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

By continuously sending SYN-FIN 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.

SYN-FIN 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 SYN-FIN flood is shown. The following images depict a high rate of SYN-FIN packets being sent from a single source IP towards a single destination IP.
In Image 1 below, you can see the flood of SYN-FIN packets coming from a single source. Notice the rate at which the packets are sent.

“Image 1 – example of single SYN-FIN packet being sent to port 80”.

As seen in Image 2. The capture analyzed is 14 seconds long and the average number of packets per second are at 118, with a rate of around 51Kbps. Attack rates could be much higher.

“Image 2 – SYN-FIN Flood stats”

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

Analysis of an SYN-FIN flood in Wireshark – Filters

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

Download Example PCAP of SYN-FIN Flood

*Note: IP’s have been randomized to ensure privacy.