External authentication

When you need to integrate with external services, you can use the Slack CLI to encrypt and to store OAuth2 credentials.

What is OAuth2, and why should you use it?

OAuth2 stands for Open Authorization 2.0. It is a standard designed to allow apps to access resources hosted by other apps on behalf of a user. Unlike basic authorization, where you share a password with a user, OAuth uses access tokens to verify a user's identity.

The following steps guide you through integrating your app with an external service using Google as an example.

1. Obtain your OAuth2 credentials

The first step is to obtain your OAuth credentials. To do that, create a new OAuth2 credential with the external service you'll be integrating with.

For our example, navigate to the Google API Console to obtain your OAuth2 client ID and client secret.

If you're asked to provide a redirect URL, use the following:

https://oauth2.slack.com/external/auth/callback

When you're done creating your credential, copy the credential's client ID and client secret, then head to your manifest file (manifest.ts).

2. Define your OAuth2 provider

Next, tell your app about your OAuth2 provider by defining an OAuth2 provider within your app. Inside your app's manifest, import DefineOAuth2Provider. Then, create a new provider instance.

An example provider instance for Google is below. You can define it right in your manifest, or put it in its own file and import it into the manifest.

// manifest.ts 
import { DefineFunction, DefineOAuth2Provider, DefineWorkflow, Manifest, Schema,} from "deno-slack-sdk/mod.ts";
// ...

// Define a new OAuth2 provider
// Note: replace <your_client_id> with your actual client ID
// if you're following along and creating an OAuth2 provider for 
// Google.
const GoogleProvider = DefineOAuth2Provider({
  provider_key: "google",
  provider_type: Schema.providers.oauth2.CUSTOM,
  options: {
    "provider_name": "Google",
    "authorization_url": "https://accounts.google.com/o/oauth2/auth",
    "token_url": "https://oauth2.googleapis.com/token",
    "client_id": "<your_client_id>.apps.googleusercontent.com",
    "scope": [
      "https://www.googleapis.com/auth/spreadsheets.readonly",
      "https://www.googleapis.com/auth/userinfo.email",
      "https://www.googleapis.com/auth/userinfo.profile",
    ],
    "authorization_url_extras": {
      "prompt": "consent",
      "access_type": "offline",
    },
    "identity_config": {
      "url": "https://www.googleapis.com/oauth2/v1/userinfo",
      "account_identifier": "$.email",
    },
  },
});
// ...

OAuth2 provider properties are described in the table below.

OAuth2 provider properties

Field Type Description
provider_key string The unique string identifier for a provider. An app cannot have two providers with the same unique identifier. Changing unique identifiers will be treated as the deletion of a provider. Providers with active tokens cannot be deleted.
provider_type Schema.providers.oauth2 The only supported provider type value at this time is Schema.providers.oauth2.CUSTOM.
provider_name string The name of your provider.
client_id string The client ID from your provider.
authorization_url string An OAuth2 requirement to complete the OAuth2 flow and to direct the user to the provider consent screen.
token_url string An OAuth2 requirement to complete the OAuth2 flow and to exchange the code with an access token.
scope array An OAuth2 requirement to complete the OAuth2 flow and to grant only the scopes provided to the access token.
identity_config object Used to obtain user identity by finding the account associated with the access token. Must include url (the endpoint the provider exposes to fetch user identity) and account_identifier (the field in the response that represents the identity). May also include headers (object), in case your OAuth2 provider requires additional headers to fetch user identity.
authorization_url_extras object (Optional) This is for providers who require additional query parameters in their authorize_url.

If the same user has two different accounts for the the same provider, but the identity_config is configured such that the same external_user_id value (obtained via the identity_config property) is returned in case of both the accounts, the user will not be able to issue multiple tokens for multiple accounts as existing tokens will be overwritten if the external user IDs are the same. This identity is what is used to identify different accounts for a user for the same provider, and only one token per identity is stored for a user, team, and provider combination.

