What are some security vulnerabilities I can hunt for when combing error code 400 responses?

Hardening my NGINX HTTP servers and i've been combing over logs looking for items that are returning 200 codes and filtering out the known HTTP routes that I know should be returning 200's, thats exposing the ones that houldnt be there and I am then able to go to my servers and prevent them,

I then went through looking for anything with 500 error code responses to fix any of that I could find. I had multiple webservers up that …

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