Slack OpenID Connect Wrapper for Cognito

Do you want to add Slack as an OIDC (OpenID Connect) provider to an AWS Cognito User Pool? Have you run in to trouble because Slack only provides OAuth2.0 endpoints, and doesn't support OpenID Connect?

This project allows you to wrap your Slack OAuth App in an OpenID Connect layer, allowing you to use it with AWS Cognito.

Here are some questions you may immediately have:

• Why does Cognito not support federation with OAuth? Because OAuth provides no standard way of requesting user identity data. (see the background section below for more details).

• Why has no one written a shim to wrap general OAuth implementations with an OpenID Connect layer? Because OAuth provides no standard way of requesting user identity data, any shim must be custom written for the particular OAuth implementation that's wrapped.

• Slack is very popular, has someone written this specific custom wrapper before? As far as I can tell, if it has been written, it has not been open sourced. Until now!

Project overview

When deployed, this project sits between Cognito and Slack:

This allows you to use Slack as an OpenID Identity Provider (IdP) for federation with a Cognito User Pool.

The project implements everything needed by the OIDC User Pool IdP authentication flow used by Cognito.

It implements the following endpoints from the OpenID Connect Core Spec:

• Authorization - used to start the authorisation process (spec)
• Token - used to exchange an authorisation code for an access and ID token (spec)
• UserInfo - used to exchange an access token for information about the user (spec)
• jwks - used to describe the keys used to sign ID tokens (implied by spec)

It also implements the following OpenID Connect Discovery endpoint:

• Configuration - used to discover configuration of this OpenID implementation's endpoints and capabilities. (spec)

Out of the box, you can deploy it as a CloudFormation stack, or run it as a web server with node.

Getting Started

This project is intended to be deployed as a series of lambda functions alongside an API Gateway. This means it's easy to use in conjunction with Cognito, and should be cheap to host and run.

You can also deploy it as a http server running as a node app. This is useful for testing, exposing it to Cognito using something like ngrok.

1: Setup

You will need to:

• Create a Cognito User Pool (instructions).
• Configure App Integration for your User Pool (instructions). Note down the domain name.
• Create a Slack OAuth App (instructions, with the following settings:
  • Authorization callback URL: https://<Your Cognito Domain>/oauth2/idpresponse
  • Note down the Client ID and secret

2: Deployment with lambda and API Gateway

• Install the aws and sam CLIs from AWS:

  • aws (install instructions) and configured
  • sam (install instructions)

• Run aws configure and set appropriate access keys etc

• Set environment variables for the OAuth App client/secret, callback url, stack name, etc:

   cp example-config.sh config.sh
   vim config.sh # Or whatever your favourite editor is
  

• Run npm install and npm run deploy

• Note down the DNS of the deployed API Gateway (available in the AWS console).

3: Finalise Cognito configuration

• Configure the OIDC integration in AWS console for Cognito (described below, but following these instructions). The following settings are required:
  • Client ID: The Slack Client ID above
  • Authorize scope: openid users:read users:read.email
  • Issuer: either https://<Your API Gateway DNS name>/Prod (for lambda with API gateway, replace Prod with the correct stage name) or https://<your webserver>/ (for the node server).
  • If you have deployed the web app: Run discovery (big blue button next to Issuer).
  • If you have deployed the lambda/Gateway: For some reason, Cognito is unable to do OpenID Discovery. You will need to configure the endpoints manually. They are:
    • Authorization endpoint: https://<Your API Gateway DNS name>/Prod/authorize
    • Token endpoint: https://<Your API Gateway DNS name>/Prod/token
    • Userinfo endpoint: https://<Your API Gateway DNS name>/Prod/userinfo
    • JWKS uri: https://<Your API Gateway DNS name>/Prod/.well-known/jwks.json
• Configure the Attribute Mapping in the AWS console:

• Ensure that your new provider is enabled under Enabled Identity Providers on the App Client Settings screen under App Integration.

That's it! If you need to redeploy the lambda/API gateway solution, all you need to do is run npm run deploy again.

Logging

This shim also supports logging with Winston. By default, all logging goes to STDOUT. Beware that if you set the log level to DEBUG, then sensitive user information may be logged.

If you're using the node server, you can also use Splunk for logging. Environment variables configuring splunk are commented in example-config.sh. The Splunk HEC URL and access token are required, and you can also set the source, sourcetype & index for all logged events.

The details

Background

There are two important concepts for identity federation:

• Authentication: Is this user who they say they are?
• Authorisation: Is the user allowed to use a particular resource?

OAuth

OAuth2.0 is an authorisation framework, used for determining whether a user is allowed to access a resource (like private user profile data). In order to do this, it's usually necessary for authentication of the user to happen before authorisation.

