Hiding TCP Traffic: Threats and Counter-measures

Law Interception, data hiding, TCP, covert channels.

Computer networks were designed to be simple and routers do not validate the integrity of the processed traffic. Consequently, an attacker can modify his or her traffic with the aim of confusing any analyser that intercepts the traffic, e.g. monitoring and security software or lawful interception. This paper studies the attack that is based on sending additional colliding TCP segments with the same sequential number but different content. The segments with the correct message are delivered to the other communicating party of the TCP connection while the fake segments are dropped en route. The goal of the fake segments is to confuse analysers into decoding a different message to the one that is received by the other communicating party. The other communicating party does not need to be aware of the attack and therefore does not need any specific software. Although this paper discuss the advantages and disadvantages of the attack for an attacker, our ultimate goal was to find counter-measures against the attack. Our contribution can be divided into four following parts. 1) We converted the attack to IPv6 and searched for possibilities that may force a middle box to drop fake packets. 2) We developed a tool called LDP, which behaves as a TCP proxy server that masks outbound TCP traffic of a whole network. 3) We identified several counter-measures. In addition, we implemented LNC, a tool that identifies the attack in pcap files and removes the fake segments. Since LNC is a stand-alone tool, it also deals with traces generated by other software than LDP as long as it is based on the same attack vector. 4) LDP and LNC were tested in both laboratory environment and on the Internet. The experiments validated that the attack is applicable for a communication with a server that is not under the control of an attacker. Several parameters of the attack were evaluated during the experiments; mainly the number and the length of fake packets and their influence on the performance of the attack and counter-measures.