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Testing for Web Application Fingerprint (OWASP-IG-004)

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This article is part of the new OWASP Testing Guide v4.

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Summary

Web server fingerprinting is a critical task for the Penetration tester. Knowing the version and type of a running web server allows testers to determine known vulnerabilities and the appropriate exploits to use during testing.

There are several different vendors and versions of web servers on the market today. Knowing the type of web server that you are testing significantly helps in the testing process, and will also change the course of the test. This information can be derived by sending the web server specific commands and analyzing the output, as each version of web server software may respond differently to these commands. By knowing how each type of web server responds to specific commands and keeping this information in a web server fingerprint database, a penetration tester can send these commands to the web server, analyze the response, and compare it to the database of known signatures. Please note that it usually takes several different commands to accurately identify the web server, as different versions may react similarly to the same command. Rarely, however, different versions react the same to all HTTP commands. So, by sending several different commands, you increase the accuracy of your guess.

Test Objectives

How to Test

Black Box testing and example

The simplest and most basic form of identifying a Web server is to look at the Server field in the HTTP response header. For our experiments we use netcat. Consider the following HTTP Request-Response: 

$ nc 202.41.76.251 80
HEAD / HTTP/1.0

HTTP/1.1 200 OK
Date: Mon, 16 Jun 2003 02:53:29 GMT
Server: Apache/1.3.3 (Unix)  (Red Hat/Linux)
Last-Modified: Wed, 07 Oct 1998 11:18:14 GMT
ETag: "1813-49b-361b4df6"
Accept-Ranges: bytes
Content-Length: 1179
Connection: close
Content-Type: text/html

From the Server field, we understand that the server is likely Apache, version 1.3.3, running on Linux operating system.

Four examples of the HTTP response headers are shown below.

From an Apache 1.3.23 server: 

HTTP/1.1 200 OK 
Date: Sun, 15 Jun 2003 17:10: 49 GMT 
Server: Apache/1.3.23 
Last-Modified: Thu, 27 Feb 2003 03:48: 19 GMT 
ETag: 32417-c4-3e5d8a83 
Accept-Ranges: bytes 
Content-Length: 196 
Connection: close 
Content-Type: text/HTML

From a Microsoft IIS 5.0 server: 

HTTP/1.1 200 OK 
Server: Microsoft-IIS/5.0 
Expires: Yours, 17 Jun 2003 01:41: 33 GMT 
Date: Mon, 16 Jun 2003 01:41: 33 GMT 
Content-Type: text/HTML 
Accept-Ranges: bytes 
Last-Modified: Wed, 28 May 2003 15:32: 21 GMT 
ETag: b0aac0542e25c31: 89d 
Content-Length: 7369

From a Netscape Enterprise 4.1 server: 

HTTP/1.1 200 OK 
Server: Netscape-Enterprise/4.1 
Date: Mon, 16 Jun 2003 06:19: 04 GMT 
Content-type: text/HTML 
Last-modified: Wed, 31 Jul 2002 15:37: 56 GMT 
Content-length: 57 
Accept-ranges: bytes 
Connection: close

From a SunONE 6.1 server: 

HTTP/1.1 200 OK
Server: Sun-ONE-Web-Server/6.1
Date: Tue, 16 Jan 2007 14:53:45 GMT
Content-length: 1186
Content-type: text/html
Date: Tue, 16 Jan 2007 14:50:31 GMT
Last-Modified: Wed, 10 Jan 2007 09:58:26 GMT
Accept-Ranges: bytes
Connection: close

However, this testing methodology is not so good. There are several techniques that allow a web site to obfuscate or to modify the server banner string. For example we could obtain the following answer: 

403 HTTP/1.1 Forbidden 
Date: Mon, 16 Jun 2003 02:41: 27 GMT 
Server: Unknown-Webserver/1.0 
Connection: close 
Content-Type: text/HTML; charset=iso-8859-1

In this case, the server field of that response is obfuscated: we cannot know what type of web server is running.

