have a viewer relation to #general)

The requirements are:

• Amy, Bob, Catherine and Emily, are normal members of the Sandcastle workspace, they can view all public channels, in this case: #general and #proj-marketing-campaign
• David, a guest user, has only view and write access to the #proj-marketing-campaign channel
• Bob and Emily are the only ones with either view or write access to the #marketing-internal channel
• Amy and Emily are the only ones with write access to the #general channel

The possible relations to channels are:

• Workspace includes the channel, consider the relation that of a parent workspace
• A user can be a viewer and/or writer on a channel

The authorization model already has a section describing the workspace, what remains is describing the channel. That can be done by adding the following section to the configuration above:

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Implied relation

There is an implied relation that anyone who can write to a channel can also read from it, so the authorization model can be modified to be: As a result, the authorization model is:

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Updating relationship tuples

What remains is to add the relationship tuples to indicate the relation between the users, workspace and the channels. The Sandcastle workspace is a parent workspace of the #general, #marketing-internal and #proj-marketing-campaign channels.

#general channel

The #general channel is a public channel visible to all the members of the workspace. In OpenFGA, you represent this relation in the form of the following relationship tuple: And to indicate that Amy and Emily can write to it:

#marketing-internal channel

The #marketing-internal is visible to only Bob and Emily. They can view and write in it.

#proj-marketing-campaign channel

The #proj-marketing-campaign is public to all members of the Sandcastle workspace. They can view and write in it. David is a guest user who can also view and write to #proj-marketing-campaign

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Verification

Now that you have added the necessary relationship tuples, you will check to make sure that your configuration is valid. First, we want to ensure david is not related to channel:general as viewer. David should be related to channel:proj_marketing_campaign as viewer. Repeat this for the following relations

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Summary

• Have a basic understanding of authorization and OpenFGA Concepts.
• Understand how to model authorization for a communication platform like Slack using OpenFGA.

In this tutorial, you:

• were introduced to fine grain authentication and OpenFGA.
• learned how to build and test an OpenFGA authorization model for a communication platforms like Slack.

Upcoming tutorials will dive deeper into OpenFGA, introducing concepts that will improve on the model you built today, and tackling different permission systems, with other relations and requirements that need to be met. If you are interested in learning more about Authorization and Role Management at Slack, check out the Auth0 Fine-Grained Authorization (FGA) team’s chat with the Slack engineering team.

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Exercises for you

• Try adding more relationship tuples to represent other users and channels being added. Then run queries to make sure that the authorization model remains valid.
• Update the configuration to model more Slack permissions (workspace owners, Slack orgs), then add the relationship tuples necessary and run some queries to validate your configuration.