CORRECTION: Remote IIS 5.x and IIS 6.0 Server Name Spoof

[Mark Burnett <mb () xato net>]

I just wanted to clarify some issues here. First of all, this is by no means a new discovery. Others have mentioned it 
and I even used it as an example in my book "Hacking the Code". It is not a flaw in the handling of the server 
variable, but a bug in the 500-100.asp script. It is also important to note that this issue does not affect IIS 6 as 
Inge claims. 

The original code in IIS 5 used the SERVER_NAME variable to see if the user was local with this code:

                If (strServername = "localhost" or strServerIP = strRemoteIP)  And .....

In IIS 6 they updated the code to this:

                If (strServerIP = strRemoteIP) And...

A quick fix for this is to pull the 500-100.asp script from IIS 6 and use it for IIS 5. A better fix is to simply 
replace that error with a static html page.

So if you maintain a vulnerability database, write vulnerability scanners, assign vulnerability ID numbers, or if you 
somehow document this, you should correct this so we all don't have to explain this every time in the next 10 years 
that someone runs vulnerability scanner on IIS 6 and this shows up in the report.

Incidently, you do not need to use telnet to manually enter the URL to exploit this. An easier solution is to change 
your HOSTS file so that localhost points to the remote IP address of the target IIS server. Then you can use your 
browser to hammer away at it.


Mark Burnett