There is a web app in our institute that anyone from outside world can access and it was running fine until today. The site is down currently and displays 404 error, According to Linux admin, it is because Mod security is blocking due to insecure in the HTML body that it returns to the user. So, it is down there is something in that was detected as insecure so it is being blocked with a 404 returned to the user.
The security functions get updated to check for newly found exploits and vulnerabilities and maybe it is possible that a vulnerability existed before that was not covered by mod security up until now.
My question is how do I figure out which part of html body is insecure?