Protocol behaviour

More refined techniques take in consideration various characteristics of the several web servers available on the market. We will list some methodologies that allow us to deduce the type of web server in use.

HTTP header field ordering

The first method consists of observing the ordering of the several headers in the response. Every web server has an inner ordering of the header. We consider the following answers as an example:

Response from Apache 1.3.23 

$ nc apache.example.com 80 
HEAD / HTTP/1.0 

HTTP/1.1 200 OK 
Date: Sun, 15 Jun 2003 17:10: 49 GMT 
Server: Apache/1.3.23 
Last-Modified: Thu, 27 Feb 2003 03:48: 19 GMT 
ETag: 32417-c4-3e5d8a83 
Accept-Ranges: bytes 
Content-Length: 196 
Connection: close 
Content-Type: text/HTML

Response from IIS 5.0 

$ nc iis.example.com 80 
HEAD / HTTP/1.0 

HTTP/1.1 200 OK 
Server: Microsoft-IIS/5.0 
Content-Location: http://iis.example.com/Default.htm 
Date: Fri, 01 Jan 1999 20:13: 52 GMT 
Content-Type: text/HTML 
Accept-Ranges: bytes 
Last-Modified: Fri, 01 Jan 1999 20:13: 52 GMT 
ETag: W/e0d362a4c335be1: ae1 
Content-Length: 133

Response from Netscape Enterprise 4.1 

$ nc netscape.example.com 80 
HEAD / HTTP/1.0 

HTTP/1.1 200 OK 
Server: Netscape-Enterprise/4.1 
Date: Mon, 16 Jun 2003 06:01: 40 GMT 
Content-type: text/HTML 
Last-modified: Wed, 31 Jul 2002 15:37: 56 GMT 
Content-length: 57 
Accept-ranges: bytes 
Connection: close

Response from a SunONE 6.1 

$ nc sunone.example.com 80 
HEAD / HTTP/1.0

HTTP/1.1 200 OK
Server: Sun-ONE-Web-Server/6.1
Date: Tue, 16 Jan 2007 15:23:37 GMT
Content-length: 0
Content-type: text/html
Date: Tue, 16 Jan 2007 15:20:26 GMT
Last-Modified: Wed, 10 Jan 2007 09:58:26 GMT
Connection: close

We can notice that the ordering of the Date field and the Server field differs between Apache, Netscape Enterprise, and IIS.

Malformed requests test

Another useful test to execute involves sending malformed requests or requests of nonexistent pages to the server. Consider the following HTTP responses.

Response from Apache 1.3.23 

$ nc apache.example.com 80 
GET / HTTP/3.0 

HTTP/1.1 400 Bad Request 
Date: Sun, 15 Jun 2003 17:12: 37 GMT 
Server: Apache/1.3.23 
Connection: close 
Transfer: chunked 
Content-Type: text/HTML; charset=iso-8859-1

Response from IIS 5.0 

$ nc iis.example.com 80 
GET / HTTP/3.0 

HTTP/1.1 200 OK 
Server: Microsoft-IIS/5.0 
Content-Location: http://iis.example.com/Default.htm 
Date: Fri, 01 Jan 1999 20:14: 02 GMT 
Content-Type: text/HTML 
Accept-Ranges: bytes 
Last-Modified: Fri, 01 Jan 1999 20:14: 02 GMT 
ETag: W/e0d362a4c335be1: ae1 
Content-Length: 133

Response from Netscape Enterprise 4.1 

$ nc netscape.example.com 80 
GET / HTTP/3.0 

HTTP/1.1 505 HTTP Version Not Supported 
Server: Netscape-Enterprise/4.1 
Date: Mon, 16 Jun 2003 06:04: 04 GMT 
Content-length: 140 
Content-type: text/HTML 
Connection: close

Response from a SunONE 6.1 

$ nc sunone.example.com 80 
GET / HTTP/3.0

HTTP/1.1 400 Bad request
Server: Sun-ONE-Web-Server/6.1
Date: Tue, 16 Jan 2007 15:25:00 GMT
Content-length: 0
Content-type: text/html
Connection: close

