iis4.webhits.txt

`Cerberus Information Security Advisory (CISADV000126)  
http://www.cerberus-infosec.co.uk/advisories.html  
  
Released : 26th January 2000  
Name : Webhits.dll buffer truncation  
Affected Systems: Microsoft Windows NT 4 running Internet Information  
Server 4 All service Packs  
Issue : Attackers can access files outside of the web virtual  
directory system and view ASP source  
Author : David Litchfield ([email protected])  
Microsoft Advisory :  
http://www.microsoft.com/technet/security/bulletin/ms00-006.asp  
  
  
Internet Information Server 4.0 ships with an ISAPI application webhits.dll  
that provides hit-highlighting functionality for Index Server. Files that  
have the extention .htw are dispatched by webhits.dll.  
  
A vulnerability exists in webhits however that allows an attacker to break  
out  
of the web virtual root file system and gain unathorized access to  
other files on the same logical disk drive, such as customer databases,  
log files or any file they know or can ascertain the path to. The same  
vulnerability can be used to obtain the source of Active Server Pages or  
any other server side script file which often contain UserIDs and  
passwords as well as other sensitive information.  
  
  
*** WARNING ****  
Even if you have no .htw files on your system you're probably  
still vulnerable! A quick test to show if you are vulnerable:  
go to http://YOUR_WEB_SERVER_ADDRESS_HERE/nosuchfile.htw  
If you receive a message stating the "format of the QUERY_STRING  
is invalid" you _are_ vulnerable. Cerberus Information Security's  
free vulnerability scanner - CIS - now contains a check for this  
issue - available from the website http://www.cerberus-infosec.co.uk/  
*** WARNING ****  
  
  
  
Details  
*******  
  
This vulnerability exploits two problems and for the sake of clarity  
this section will be spilt into two.  
  
1) If you DO have .htw files on your system  
****************************************  
The hit-highlighting functionality provided by Index Server allows  
a web user to have a document returned with their original search  
terms highlighted on the page. The name of the document is passed  
to the .htw file with the CiWebHitsFile argument. webhits.dll,  
the ISAPI application that deals with the request, opens the file  
highlights accordingly and returns the resulting page. Because  
the user has control of the CiWebHitsFile argument passed to the  
.htw file they can request pretty much anything they want. A secondary  
problem to this is the source of ASP and other scripted pages can  
be revealed too.  
  
However, webhits.dll will follow double dots and so an attacker is able  
to gain access to files outside of the web virtual root.  
  
For example to view the web access logs for a given day the attacker would  
build the following URL  
  
http://charon/iissamples/issamples/oop/qfullhit.htw?CiWebHitsFile=/../../win  
nt/system32/logfiles/w3svc1/ex000121.log&CiRestriction=none&CiHiliteType=Ful  
l  
  
Sample .htw files often installed and left on the system are  
/iissamples/issamples/oop/qfullhit.htw  
/iissamples/issamples/oop/qsumrhit.htw  
/iissamples/exair/search/qfullhit.htw  
/iissamples/exair/search/qsumrhit.htw  
/iishelp/iis/misc/iirturnh.htw (this .htw is normally restricted to  
loopback)  
  
2) If you DON'T have any .htw files on your system  
**************************************************  
To invoke the webhits.dll ISAPI application a request needs to be made  
to a .htw file but if you don't have any on your web server you might wonder  
why you are still vulnerable - requesting a non-existent .htw file will  
fail.  
  
The trick is to be able to get inetinfo.exe to invoke webhits.dll but  
then also get webhits.dll to access an existing file. We achevie this  
by crafting a special URL.  
  
First we need a valid resource. This must be a static file such as a .htm,  
.html, .txt or even a .gif or a .jpg. This will be the file opened by  
webhits.dll as the template file.  
  
Now we need to get inetinfo.exe to pass it along to webhits for dispatch and  
the only way we can do this is by requesting a .htw file.  
  
http://charon/default.htm.htw?CiWebHitsFile=/../../winnt/system32/logfiles/w  
3svc1/ex000121.log&CiRestriction=none&CiHiliteType=Full  
  
will fail. Obviously. There is no such file on the system with that name.  
  
Notice we've now invoked webhits, however, and by placing a specific number  
of spaces (%20s) between the exisiting resource and the .htw it is then  
possible to trick the web service: The buffer that holds the name of the  
.htw  
file to open is truncated, causing the .htw part to be removed and therefore  
when it comes to webhits.dll attempting to open the file it succeeds and we  
are then returned the contents of the file we want to access without there  
actually being a real .htw file on the system.  
  
  
The code is probably doing something similar to this:  
  
FILE *fd;  
int DoesTemplateExist(char *pathtohtwfile)  
{  
  
// Just in case inetinfo.exe passes too long a string  
// let's make sure it's of a suitable length and not  
// going to open a buffer overrun vulnerability  
  
char *file;  
  
file = (char *)malloc(250);  
strncpy(file,pathtohtwfile,250);  
fd = fopen(file,"r");  
  
// Success  
if(fd !=NULL)  
{  
return 1;  
}  
// failed  
else  
{  
return 0;  
}  
}  
  
Here webhits.dll "contains" a function called DoesTemplateExist() and is  
passed  
a pointer to a 260 byte long string buffer containing the path to the .htw  
file  
to open but this buffer is further reduced in length by the strncpy()  
function  
removing whatever was stored in the last ten bytes (in this case the .htw of  
the  
HTTP REQUEST_URI) so when fopen() is called it succeeds. This happens  
because  
Windows NT will ignore trailing spaces in a file name.  
  
  
Solution  
********  
.htw needs to be unassociated from webhits.dll  
To do this open the Internet Server Manager (MMC). In the left hand pane  
right click the computer you wish to administer and from the menu that pops  
up choose Properties.  
  
>From the Master Properties select the WWW Service and then click Edit. The  
WWW Service Master properties window should open. From here click on the  
Home Directory tab and then click the Configuration button. You should  
be presented with an App Mappings tab in the Application Mappings window.  
Find the .htw extention and then highlight it then click on remove. If a  
confirmation  
window pops up selected Yes to remove. Finally click on Apply and select  
all of the child nodes this should apply to and then OK that. Now close all  
of the WWW Service property windows.  
  
  
About Cerberus Information Security, Ltd  
****************************************  
Cerberus Information Security, Ltd, a UK company, are specialists in  
penetration testing and other  
security auditing services. They are the developers of CIS (Cerberus'  
Internet  
security scanner) available for free from their website:  
http://www.cerberus-infosec.co.uk  
  
To ensure that the Cerberus Security Team remains one of the strongest  
security audit teams available globally  
they continually research operating system and popular service software  
vulnerabilites  
leading to the dicovery "world first" issues. This not only keeps the team  
sharp  
but also helps the industry and vendors as a whole ultimately protecting the  
end consumer.  
As testimony to their ability and expertise one just has to look at exactly  
how many major  
vulnerabilities have been discovered by the Cerberus Security Team - over 40  
to date,  
making them a clear leader of companies offering such security services.  
  
Founded in late 1999, by Mark and David Litchfield, Cerberus Information  
Security, Ltd  
are located in London, UK but serves customers across the World. For more  
information  
about Cerberus Information Security, Ltd please visit their website or call  
on  
+44(0) 181 661 7405  
  
Permission is hereby granted to copy or redistribute this advisory but only  
in its entirety.  
  
Copyright (C) 2000 by Cerberus Information Security, Ltd  
  
  
`