Mass Assignment is a security vulnerability that allows attackers to manipulate an API's data structures by modifying input parameters, potentially leading to unauthorized access, data leaks, or a complete takeover of the API's functionality. This issue arises when APIs transform incoming data into objects without proper filtering, which can be exploited by attackers to add or change fields unintentionally. The text provides examples, such as an API for booking systems, passenger lists, crew rosters, cargo manifests, and navigation systems, illustrating how Mass Assignment attacks can occur in various scenarios. To mitigate these risks, developers are advised to use whitelists for allowed fields, validate input parameters, and employ tools like Pynt to detect vulnerabilities, ensuring that only authorized modifications are permitted and the API remains secure.