Actively sniffing and spoofing using Cain and Abel

In Part 1 (link) active sniffing and performing a man-in-the-middle attack were introduced. It allows you to intercept between the communication of two clients and thus able to read all their conversation. In this part tools will be demonstrated with which such an attack can be performed.

We will begin with one of the most easy to use and robust tools called Cain and Abel. It is composed of two components, first being Cain which is can perform a host of features such as:

a) Sniffing
b) Man-in-the-middle attack
c) Bruteforce and dictionary based hash cracking
d)Windows logon password cracking

The lesser known component, Abel can be also be pretty useful. The two utilities compliment each other in the sense that Cain is used to remotely execute attacks whereas Abel has to be installed on the victim machine as a service after which Cain's frontend can be used to remotely control the machine.

Brief Overview:

This post will cover the first two aspects of Cain only. Please note that this tool is available for Windows only. It uses the Winpcap (a port of libpcap) library to inject and monitor packets at link layer.

1. Install Cain (link)and run a quick Arp scan to find out all the local machines within your network.

2. Now start a man in the middle attack between two victim nodes. Obviously the best strategy is to attack a victim PC and your gateway router. This will allow you to read or sniff all the data between your victim PC and the outside world which is the requirement in most of the cases.

Options and Basic Configuration:

To configure which protocols/ports to sniff and route through your machine, go to configure->filter and ports and then tick all the protocols. Nowadays modern browser display a warning when the connection is under MitM. I would suggest that you uncheck port 443 which denotes ssl/https traffic. There are other interesting ways to decipher ssl traffic.

The other important tab to go is the HTTP Fields tab. It defines which keywords to grab and display the corresponding values. So if you are not able to capture username and/or password of a particular webpage, chances are that they use different value and name pair variables.

Finally the option which makes Cain and Abel worth using is the "Spoofed ip and mac option". It basically allows you to do all the above using some other IP from your subnet. So even if there is a MitM or sniffing detector, it will catch that imaginary IP that you have chosen. Simply go to ARP tab and choose a spoofed IP and MAC address. Make sure it is not in use by any other machine in your subnet.

Initiating the Attack:

Now that we are all set, click on the sniffer button as illustrated below. Click on the + button to ennumerate all the devices. Click on the button next to it and you will start poisoning the victims ARP table and thus perform the MitM attack. Once the whole process has started, the tabs at the bottom come in handy. According to me, the most important ones to look at is the APR tab and Passwords tab. They will in all likelihood contain all the details that you wanted to know, for which you started this whole process! Passwords found in all the major protocols- HTTP,HTTPS, FTP,RADIUS,SMB,SSH-1,Telnet to name a few!

Cain can process protocol specific traffic

Cain And Abel is one of the best sniffing tools around for several years now. It is easy to configure and quite stable. The only drawback is that it is Windows based tool.

Here is a short video exhibiting how powerful yet easy this tool is to use.

In the next part of this tutorial I will be describing how we can use open source tools to achieve the same. I will also explain how we can thwart such techniques. Till then, happy packet sniffing!

Important Links:

1.Cain and Abel homepage
2. Uses of Abel
3. Promiscuous mode

Sniffing & man-in-the-middle/mitm using network sniffers

A long time ago I had written an article (http://anadisays.blogspot.in/2010/06/wireshark-as-sniffer.html) on network sniffers, primarily wireshark, which described what sniffing is and how one can passively sniff other users' data by setting Wireshark in promiscuous mode. Such a technique is known as passive sniffing and is really useful if your local network uses hubs instead of switches. But if your network uses the latter then you will not be able to capture all the traffic. The difference is due to the way these devices forward packets to the destination.

When a node sends a packet to a hub it quickly makes several copies and forwards it to all the other physical ports. The figure below illustrates this.

As you can see, the network packet denoted by the envelope is supposed to be delivered to PC0. But as soon as it receives the packet from Router0 it makes several copies and pushes it through all the links.

So if PC1 were to be running Wireshark then it will always receive all the traffic meant for PC0. It should be clear that it becomes very simple to sniff data when using hub.

Now let us consider a network using a switch.

 In the previous article mentioned above I have explained how a switch maintains a table which has MAC address and physical port mapping.

Briefly speaking, any packet meant for PC0 will be forwarded only to that particular link and the other links will never get a copy. It is obvious that Wireshark running on PC1 will not be able to capture any other data.

Now comes the interesting part! A technique known as Active sniffing is all you need to overcome this problem! It requires you to perform a Man in the Middle attack aka MitM which will be described below. But before I delve into what is active sniffing and mitm let me talk a little about ARP and ARP spoofing.

ARP 

(NOTE: You may skip this section if you are well aware of what ARP is)

I assume you are already a little familiar with ARP (Address Resolution Protocol ). It is basically a protocol designed to allow two machines to communicate. It works at the link layer of the OSI model. You should also be aware of the arp or MAC table. It can be viewed by running the arp -a command in Windows and a simple arp command in Linux. It is a mapping of the IP address of a machine and its MAC address present in the local network.

Very briefly, if a node wants to send a packet it uses the destination IP address to look up its arp table to find the MAC address of that machine. When the packet is processed by a receiving node, it validates whether the destination MAC address matches or not, otherwise the packet is discarded.

ARP Spoofing/Poisoning

What if an attacker machine is somehow able to convince that it is actually the destination machine the victim wants to talk to? Simultaneously if it is able to convince the destination machine that it is actually the victim pc? Basically it will be able to read all their conversation.

Technically speaking, what if the attacking machine keeps on sending false ARP updates telling the victim pc that the destination pc's MAC address is its own?

Refer to the illustration above. Here the attacker PC has crafted and sent false ARP updates telling the victim PC to update its arp table to point the destination PC's arp table with bb:bb:bb:bb . The poor victim PC would always put the destination MAC address as bb:bb:bb:bb. The attacker after reading the packet will then quietly forward it to the destination using the correct MAC address. At the same time the attacker would attack the destination node as well.

Please keep in mind is that if the destination node is not in the local network then it will never be able to know its MAC address. This is because MAC addresses are not forwarded outside the local network.In such a situation you will have to attack the local gateway/router since all the traffic intended for outside the network is forwarded to it.

Finally even though physically the attacker may be anywhere within the local network, logically all data will travel through it. The attacker acts as a middle man between the victim and the destination PC, thus the term man in the middle attack.

To summarize, we achieved a man in the middle attack by ARP spoofing or ARP poisoning. This is active sniffing because unlike passive sniffing, where the attacker simply sits and silently reads all the packets, proactive measures are taken and network configuration or setting is stealthily disturbed.

Limitations of this attack

It should become clear that in order to perform this attack the victim PC and the attacker MUST be within the same local network. Also some protocols have been built to detect Mi-t-M attacks. HTTPS/SSL is one such protocol (I hope now you realize why it is advised to look for the small padlock when communicating sensitive information!). Most modern browsers will detect errors in digital certificates and promptly display an error. There are other techniques which have been developed to specifically attack SSL and HTTPS which will be dealt with in future.

Conclusion

Using these concepts, tools have been built which allow you to actively sniff packets. Such tools such as Wireshark combined with Arpspoof, Cain and Abel and Ettercap will be discussed in the next post of this series. By the next article you will be able to actively sniff and read data from packets intended for others! Prevention techniques will also be discussed.

IMPT: the next part of this tutorial *Actively network sniffing uing cain and abel*

Important Links:

1. Understanding passive sniffing
2. Wiki on arp spoofing
3. Wiki on Packet sniffing
4. Wiki on promiscuous mode