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 (mnemonixat_private) 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
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