What is a significant downside of using cleartext transmission over HTTP for authentication?

Using cleartext transmission over HTTP for authentication poses a significant downside primarily because it can be intercepted by unauthorized users. When information, such as usernames and passwords, is transmitted in cleartext, it is sent openly over the network. This means that anyone with access to the network, including potential attackers, can easily capture and read this sensitive data using various tools and techniques. Such exposure can lead to account compromises, identity theft, and unauthorized access to secure systems.

In contrast, secure communication methods, like HTTPS, encrypt the data being transmitted, making it extremely difficult for an interceptor to gain any useful information even if they are able to capture the data packets. Ensuring the confidentiality of authentication data is critical in maintaining the integrity and security of web applications and user accounts. Hence, the risk of interception is a central concern when using cleartext transmission for authentication purposes.