This means that most OAuth2.0 implementations (including Slack) include authentication in a step of the authorisation process. For all practical purposes, most OAuth2.0 implementations (including Slack)can be thought of as providing both authorisation and authentication.

Below is a diagram of the authentication code flow for OAuth:

(The solid lines are http requests from the browser, and then dashed lines are back-channel requests).

As you can see in the diagram, a drawback of OAuth is that it provides no standard way of finding out user data such as name, avatar picture, email address(es), etc. This is one of the problems that is solved by OpenID.

OpenID Connect

To provide a standard way of learning about users, OpenID Connect is an identity layer built on top of OAuth2.0. It extends the token endpoint from OAuth to include an ID Token alongside the access token, and provides a userinfo endpoint, where information describing the authenticated user can be accessed.

OpenID Connect describes a standard way to get user data, and is therefore a good choice for identity federation.

A custom shim for Slack

This project provides the OpenID shim to wrap Slack's OAuth implementation, by combining the two diagrams:

The userinfo request is handled by joining two Slack API requests: /user and /user/emails.

You can compare this workflow to the documented Cognito workflow here

Code layout

├── scripts             # Bash scripts for deployment and key generation
├── src                 # Source code
│    ├── __mocks__      # Mock private key data for tests
│    └── connectors     # Common code for both lambda and web handlers
│         ├── lambda    # AWS lambda handlers
│         │    └── util # Helper functions for lambdas
│         └── web       # Express.js webserver (useful for local deployment)
├── docs                # Documentation images
├── config              # Configuration for tests
├── dist-web            # Dist folder for web server deployment
└-- dist-lambda         # Dist folder for lambda deployment

npm targets

• build and build-dist: create packages in the dist-lambda folder (for the lambda deployment) and the dist-web folder (for the node web server).
• test: Run unit tests with Jest
• lint: Run eslint to check code style
• test-dev: Run unit tests continuously, watching the file system for changes (useful for development)
• deploy: This script builds the project, then creates and deploys the cloudformation stack with the API gateway and the endpoints as lambdas

Scripts

• scripts/create-key.sh: If the private key is missing, generate a new one. This is run as a preinstall script before npm install
• scripts/deploy.sh: This is the deploy part of npm run deploy. It uploads the dist folder to S3, and then creates the cloudformation stack that contains
  the API gateway and lambdas

Tests

Tests are provided with Jest using chai's expect, included by a shim based on this blog post.

Pact consumer tests for the Slack API connection are provided in src/slack.pact.test.js. There is currently no provider validation performed.

Private key

The private key used to make ID tokens is stored in ./jwtRS256.key once scripts/create-key.sh is run (either manually, or as part of npm install). You may optionally replace it with your own key - if you do this, you will need to redeploy.

Missing features

This is a near-complete implementation of OpenID Connect Core. However, since the focus was on enabling Cognito's authentication flow, you may run in to some missing features if you wish to use it with a different client.

Missing Connect Core Features:

• Private key rotation (spec)
• Refresh tokens (spec)
• Passing request parameters as JWTs (spec)

If you don't know what these things are, you are probably ok to use this project.

Missing non-core features:

A full OpenID implementation would also include:

• The Dynamic client registration spec
• The OpenID Connect Discovery endpoints beyond openid-configuration

Known issues

See the issue tracker for an up to date list.

Extending

This section contains pointers if you would like to extend this shim.

Using other OAuth providers

If you want to use a provider other than Slack, you'll need to change the contents of userinfo in src/openid.js.

Using a custom Slack location

If you're using an on-site Slack install, you will need to change the API endpoints used when the slack object is initialised.

Including additional user information

If you want to include custom claims based on other Slack data, you can extend userinfo in src/openid.js. You may need to add extra API client calls in src/slack.js

Contributing

Contributions are welcome, especially for the missing features! Pull requests and issues are very welcome.

FAQ

How do I use this to implement Cognito logins in my app?

Login requests from your app go directly to Cognito, rather than this shim. This is because the shim sits only between Cognito and Slack, not between your app and Slack. See the Cognito app integration instructions for more details.

Can I use this shim to connect to Slack directly from another OpenID client?

Yes. This implementation isn't complete, as it focusses exclusively on Cognito's requirements. However, it does follow the OpenID spec, and is complete enough to be able to use it as an OpenID connect provider. See the missing features section above for one or two caveats.

How do I contact you to tell you that I built something cool with this code?

If you build anything cool, ping me @JonesTim on twitter (or open an issue if you have any problems).

License

BSD 3-Clause License