What does this attack string do? How do I mitigate it?

Question

Today I decided to run the apache-scalp script and it found a few issues. One was ranked as Impact 5, and I'd like to know if someone could explain what this does, and how to disable it?

The request was:

- - [25/Jun/2015:20:08:26 -0500] "GET /xc9/xbf/xe1/xf9/xa0/xbb/xd4./xf6/xe8%5D/x15K/xa3Tn/xe0;/xb4/xa2/xdd/xe4%+/x1e/x0e/%5Cxbfp%5Cxd7%5Cxd1%5Cxa2.%5Cxd0%5Cxb7%5C%5CA~%20%5Cx1a%5Cxec%5D8%5Cxd4%5Cx98%5Cxec HTTP/1.1" 400 471 "-" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0"

From what I see, it returns a 400. This seems correct to me and I'm hoping this is just a false positive. But I just wanted to be sure.

Answer 1

I don't know what he is trying to do but returning 400 is fine. It stands for "Bad Request" so you server did not process the request further. You don't need to do anything as this request did just nothing.