Category: Footprinting

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Banner grabbing for beginners

Hello, aspiring ethical hackers. In this blogpost, you will learn about banner grabbing. Banner grabbing plays a very important role in ethical hacking for penetration testers and hackers alike. To understand what is banner grabbing, you need to understand what is a banner first.

What is a Banner?

A banner is the information displayed by software or service running on a specific port. This information involves the type of software running, version of the software running etc. This information is displayed by default by every software running for marketing purposes.

What is Banner grabbing?

Banner grabbing as its name implies, is grabbing this banner. A banner when displayed to a common user may provide information to the user. In the same way, by grabbing this banner, hackers and penetration testers can get information about the software running on it and the version of the software running. This allows them to search or research for any vulnerabilities in the software.

Types of Banner grabbing

Banner grabbing can be performed in two ways: Active & passive.

  1. Active banner grabbing: In active banner grabbing, a hacker or penetration tester interacts with the software & target services to grab the banner.
  2. Passive banner grabbing: In passive banner grabbing, a hacker or penetration tester doesn’t interact with the target service while grabbing the banner. This can be done by packet sniffing on the network traffic of the network.

Although banner grabbing can be performed on almost all services running on all ports, the most common services that are used for banner grabbing are,

FTP-Port 21
SSH-Port 23
SMTP-Port 22
HTTP-Port 80

Tools used for Banner grabbing:

  1. Telnet, wget, curl etc

Apache:

Imagine I have set up a website named www.shunya.com on an Apache server. A hacker can easily find Information about the web server in different ways. For example, a hacker can visit the website and and try to open a webpage which is not existent on my server, like below.

banner grabbing

In the above example, hacker tried to open page named “admin.php” which was not available on my server and in turn the server responded with a type of web server, the target OS and the scripting language. This is giving out too much information.

The traditional and popular way of fingerprinting is through telnet. A hacker opens command line or terminal. and types the command “telnetwww.shunya.com 80″. When the screen goes black, type “HEAD / HTTP/1.0″and this will give the server information.

There are also many fingerprinting tools available. I am gonna show you only one, Id serve. Let’s see how to banner grab using Id serve.

Now what are the preventive measures we can take in Apache server to disable or atleast prevent fingerprinting to some extent. Apache web server has a configuration file called “httpd.conf” where we can make changes to fight fingerprinting. Go to httpd.conf and change the value of the option “Server Signaturetooff”.This will not display any information about server when an nonexistent page has been accessed.

In the httpd.conf file, changing the value of “Server Tokens”from “Full” to “Prod” will only show the minimum server information as shown below.

This still discloses that our web server is Apache but it doesn’t show the version. In Kautilya’s words this is delaying the march of enemy. Here are the options we set.

IIS 8:

Now imagine we changed our www.shunya.com website from Apache server to the latest version of Microsoft web server, IIS 8. To prevent error pages form revealing any information in IIS server, we can set custom error pages. Now let’s use IDserve tool to fingerprint the IIS 8 server.

It shows the server version. Now how can we prevent this. Microsoft provides a tool named UrlScan freely available for download which can be used easily to process HTTP requests. Download this tool and install it. ( See how to configure Urlscan for IIS 7.5 and IIS 8 ). Then go to the configuration file of UrlScan, “UrlScan.ini” located at “C:WindowsSystem32inetservUrlscan”by default and change the value of “RemoveServerHeader’ from “0″ to “1″.

This will not reveal the server version information as shown below.

We can further mislead the attacker by setting our server name to some other value different than our original one. This can be done by setting the value of “RemoveServerHeader” to “0 “and changing the value of “AlternateServerName” to the value we want to specify ( in our example Nginx ).

So when the attacker tries to fingerprint our website, he will be misleaded.

Note: Taking this preventive measures will not stop a determined hacker to find out our server information.

That’s all in webserver banner grabbing and countermeasures.

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Port Scan Results explained for beginners

Hello aspiring Ethical Hackers. In this blogpost you will learn how to analyze port scan results. Scanning plays a very important role in hacking a system. Scanning is a phase in which we find out the ports which are open and the services listening on those ports. NMap is the most popular port scanner being used security guys nowadays. However it is very important to understand classification of ports by Nmap while scanning. Nmap classifies ports into six states. They are, open, closed, filtered, unfiltered, open | filtered and closed | filtered. Let us find out when Nmap classifies ports into specific states. For this, I use two virtual machines,

1. Kali Linux as attacker (with IP 10.10.10.2)

2. XP as victim (with IP 10.10.10.3)

On the victim machine, Telnet server is running and an exception is provided for it in windows firewall.

1. Open

Nmap classifies a port as open if an application is actively accepting TCP connections, UDP datagrams or SCTP associations on this port.

When I perform a default Nmap scan from the attacker of port 23 of the victim,

Nmap –p 23 10.10.10.3

The result I get is open. This is because the Telnet server is actively accepting connections.

2. Closed

Nmap classifies a port as closed when the port is accessible but there is no application listening on it. On our victim machine, let’s stop the the telnet service as shown below.

Now when we perform the above scan again, the port is shown as closed because although the port is accessible we don’t have any application listening on it.(i.e telnet is stopped)

3. filtered

Nmap classifies a port as filtered when it can’t determine whether the port is open or closed because packet filtering prevents its probes from reaching the port. On our victim machine, let’s select ‘Don’t Allow Exceptions’ option in the firewall settings.

When we perform the above scan once again, the port is classified as filtered because firewall filtering blocks the probes of Nmap. When Nmap classifies a port as filtered, it is most likely that a firewall is blocking our probes.

Classification of ports by Nmap

4. Unfiltered.

Nmap classifies a port as unfiltered when a port is accessible but it can’t determine whether it is open or closed. A port is classified as unfiltered only with the ACK scan.

Let’s start the telnet service again on our victim machine and allow an exception for telnet in the firewall.

Then let us perform the ACK scan.

nmap -sA –p 23 10.10.10.3

The scan couldn’t determine whether the port is open or closed.

5. open | filtered

A port is classified as open | filtered when Nmap is unable to determine whether a port is open or filtered. This happens for scan types in which open ports give no response. The UDP,IP protocol, FIN, NULL and XMAS scans classify ports this way. Let’s go to our machine and once again block telnet using firewall.

And then perform FIN scan and NULL scan respectively.

The port is classified as open | filtered in both cases because Nmap can’t determine whether the port is open or filtered.

6. closed | filtered

Nmap can’t find out whether a port is closed or filtered. A port is classified this way only for IP IDLE scan. Now what is IDLE scan? Idle scan is a scan in which we use a zombie host to scan the victim. In our example, we use another host with IP 10.10.10.3 as a zombie to perform IDL scan on our victim.

In our victim, firewall is still blocking telnet. Let’s perform a IP IDLE scan.

nmap –sI 10.10.10.1 –p 23 10.10.10.3

The scan shows result as closed | filtered because it couldn’t determine whether a port is closed or filtered. With this, I am sure you have understood port scan results.