We notice that every server answers in a different way. The answer also differs in the version of the server. Similar observations can be done we create requests with a non-existent protocol. Consider the following responses:

Response from Apache 1.3.23 

$ nc apache.example.com 80 
GET / JUNK/1.0 

HTTP/1.1 200 OK 
Date: Sun, 15 Jun 2003 17:17: 47 GMT 
Server: Apache/1.3.23 
Last-Modified: Thu, 27 Feb 2003 03:48: 19 GMT 
ETag: 32417-c4-3e5d8a83 
Accept-Ranges: bytes 
Content-Length: 196 
Connection: close 
Content-Type: text/HTML

Response from IIS 5.0 

$ nc iis.example.com 80 
GET / JUNK/1.0 

HTTP/1.1 400 Bad Request 
Server: Microsoft-IIS/5.0 
Date: Fri, 01 Jan 1999 20:14: 34 GMT 
Content-Type: text/HTML 
Content-Length: 87

Response from Netscape Enterprise 4.1 

$ nc netscape.example.com 80 
GET / JUNK/1.0 

<HTML><HEAD><TITLE>Bad request</TITLE></HEAD> 
<BODY><H1>Bad request</H1> 
Your browser sent to query this server could not understand. 
</BODY></HTML>

Response from a SunONE 6.1 

$ nc sunone.example.com 80 
GET / JUNK/1.0

<HTML><HEAD><TITLE>Bad request</TITLE></HEAD>
<BODY><H1>Bad request</H1>
Your browser sent a query this server could not understand.
</BODY></HTML>

Tools

Automated Testing

Rather than rely on manual bannering and analysis of the web server headers, a tester can use automated tools to achieve the same purpose. The tests to carry out in order to accurately fingerprint a web server can be many. Luckily, there are tools that automate these tests. "httprint" is one of such tools. httprint has a signature dictionary that allows one to recognize the type and the version of the web server in use.
An example of running httprint is shown below:

Httprint.jpg


Nmap version detection offers a lot of advanced features that can help in determining services that are running on a given host, it obtains all data by connecting to open ports and interrogating them by using probes that the specific services understand, the following example shows how Nmap connected to port 80 in order to fingerprint the service and its current version 

localhost$ nmap -sV example.com
Starting Nmap 6.40 ( http://nmap.org ) at 2013-09-21 13:20 GST
Nmap scan report for example.com (127.0.0.1)
Host is up (0.028s latency).
Not shown: 997 filtered ports
PORT     STATE  SERVICE    VERSION
80/tcp   open   http       Microsoft IIS httpd 6.0
Service Info: OS: Windows; CPE: cpe:/o:microsoft:windows

Online Testing

Online tools can be used if the tester wishes to test more stealthily and doesn't wish to directly connect to the target website. An example of online tool that often delivers a lot of information on target Web Server, are Netcraft and SHODAN

With Netcraft we can retrieve information about operating system, web server used, Server Uptime, Netblock Owner, history of change related to Web server and O.S.
An example is shown below:

Netcraft2.png


SHODAN combines an HTTP port scanner with a search engine index of the HTTP responses, making it trivial to find specific web servers. Shodan collects data mostly on web servers at the moment (HTTP port 80), but there is also some data from FTP (21), SSH (22) Telnet (23), SNMP (161) and SIP (5060) services.
An example is shown below:

Shodan.png


OWASP Unmaskme Project expect becomes another online tool to do fingerprinting in any website with an overall interpretation of all the Web-metadata extracted. The idea behind this project is that anyone in charge of a website could test the metadata their site is showing to the world and assess it from a security point of view. While this project is being developed, you can test a Spanish Proof of Concept of this idea.

Vulnerability References

Whitepapers

Remediation

Protect the presentation layer web server behind a hardened reverse proxy.

Obfuscate the presentation layer web server headers.

  • Apache
  • IIS
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