Defending TCP Against Spoofing Attacks

Abstract

Recent analysis of potential attacks on core Internet infrastructure indicates an increased vulnerability of TCP connections to spurious resets (RSTs), sent with forged IP source addresses (spoofing). TCP has always been susceptible to such RST spoofing attacks, which were indirectly protected by checking that the RST sequence number was inside the current receive window, as well as via the obfuscation of TCP endpoint and port numbers. For pairs of well-known endpoints often over predictable port pairs, such as BGP or between web servers and well-known large-scale caches, increases in the path bandwidthdelay product of a connection have sufficiently increased the receive window space that off-path third parties can brute-force generate a viable RST sequence number. The susceptibility to attack increases with the square of the bandwidth, and thus presents a significant vulnerability for recent high-speed networks. This document addresses this vulnerability, discussing proposed solutions at the transport level and their inherent challenges, as well as existing network level solutions and the feasibility of their deployment. This document focuses on vulnerabilities due to spoofed TCP segments, and includes a discussion of related ICMP spoofing attacks on TCP connections.

Improving TCP's Robustness to Blind In-Window Attacks

“…This is because [RFC0793] specifies that any RST within the current window is acceptable. Also, [RFC4953] talks about the probability of a successful attack with varying window sizes and bandwidth.…”

“…For further details regarding the attacks and the existing techniques, please refer to [RFC4953]. It also needs to be emphasized that, as suggested in [TSVWG-PORT] and [RFC1948], port randomization and initial sequence number (ISN) randomization would help improve the robustness of the TCP connection against off-path attacks.…”

ICMP Attacks against TCP

“…However, as discussed in [Watson] and [RFC4953], there are a number of scenarios (notably that of TCP connections established between two BGP routers [RFC4271]) in which an attacker may be able to know or guess the four-tuple that identifies a TCP connection. In such a case, if we assume the attacker knows the two systems involved in the TCP connection to be attacked, both the client-side and the server-side IP addresses could be known or be within a reasonable number of possibilities.…”

A Selective Survey of Distributed Denial-of-Service (Ddos) Attacks and Defense Mechanisms

“…However, because the sender address is forged, the response never comes. These half-open connections consume resources on the server and limit the number of connections the server is able to make, reducing the server's ability to respond to legitimate requests until after the attack ends [18,19,20,21].…”