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

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Brief 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.

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 Apache, version 1.3.3, running on Linux operating system. Three 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 

This testing methodology however is not so good. There are several techniques that allow 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

Refined techniques of testing 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 in observing the ordering of the several headers in the response. Every web server has just 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 

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 consists in sending malformed requests to the server, or requests of nonexistent pages. We consider the following HTTP response:

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 

We notice that every server answers in a different way. The answer differs also by the version of the server. An analogous issue comes if we create requests with a not existing 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> 

Automated Testing

The tests to carry out can be several. A tool that automates these tests is "httprint" that allows, through a signature dictionary, to recognize the type and the version of the web server in use.
An example of such tool is shown below:

References

Whitepapers

• Saumil Shah: "An Introduction to HTTP fingerprinting" - http://net-square.com/httprint/httprint_paper.html

Tools

• httprint - http://net-square.com/httprint/index.shtml

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OWASP Testing Guide v2

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