401 - invalid authorization header format on client type application

[Snowwhitee]

I am developing an Android app where I use Fitbit to show sleep data to users.

At some point I was retrieving data from Fitbit API with no problems, but now I noticed that the login no longer works. Redirect happens correctly and I get the authorization code and state. When I try to exchange these for access and refresh tokens, I get this error: 

{"errors":[{"errorType":"invalid_client","message":"Invalid authorization header format. The header was not recognized to be a valid header for any of known implementations or a client_id was not specified in case of a public client Received header = null. Visit https://dev.fitbit.com/docs/oauth2 for more information on the Fitbit Web API authorization process."}],"success":false}

I also tried to debug this with the help of the troubleshooting guide on the website https://dev.fitbit.com/build/reference/web-api/troubleshooting-guide/oauth2-tutorial , but the website also returns the same error. I also tried to change my values (code challenge, state etc.) to be the same as on the site and still the same error occurs. 

Here is the POST request I am sending (I redacted the values): 

POST https://api.fitbit.com/oauth2/token
Body : client_id=CLIENT_ID&code=AUTH_CODE&code_verifier=CODE_VERIFIER&redirect_uri=REDIRECT_URL&grant_type=authorization_code
Headers : (1)
Content-Type : application/x-www-form-urlencoded

I have been searching for days for an answer to this and can not seem to find one. Am I missing something? I feel like it's weird that I even get the same error when trying it with the troubleshooting tool on the website so has something changed?

[GordonFitbit · Accepted solution]

Hi @Snowwhitee 

Would you please provide your authorization URL (i.e. https://www.fitbit.com/oauth2/authorize) so I can see what parameters and the values are being used when someone consents to your application?   I'll also try to consent to your application to see if I can replicate the problem.

Thanks!

Gordon

[Snowwhitee]

The authorization URL is: https://www.fitbit.com/oauth2/authorize?client_id=CLIENT_ID&response_type=code&redirect_uri=REDIRECT...

I sent CLIENT_ID and REDIRECT_URL to you as personal message

[GordonFitbit]

Thank you!   When you exchange the authorization code for the access token and refresh token using the /oauth2/token endpoint, are you providing the authorization header along with the Basic token?  For example

POST https://api.fitbit.com/oauth2/token
Authorization: Basic QUJDMTIzOkRFRjQ1Ng==
Content-Type: application/x-www-form-urlencoded

client_id=ABC123&code=<authorization_code>&code_verifier=<code_verifier>&grant_type=authorization_code

[Snowwhitee]

Currently, no I am not. I though it was not needed for client type application, the request I'm making is posted on the original message I posted. Also, I did try it with adding a header like described on the documentation, which is Authorization: Basic [base64encodedString(CLIENT_ID:CLIENT_SECRET)] but it still returned the same error. Additionally, the code I have has been working earlier last week even though I did not have the authorization header with the basic token in the exchange request.

[GordonFitbit]

 You are correct.   Is your application type set to "client" or "server" at https://dev.fitbit.com/apps?

[Snowwhitee]

Ugh. Apparently when debugging the issue I changed it to 'server' and thought I changed it back to 'client' but it was still set to 'server'. So the issue resolved by changing the application type. Sorry for the trouble! I don't know where the original issue came from though, because I didn't make the change until I received the error and was trying out different things to resolve it.

But it's working now!