An ACK-RST-SYN flood is a DDoS attack designed to disrupt network activity by saturating bandwidth and resources on stateful devices in its path.

By continuously sending ACK-RST-SYN packets towards a target, stateful defenses can go down (In some cases into a fail open mode). This flood could also be used as a smoke screen for more advanced attacks. This is true for other out of state floods too.

ACK-RST-SYN Packets are considered an illegal packet by the Original TCP RFC. While it left room for customized behavior it is virtually unused today. Thus different systems can react differently to these packets and may cause unexpected issues and behavior.

Technical Analysis

Below an analysis of an ACK-RST-SYN flood is shown. The following images depict a high rate of ACK-RST-SYN packets being sent from a single source IP towards a single destination IP.

In Image 1 below, you can see the flood of ACK-RST-SYN packets coming from a single source. Notice the rate at which the packets are sent.

“Image 1 – example of single ACK-RST packet being sent to port 80”

As seen in Image 2. The capture analyzed is 10 seconds long and the average number of packets per second are at 2335, with a rate of around 126Kbps. Attack rates could be much higher.

“Image 2 – ACK-RST-SYN Flood stats”

A typical ACK-RST-SYN flood running against an unsuspecting host will look similar to the above analysis. Generally, what is seen is a high rate of ACK-RST-SYN packets (not preceded by a TCP handshake).

Analysis of an ACK-RST-SYN flood in Wireshark – Filters

Filter ACK-RST-SYN packets – “tcp.flags.ack && tcp.flags.reset && tcp.flags.syn ”.
Goto Statistics -> Summary on the menu bar to understand the rate you are looking at.

Download Example PCAP of ACK-RST-SYN Flood

*Note: IPs have been randomized to ensure privacy.