Q3 2021 brought two new DDoS attack vectors, potentially posing a serious threat, including for major web resources. A team of researchers from the University of Maryland and the University of Colorado Boulder found a way to spoof the victim’s IP address over TCP. To date, amplification attacks have mostly been carried out using the UDP protocol, since it does not require connection establishment procedures and allows IP spoofing. In contrast, the TCP protocol implements a three-way handshake in which the client and the server establish a connection and confirm they are ready to exchange traffic. If the victim receives a response from the server to a request they did not send, they simply discard this response.
The new attack, as described by the researchers, targets security devices located between the client and the server (so-called middleboxes) — firewalls, load balancers, network address translators (NAT), deep packet inspection (DPI) tools and others. Many of them can interfere with a TCP connection, for example, by blocking a connection to a banned resource, and they often react to packets received from one of the parties without seeing the full picture or monitoring the validity of the TCP session.