3. Add your OAuth2 provider to your manifest

In your manifest file, insert the newly-defined provider into the externalAuthProviders property (if that property doesn't exist yet, go ahead and create it):

export default Manifest({
  //...
  // Tell your app about your OAuth2 providers here:
  externalAuthProviders: [GoogleProvider],
  //...
});

Now, with your OAuth2 provider defined and your manifest configured to use it, you can encrypt and store your client secret so that your app's users can utilize the OAuth2 authorization flow.

4. Encrypt and store your client secret

Your app needs to be deployed to Slack once in order to create a place to store your encrypted client secret. Run the slack deploy command in your terminal window:

slack deploy

This command will bring up a list of currently authorized workspaces. Select the workspace where your app will exist, and wait for the CLI to finish deploying.

When finished, stay in your terminal window to add your client secret for the newly-defined provider, ensuring that you wrap the secret string in double quotes as follows:

slack external-auth add-secret --provider google --secret "GOCSPX-abc123..."

Running the add-secret command will bring up a list of workspaces available to you. Find and select the workspace you recently deployed your app to; you'll know it's the workspace you recently installed the app in by locating the item in the list with your app's name and ID (e.g., myapp A01BC...) rather than "App is not installed to this workspace."

If you get a provider_not_found error, go back to your manifest file and check to make sure that you included your OAuth2 provider in the externalAuthProviders properties of your manifest definition.

If everything was successful, the CLI will let you know:

✨  successfully added external auth client secret for google

Great! With your app configured to interact with your defined OAuth2 provider, we can now initialize the OAuth2 sign-in flow, connecting your external provider to your Slack app.

5. Initialize the OAuth2 flow

Once your provider's client secret has been added, it's time to create a token for your app to interact with your OAuth2 provider with external-auth add.

Run the following command:

slack external-auth add

This will display a list of workspaces your app is deployed to. Select the one you're currently working in. Upon selection, you'll be provided a list of all providers that have been defined for this app, along with whether there's a secret and token.

$ slack external-auth add

? Select a provider  [Use arrows to move, type to filter]
> Provider Key: google
  Provider Name: Google
  Client ID: <your_id>.apps.googleusercontent.com
  Client Secret Exists? Yes
  Token Exists? No

If you have just created a provider, you'll notice that it reports no tokens existing. Let's go ahead and create a token by initializing the OAuth2 sign-in flow.

Select the provider you're working on, which will open a browser window for you to complete the OAuth2 sign-in flow according to your provider's requirements. You'll know you're successful when your browser sends you to a oauth2.slack.com page stating that your account was successfully connected.

Verify that a valid token has been created by re-running the external-auth add command:

slack external-auth add

? Select a provider  [Use arrows to move, type to filter]
> Provider Key: google
  Provider Name: Google
  Client ID: <your_id>.apps.googleusercontent.com
  Client Secret Exists? Yes
  Token Exists? Yes

If you see Token Exists? Yes, that means a valid auth token has been created, and you're ready to use OAuth2 in your app! Exit out of this command flow by entering Ctrl+C in your terminal β€” otherwise you'll be guided through the OAuth2 sign-in flow again.

6. Add OAuth2 to your function

Your custom functions can leverage your provider's token by configuring it to receive a Schema.slack.types.oauth2 type as an input parameter to your function's definition.

Here's how that might look if we were to use the sample function from the starter template:

// functions/sample_function.ts
import { DefineFunction, Schema, SlackFunction } from "deno-slack-sdk/mod.ts";

export const SampleFunctionDefinition = DefineFunction({
  callback_id: "sample_function",
  title: "Sample function",
  description: "A sample function",
  source_file: "functions/sample_function.ts",
  input_parameters: {
    properties: {
      message: {
        type: Schema.types.string,
        description: "Message to be posted",
      },
      // Define token here
      googleAccessTokenId: {
        type: Schema.slack.types.oauth2,
        oauth2_provider_key: "google",
      },
      user: {
        type: Schema.slack.types.user_id,
        description: "The user invoking the workflow",
      },
    },
    required: ["user"],
  },
  output_parameters: {
    properties: {
      updatedMsg: {
        type: Schema.types.string,
        description: "Updated message to be posted",
      },
    },
    required: ["updatedMsg"],
  },
});

export default SlackFunction(
  SampleFunctionDefinition, // Define custom function
  async ({ inputs, client }) => {
    // Get the token:
    const tokenResponse = await client.apps.auth.external.get({
      external_token_id: inputs.googleAccessTokenId,
    });
    if (tokenResponse.error) {
      const error =
        `Failed to retrieve the external auth token due to ${tokenResponse.error}`;
      return { error };
    }

