A penetration tester is receiving an HTTP code 403 when using Postman to test an API directly. However, the penetration tester can normally interact with the web application when using the browser…

Configuring Burp Suite to use Postman as a proxy is an incorrect proxy chain setup and not relevant to the authorization issue.

Configuring Postman to use Burp Suite as a proxy would only route Postman's requests through Burp but wouldn't automatically add missing authentication headers like session cookies unless manually configured.

When a browser interacts with a web application, it automatically manages and sends session cookies, which authenticate the user. A 403 Forbidden error in Postman, contrasted with successful interaction via a browser (even proxied through Burp Suite), strongly suggests that Postman is missing the required session cookie for authentication or authorization. Therefore, configuring Postman to send the correct session cookie, obtained from the successful browser interaction, would resolve the 403 error and allow API access.

Configuring Burp Suite to send the Content-Type: application/json header is irrelevant because the issue is with Postman directly, and the 403 error is typically an authorization issue, not a content type issue.