Glen Mazza's Weblog

Salesforce's Mohith Shrivastava's second Connected App tutorial shows how the JWT Bearer Token Flow can be used with Postman to obtain access tokens. Separately, SFDCStop shows how to use an access token to retrieve data from Salesforce. Summary of the process for getting an access token via JWT Bearer Token flow is given below, along with the alternative Username-Password flow.

Obtaining Access Token via JWT Bearer Token Flow

Mohith's second tutorial largely follows the Trailblazer documentation for creating the JWT necessary for obtaining an access token. First, within Postman, open a POST request to https://test.salesforce.com/services/oauth2/token (or login.salesforce.com for production) with two query attributes, grant_type hardcoded to urn:ietf:params:oauth:grant-type:jwt-bearer and assertion with a value that can be generated at JWT.io as follows:

1. Place {"alg":"RS256"} in the JWT header.

2. Place the following in the JWT payload, updating the iss value with the Client ID of the connected app, the aud value (should be either https://test.salesforce.com for sandbox instances, https://login.salesforce.com for all others, including developer instances), the sub with the Salesforce resource owner (the user ID, usually expressed as an email), and exp value being the ten-digit current UNIX timestamp. Ensure there are no spaces or carriage returns in the payload:

   {"iss":"--client id--","sub":"--salesforce user--","aud":"https://test.salesforce.com","exp":"1616865336"}

   You may also wish to add a "jti" field (JWT ID), the value of which can be a randomly generated UUID. As stated in the Trailblazer documentation linked above, if a "jti" field is added, Salesforce will make sure the JWT hasn't been used to make a prior access token request, serving as a guard against replay attacks. If it has been used, it will return a 400 response code with message { "error": "invalid_request", "error_description": "jti already in use"}.

3. Finally, in the Verify Signature portion, as generated in the previous tutorial, place the PublicCert.crt contents and the private key found in key.pem (including the BEGIN/END delimiters) in the corresponding boxes. JWT.io should report "Signature Verified", upon which you can copy the left-side as the assertion value into the Postman call. Make the call and you should get an access token in response.

   

Obtaining Access Token via Username-Password Flow

This process is much more straightforward as no keys are used. As shown in the Salesforce documentation, you can simply make a POST request with the following five query parameters: grant_type, client_id, client_secret, username, and password.

Making Salesforce calls using the Access Token

As demo'ed in the SFDCStop video, can create a GET request to say https://yoursalesforceinstance.salesforce.com/services/data/v50.0/query?q=SELECT+Name,Type+FROM+Account. Under the Authorization tab, select "Bearer", copy-and-paste the access token in, and make the API call.

More information on SOQL calls are available here.