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Welcome to my 2nd tutorial of the series of networking. In this video I've briefly described peer to peer network (P2P). Moreover, you'll see how to make a peer to peer network? How it's working? How we can intercept traffic over the network by using Wireshark? and many more. Wireshark tool is integrated with eNSP so it'll be installed automatically when you install the eNSP. On the other hand, you can install the Wireshark for your personal use from its website.

What is Peer to Peer (P2P) network?

As when devices are connected with each other for the sake of communication that'll be known as a Network. Now what is peer to peer network? In P2P network each and every device is behaving like a server and a client as well. Moreover They are directly connected with each other in such a way that they can send and received data to other devices at the same time and there is no need of any central server in between them.

There is a question that mostly comes up into our minds that Is it possible to capture data from the network? So the answer is yes. We can easily captured data from the network with the help of tools that have been created for network troubleshooting, so whenever there will be some issues happening to the network so we fixed that issues with the help of tools. Most usable tool for data capturing that every network analyst used named Wireshark but there are so many other tools available over the internet like SmartSniff, Ethereal, Colasoft Capsa Network Analyze, URL Helper, SoftX HTTP Debugger and many more.

What is Wireshark?

Wireshark is an open source network analyzer or sniffer used to capture packets from the network and tries to display the brief information about the packets. It is also used for software and communication protocol development. Moreover, Wireshark is the best tool to intercept the traffic over the network.

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It's a 32bits elf binary of some version of vsftpd, where it have been added a backdoor, they don't specify is an authentication backdoor, a special command or other stuff.

I started looking for something weird on the authentication routines, but I didn't found anything significant in a brief period of time, so I decided to do a bindiff, that was the key for locating the backdoor quickly. I do a quick diff of the strings with the command "strings bin | sort -u" and "vimdiff" and noticed that the backdoored binary has the symbol "execl" which is weird because is a call for executing elfs, don't needed for a ftp service, and weird that the compiled binary doesn't has that symbol.

Looking the xrefs of "execl" on IDA I found that code that is a clear backdoor, it create a socket, bind a port and duplicate the stdin, stdout and stderr to the socket and use the execl:

There are one xrefs to this function, the function that decides when trigger that is that kind of systems equations decision:

The backdoor was not on the authentication, it was a special command to trigger the backdoor, which is obfuscated on that systems equation, it was no needed to use a z3 equation solver because is a simple one and I did it by hand.

The equation:

cmd[0] = 69

cmd[1] = 78

cmd[1] + cmd[2] = 154

cmd[2] + cmd[3] = 202

cmd[3] + cmd[4] = 241

cmd[4] + cmd[5] = 233

cmd[5] + cmd[6] = 217

cmd[6] + cmd[7] = 218

cmd[7] + cmd[8] = 228

cmd[8] + cmd[9] = 212

cmd[9] + cmd[10] = 195

cmd[10] + cmd[11] = 195

cmd[11] + cmd[12] = 201

cmd[12] + cmd[13] = 207

cmd[13] + cmd[14] = 203

cmd[14] + cmd[15] = 215

cmd[15] + cmd[16] = 235

cmd[16] + cmd[17] = 242

The solution:

cmd[0] = 69

cmd[1] = 75

cmd[2] = 79

cmd[3] = 123

cmd[4] = 118

cmd[5] = 115

cmd[6] = 102

cmd[7] = 116

cmd[8] = 112

cmd[9] = 100

cmd[10] = 95

cmd[11] = 100

cmd[12] = 101

cmd[13] = 106

cmd[14] = 97

cmd[15] = 118

cmd[16] = 117

cmd[17] = 125

The flag:

EKO{vsftpd_dejavu}

The binary:
https://ctf.ekoparty.org/static/pre-ekoparty/backdoor

Neuro-Oncology

Wednesday, May 31, 2023

Hackerhubb.blogspot.com

Networking | Routing And Switching | Tutorial 2 | 2018

What is Peer to Peer (P2P) network?

What is Wireshark?

Hackerhubb.blogspot.com

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Tuesday, May 30, 2023

Vsftpd Backdoor - Ekoparty Prectf - Amn3S1A Team

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