Detection Bypass
If as a server side you break a normal TCP 3 way handshake packets order and inject some response data before 3whs is complete then data still will be received by the a client but some IDS engines may skip content checks on that. Attack scenario TCP flow scheme: Client -> [SYN] [Seq=0 Ack= 0] -> Evil Server Client <- [SYN, ACK] [Seq=0 Ack= 1] <- Evil Server Client <- [PSH, ACK] [Seq=1 Ack= 1] <- Evil Server # Injection before the 3whs is completed Client <- [FIN, ACK] [Seq=83 Ack= 1] <- Evil Server Client -> [ACK] [Seq=1 Ack= 84] -> Evil Server Client -> [PSH, ACK] [Seq=1 Ack= 84] -> Evil Server IDS signature checks for tcp stream or http response body will be skipped in the case of data injection. This attack technique requires all three packets from a malicious server to be received by a client side together before it completes 3whs. Proof of concept server was written in C to reproduce this and it works reliably in local networks. Since some network devices may affect packets transmission exploitation is not so reliable for the internet scenario. Successful exploitation of this vulnerability allows an attacker to bypass detection of malicious traffic by an IDS.
