Off-Path TCP Hijacking Attacks via the Side Channel of Downgraded IPID

In this paper, we uncover a new off-path TCP hijacking attack that can be used to terminate victim TCP connections or inject forged data into victim TCP connections by manipulating the new mixed IPID assignment method, which is widely used in Linux kernel version 4.18 and beyond. Our attack has three steps. First, an off-path attacker can downgrade the IPID assignment for TCP packets from the more secure per-socket-based policy to the less secure hash-based policy, thus building a shared IPID counter that forms a side channel in the victim. Second, the attacker detects the presence of TCP connections by observing the side channel of the shared IPID counter. Third, the attacker infers sequence and acknowledgment numbers of the detected connection by observing the side channel. Consequently, the attacker can completely hijack the connection, e.g., resetting the connection or poisoning the data stream. We evaluate the impacts of our attack in the real world, and we uncover that more than 20% of Alexa top 100k websites are vulnerable to our attack. Our case studies of SSH DoS, manipulating web traffic, and poisoning BGP routing tables show its threat on a wide range of applications. Moreover, we demonstrate that our attack can be further extended to exploit IPv4/IPv6 dual-stack networks on increasing the hash collisions and enlarging vulnerable populations. Finally, we analyze the root cause and develop a new IPID assignment method to defeat this attack. We prototype our defense in Linux 4.18 and confirm its effectiveness in the real world.