Adding Custom Header to Microsoft Teams Login Requests Using mitmproxy Causes HTTP 404 Error

Hi Team,

We are using mitmproxy to add a custom header to all requests during the login process in the Microsoft Teams Windows application. The purpose of adding this custom header is to verify that the request is coming from a specific device by checking this header on our sso server.

Unfortunately, we are unable to bypass the login URLs. Adding the custom header is a crucial part of our verification process. However, we are encountering the following issue:

    When using mitmproxy to modify requests, the Microsoft Teams app fails to connect and shows the error:
    "We can't connect you. HTTP 404 login.microsoftonline.com"

Is there a way to add a custom header to requests from the Teams app without breaking the connection to the Microsoft login endpoint?

We appreciate any guidance on how to resolve this issue or insights into how Teams validates its login requests to avoid potential interference.

Looking forward to your reply.

Thanks