Wednesday, December 9, 2015

Covert Channels - Part 2 - exfiltrating data through TCP Sequence Number field

A while back I did this blog post on transferring data via the IP ID field.
Recently, I had to do some work on this topic again, so I thought I should now publish the second part which I should have completed a lonnnnnng time ago.

In this post we will look at transferring the "/etc/shadow" file using "covert_tcp" via TCP sequence number field.

Our setup?
Everything is on one system but do note that covert_tcp has a client and server component.
Server configuration

Client configuration and execution

Above we see the client has been loaded and data being transmitted one byte at a time.

Below we see the server receiving data one byte at a time.

A "ls -al" of the files shows their similarity.

While the data was exfiltrated, I also had "tcpdump" running in the background

Below shows the packets captured.

So what does the capture traffic look like?

How to detect and or mitigate against this type of traffic?

Let's  see what stands out.
1. Large number of SYN packets
If this is a legitimate connection attempt, then we need to see corresponding  SYN-ACK and a final ACK. The fact that there is a large number of SYNs with no complete 3-way handshake should make this standout as suspicious.

2. Large number of corresponding RST-ACKs
Attempts to connect to a port which is not listening should result in a RST-ACK. Therefore the question here should be why the persistence to continue connecting to a port which is not listening. The large number of RST-ACKs as seen in the packet capture should be reason to suspect something malicious.

3. All packets have the same IP length.
This is also strange. I look at this and immediately start to think of a crafted packet or something malicious.

4. The port number is reused.
Every new client connection should have a new source port. The fact that these are all the same suggests that there is something suspicious going on here and an investigation should be performed to determine the nature of this traffic.

So we highlighted a few points above. But is there anything else which can be used to detect this type of activity?! Statistical analysis, anomaly detection, behavioural analysis are all methods that can possibly be used to detect this type of activity. However, the 4 items above are clear signs of something malicious.

Packet Capture - myShadow.pcap

No comments:

Post a Comment