    // If the token was retrieved successfully, use it:
    const externalToken = tokenResponse.external_token;
    // Make external API call with externalToken
    const response = await fetch("https://somewhere.tld/myendpoint", {
      headers: new Headers({
        "Authorization": `Bearer ${externalToken}`,
        "Content-Type": "application/x-www-form-urlencoded",
      }),
    });
    if (response.status != 200) {
      const body = await response.text();
      const error =
        `Failed to call my endpoint! (status: ${response.status}, body: ${body})`;
      return { error };
    }

    // Do something here
    const myApiResponse = await response.json();
    const updatedMsg =
      `:newspaper: Message for <@${inputs.user}>!\n\n>${myApiResponse}`;

    return { outputs: { updatedMsg } };
  },
);

7. Include OAuth2 input in a workflow step

To invoke your function that uses your token, pass an object with a credential_source to specify which user's token should be selected. For tokens added using slack external-auth add, use credential_source: "DEVELOPER".

// Somewhere in your workflow's implementation:
const sampleFunctionStep = SampleWorkflow.addStep(SampleFunctionDefinition, {
  user: SampleWorkflow.inputs.user,
  googleAccessTokenId: {
    credential_source: "DEVELOPER"
  },
});

Slack will automatically inject your app's oauth2 token ID into the oauth2 input property; you do not need to provide a token ID here.

Multiple collaborators can exist for the same app and each collaborator can create a token using the slack external-auth add command. If multiple valid tokens exist, Slack will inject the ID of the token that was created first during workflow execution.

8. Force refreshing a token programmatically

If you ever want to force a refresh of your external token as a part of error handling, retry mechanism, or something similar, you can use the sample code below:

// Somewhere in your functions error handling and retry logic:
const result = await client.apps.auth.external.get({
  external_token_id: inputs.googleAccessTokenId,
  force_refresh: true // default force_refresh is false
});

9. Deleting a token programmatically

If you ever want to delete your external token programmatically, you can use the sample code below. Bear in mind that once a token is deleted, all workflows that were previously using the token will no longer work.

This will not revoke the token from the provider's system. It will only delete the reference to the token from Slack and prevent it from being used within the external authentication system.

  // Somewhere in your function:
  await client.apps.auth.external.delete({
    external_token_id: inputs.googleAccessTokenId,
  });

Delete external auth tokens

If you'd like to delete your tokens and remove OAuth2 authentication from your Slack app, the following commands will allow you to do so:

Command Description
$ slack external-auth remove Choose a provider to delete tokens for from the list displayed.
$ slack external-auth remove --all Delete all tokens for the app by specifying the --all flag.
$ slack external-auth remove --provider provider_name --<app_name> Delete all tokens for a provider by specifying the --provider flag.

This will not revoke the token from the provider's system. It will only delete the reference to the token from Slack and prevent it from being used within the external authentication system.

Troubleshooting

You can view external authentication logs via slack activity. These logs contain information about errors encountered by users during the OAuth2 exchange and workflow execution. Below are some common errors:

Error Description
access_token_exchange_failed An error was returned from the configured token_url.
external_user_identity_not_found The configured account_identifier was not found in user identity response.
internal_error An internal system error happened. Please reach out to Slack if this occurs consistently.
invalid_identity_config_response url in the configured identity_config returned an invalid response.
invalid_token_response token_url returned an invalid response.
missing_client_secret No client secret was found for this provider.
no_refresh_token Token to refresh the expired access token does not exist.
oauth2_callback_error The OAuth2 provider returned an error.
oauth2_exchange_error There was an error while obtaining the OAuth2 token from the configured provider.
scope_mismatch_error Slack was not able to find an OAuth2 token that matched the scope configured on your provider.
token_not_found Slack was not able to find an OAuth2 token for this user and provider.

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