Cognito App Client Client Credentials Configuration

Question

I have a user pool in Cognito which has two app clients: one with Authorization Code flow (works perfectly) and another with Client Credentials flow.
The second one is not working properly. I have created the Resource Server (say with `resource-server-id` identification and `custom-scope` as custom scope).
Then, I created the Client Credentials flow app client. It does not have any authentication flows or identity providers: just default token expiration values, client id and client secret.
First I was thinking the issue was in my application but I can't execute a POST: it always returns `{"code":"BadRequest","message":"The server did not understand the operation that was requested.","type":"client"}%`.

According to the [documentation](https://docs.aws.amazon.com/cognito/latest/developerguide/token-endpoint.html), it should be as simple as below:

```
POST https://mydomain.auth.us-east-1.amazoncognito.com/oauth2/token >
                            Content-Type='application/x-www-form-urlencoded'&
                            Authorization=Basic ZGpjOTh1M2ppZWRtaTI4M2V1OTI4OmFiY2RlZjAxMjM0NTY3ODkw
                            
                            grant_type=client_credentials&
                            client_id=1example23456789&
                            scope=resourceServerIdentifier1/scope1 resourceServerIdentifier2/scope2             
```

I tried a series of combinations: removing `scope`, adding `client_secret` and I'm always getting the same response, which is not intuitive and helpless.

The only difference between the "awesome" documentation and my request is the Amazon domain pattern: https://mydomain.auth.us-east-1.amazoncognito.com/oauth2/token TO https://cognito-idp.REGION.amazonaws.com/USER_POOL_ID/oauth2/token

What can be the problem?

Accepted Answer

The error message you're receiving from Amazon Cognito, **{"code":"BadRequest","message":"The server did not understand the operation that was requested.","type":"client"}**, is a generic error message that can be caused by a variety of issues. Here are some common troubleshooting steps and considerations to help you identify and resolve the problem:

**Endpoint URL: **Ensure you're using the correct endpoint URL. The endpoint format you mentioned (**https://cognito-idp.REGION.amazonaws.com/USER_POOL_ID/oauth2/token**) is typically used for operations like signing up and signing in users, not for the OAuth 2.0 token endpoint. Stick to the format provided in the documentation (**https://mydomain.auth.us-east-1.amazoncognito.com/oauth2/token**).

**Authorization Header:** Ensure that the **Authorization** header is correctly formatted. The value should be the Base64-encoded string of **client_id:client_secret**. Double-check that you're encoding it correctly and that there are no extra characters or spaces.

**Content-Type:** Ensure that the **Content-Type** header is set to **application/x-www-form-urlencoded**.

**grant_type:** Ensure that the **grant_type** parameter is set to **client_credentials**.

**Scopes:** Ensure that the scopes you're requesting are correctly defined in the Resource Server in Cognito. If you're unsure, try the request without the scope parameter to see if that's the cause of the issue.

**Client Configuration:** Double-check the app client configuration in the Cognito User Pool:
- Ensure that the app client is enabled for the client_credentials flow.
- Ensure that the app client has the necessary scopes assigned.
- Ensure that the app client doesn't have any authentication flows or identity providers that might interfere with the client_credentials flow.

**Resource Server:** Ensure that the Resource Server and its scopes are correctly configured in the Cognito User Pool.

**HTTPS:** Ensure that you're making the request over HTTPS, not HTTP.

**Logging:** If possible, enable logging for your Cognito User Pool to get more detailed error messages. This can provide more insights into what might be causing the issue.

**Tools:** Use tools like Postman or cURL to test the token endpoint directly. This can help you isolate the issue and determine if it's related to your application or the Cognito configuration.

Remember, the **client_credentials** flow is designed for machine-to-machine authentication, where an application requests a token to access its own resources, not on behalf of a user. Ensure that this flow aligns with your use case and that the app client is correctly configured for this flow.

Cognito App Client Client Credentials Configuration

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