Off-Path Attacking the Web

TL;DR

This paper demonstrates how off-path attackers can perform various web attacks, including XSS, CSRF, site spoofing, and DoS, by exploiting TCP/IP side channels and without needing vulnerabilities in browsers or servers.

Contribution

It introduces a novel technique to learn TCP sequence numbers using IP-ID side channels, enabling practical off-path attacks on web security.

Findings

Off-path attacker can learn TCP sequence numbers using IP-ID side channel.

Attacks can bypass existing defenses and work even with SSL/TLS.

Practical firewall-level defenses are proposed.

Abstract

We show how an off-path (spoofing-only) attacker can perform cross-site scripting (XSS), cross-site request forgery (CSRF) and site spoofing/defacement attacks, without requiring vulnerabilities in either web-browser or server and circumventing known defenses. Attacker can also launch devastating denial of service (DoS) attacks, even when the connection between the client and the server is secured with SSL/TLS. The attacks are practical and require a puppet (malicious script in browser sandbox) running on a the victim client machine, and attacker capable of IP-spoofing on the Internet. Our attacks use a technique allowing an off-path attacker to learn the sequence numbers of both client and server in a TCP connection. The technique exploits the fact that many computers, in particular those running Windows, use a global IP-ID counter, which provides a side channel allowing efficient exposure of the connection sequence numbers. We present results of experiments evaluating the learning technique and the attacks that exploit it. Finally, we present practical defenses that can be deployed at the firewall level; no changes to existing TCP/IP stacks are required.