I looked up the spec and apparently absolute URIs in request lines are perfectly valid, so i cannot change the server to reject them. But, as I said, there is no security implication, from the server's point of view:
GET
http://whaterver.com/some/path
is exactly the same as
GET /some/path
this is so because the server has no way to know its own domain name, if any, so it cannot tell if whatever.com is actually a valid host name for itself.