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Introduction To Network Security - Part 2

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A Brief Walk-through of an Attack

This is an account of how an attacker in the real world might go about trying to exploit your system. There is no fixed way to attack a system, but a large number will follow the similar methodology or at least the chain of events.

This section assumes that the attacker is moderately skilled and moderately motivated to breaking into your network. He/She has targeted you due to a specific motive -- perhaps you sacked them, or didn't provide adequate customer support (D-link India are you listening ? ;)). Hopefully this will help you figure out where your network might be attacked, and what an attacker might do once they are on the inside.

Remember that attackers will usually choose the simplest way to get into the network. The path of least resistance principle always applies.

Reconnaissance & Footprinting

Here the attacker will try to gather as much information about your company and network as they can without making a noise. They will first use legitimate channels, such as google and your company webpage to find out as much about you as they can. They will look for the following information:

Technical information is a goldmine, things like a webpage to help your employees log in from home will be priceless information to them. So also will newsgroup postings by your IT department asking how to set up particular software, as they now know that you use this software and perhaps they know of a vulnerability in it.

Personal information about the company and its corporate structure. They will want information on the heads of IT departments, the CEO and other people who have a lot of power. They can use this information to forge email, or social engineer information out of subordinates.

Information about your partners. This might be useful information for them if they know you have some sort of network connection to a supplier or partner. They can then include the supplier's systems in their attack, and find a way in to your network from there.

General news. This can be useful information to an attacker as well. If your website says that it is going down for maintenance for some days because you are changing your web server, it might be a clue that the new setup will be in its teething stages and the admins may not have secured it fully yet.

They will also query the whois databases to find out what block of IP addresses you own. This will give them a general idea of where to start their network level scans.
After this they will start a series of network probes. The most basic of which will be to determine if you have a firewall, and what it protects. They will try and identify any systems you have that are accessible from the Internet.

The most important targets will be the ones that provide public services. These will be :

Webservers - usually the front door into the network. All webserver software has some bugs in it, and if you're running home made CGI scripts such as login pages etc, they might be vulnerable to techniques such as SQL injection.

Mail servers - Sendmail is very popular and most versions have at least one serious vulnerability in them. Many IT heads don't like to take down the mail server for maintenance as doing without it is very frustrating for the rest of the company (especially when the CEO doesn't get his mail).

DNS servers - Many implementations of BIND are vulnerable to serious attacks. The DNS server can be used as a base for other attacks, such as redirecting users to other websites etc.

Network infrastructure - Routers and switches may not have been properly secured and may have default passwords or a web administration interface running. Once controlled they can be used for anything from a simple Denial of Service attack by messing up their configurations, to channeling all your data through the attackers machine to a sniffer.

Database servers - Many database servers have the default sa account password blank and other common misconfigurations. These are very high profile targets as the criminal might be looking to steal anything from your customer list to credit card numbers. As a rule, a database server should never be Internet facing.

The more naive of the lot (or the ones who know that security logs are never looked at) may run a commercial vulnerability scanner such as nessus or retina over the network. This will ease their work.


Exploitation Phase

After determining which are valid targets and figuring out what OS and version of software they are using (example which version of Apache or IIS is the web server running), the attacker can look for an exploit targeting that particular version. For example if they find you are running an out of date version of Sendmail, they will look for an exploit targeting that version or below.

They will first look in their collection of exploits because they have tested these. If they cannot find one, they will look to public repositories such as They will probably try to choose common exploits as these are more likely to work and they can probably test them in their own lab.

From here they have already won half the game as they are behind the firewall and can probably see a lot more of the internal network than you ever intended for them to. Many networks tend to be very hard to penetrate from the outside, but are woefully unprotected internally. This hard exterior with a mushy interior is a recipe for trouble -- an attacker who penetrates the first line of defense will have the full run of your network.

After getting in, they will also probably install backdoors on this first compromised system to provide them with many ways in, in case their original hole gets shut down. This is why when you identify a machine that was broken into, it should be built up again from scratch as there is no way of knowing what kind of backdoors might be installed. It could be tricky to find a program that runs itself from 2:00AM to 4:00AM every night and tries to connect to the attackers machine. Once they have successfully guaranteed their access, the harder part of the intrusion is usually over.


Privilege Escalation Phase

Now the attacker will attempt to increase his security clearance on the network. He/She will usually target the administrator accounts or perhaps a CEO's account. If they are focused on a specific target (say your database server) they will look for the credentials of anyone with access to that resource. They will most likely set up a network sniffer to capture all the packets as they go through the network.

They will also start manually hunting around for documents that will give them some interesting information or leverage. Thus any sensitive documents should be encrypted or stored on systems with no connection to the network. This will be the time they use to explore your internal network.

They will look for windows machines with file sharing enabled and see what they can get out of these. Chances are if they didn't come in with a particular objective in mind (for example stealing a database), they will take whatever information they deem to be useful in some way.


Clean Up Phase

Now the attacker has either found what they were looking for, or are satisfied with the level of access they have. They have made sure that they have multiple paths into the network in case you close the first hole. They will now try to cover up any trace of an intrusion. They will manually edit log files to remove entries about them and will make sure they hide any programs they have installed in hard to find places.

Remember, we are dealing with an intruder who is moderately skilled and is not just interested in defacing your website. They know that the only way to keep access will be if you never know something is amiss. In the event that there is a log they are unable to clean up, they may either take a risk leaving it there, or flood the log with bogus attacks, making it difficult for you to single out the real attack.


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