After I finished refactoring my android app to use a 3rd-party public API for the user data, I needed a way to quickly promote my application to users.
The API I used is inherently insecure with virtually no standard REST API security in place and designed with a top-down approach to suit their own mobile app. Their whole premise being to secure the mobile app with 2FA and having an API that's quite literally a bunch of wrapper functions around database queries exposed as endpoints.
All their CRUD operations are implemented as GET methods (so much for being idempotent) and their idea of security is just using a users mobile number to get their unique user ID that they call "encrypturl". 2FA is just two separate endpoints added to send and confirm the OTP send to the user's mobile, which again just advance the mobile flow and are completely orthogonal to the API design.
So I exploited their API yet again to get the email ids of every user they have in their system by trying every 10 digit combination with the 10th digit always being either 7, 8 or 9 (majority of Indian mobile phone numbers) until I get a hit and receive the users unique id required to access the rest of the API endpoints.
Using the user-id I get their email-ID from my own API that I built around their API to have better object models. Once I have the email-id I proceed to send them an email promoting my application designed using the Campaign and Template design editor of Sendgrid with their awesome developer API and python library that made programmatically sending emails a lot more easier.
So with the brute force method I found a new email-id every 5~6 seconds that translates into a new promotional email sent with every new email discovered. And my "Continuous Marketing" is still running with over 15000 emails sent already.
If you as well every find and are willing to exploit an insecure API for at least marketing gains perhaps this python script will make that a lot more easy for you.