400 The Plain Http Request Was Sent To Https Port

This article discusses a commonly overlooked cybersecurity incident where an HTTP request is sent to the HTTPS port with 400 Bad Request responses. The presence of Nginx in the context raises suspicions that some level of web server configuration issues may be at play.

What happened

An HTTP request was sent to the HTTPS port with a 400 Bad Request response, suggesting that an error occurred in the request. This is not uncommon, but it's essential to understand why.

NGINX and web server configuration

Nginx is a popular HTTP server software that can also be used as a reverse proxy. In some cases, it may not handle HTTP requests correctly, leading to 400 Bad Request responses.

Common solutions

To fix the issue, you can try the following: