403 Forbidden when uploading file to sharepoint using REST API. Permission issue or missing header?

Question

I am new to Sharepoint and REST APIs and am currently having an issue uploading a file using REST API. I am getting error System.UnauthorizedAccessException","message":{"lang":"en-US","value":"Access denied."}}}|403 when uploading a file.

I am using OAuth and have the tenant ID, client id, and client secret. I have no issue getting the client credentials. But when calling: https://[tenant].sharepoint.com/sites/[site]/_api/web/GetFolderByServerRelativeUrl('[folder dir]')/Files/add(url='test_file.txt',overwrite=true) then I get the 403 error as response.

Here are my HTTP Headers:

"Authorization": "Bearer" + access token
"Accept": "application/json;odata=verbose"
"Content-Length": *length of file*

Is this an add-in permission issue or am I missing a header? Thanks in advance for your help.

Answer 1

You need a Form Digest - an X-RequestDigest header - to POST to SharePoint APIs.

How to get 'form digest value' to use in REST call?