Why web servers should fear their clients abusing websockets in browsers for DoS

Abstract

This paper considers exploiting browsers for attacking Web servers. We demonstrate the generation of HTTP traffic to third-party domains without the user’s knowledge, that can be used e.g. for Denial of Service attacks. Our attack is primarily possible since Cross Origin Resource Sharing does not restrict WebSocket communications. We show an HTTP-based DoS attack with a proof of concept implementation, analyse its impact against Apache and Nginx, and compare the effectiveness of our attack to two common attack tools. In the course of our work we identified two new vulnerabilities in Chrome and Safari, i.e. two thirds of all browsers in use, that turn these browsers into attack tools comparable to known DoS applications like LOIC.