L0pht Advisory: NT IIS 4.0 - showcode file viewing vulnerability

[weld () L0PHT COM (Weld Pond)]

               L0pht Security Advisory

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URL Origin:    http://www.l0pht.com/advisories.html
Release Date:  May 7th, 1999
Application:   Microsoft IIS 4.0 Web Server
Severity:      Web users can view ASP source code and other sensitive
               files on the web server
Author:        weld () l0pht com
Operating Sys: Microsoft NT Server 4.0
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I. Description

Internet Information Server (IIS) 4.0 ships with a set of sample files
to help web developers learn about Active Server Pages (ASP).  One of
these sample files, showcode.asp, is designed to view the source
code of the sample applications via a web browser. The showcode.asp
file does inadequate security checking and allows anyone with a web
browser to view the contents of any text file on the web server.  This
includes files that are outside of the document root of the web
server.

Many ecommerce web servers store transaction logs and other customer
information such as credit card numbers, shipping addresses, and
purchase information in text files on the web server.  This is the
type of data that could be accessed with this vulnerability.

The L0pht would like to thank Parcens for doing the initial research on
this problem.

II. Details

The showcode.asp file is installed by default at the URL:

http://www.someserver.com/msadc/Samples/SELECTOR/showcode.asp

It takes 1 argument in the URL, which is the file to view. The format of
this argument is:

source=/path/filename

So to view the contents of the showcode.asp file itself the URL would be:

http://www.someserver.com/msadc/Samples/SELECTOR/showcode.asp?source=/msadc/Samples/SELECTOR/showcodSamples/SELECTOR/showcode.asp

This looks like a fairly dangerous sample file. It can view the contents
of files on the system. The author of the ASP file added a security check
to only allow the viewing of the sample files which were in the '/msadc'
directory on the system. The problem is the security check does not test
for the '..' characters within the URL.  The only checking done is if the
URL contains the string '/msadc/'.  This allows URLs to be created that
view, not only files outside of the samples directory, but files anywhere
on the entire file system that the web server's document root is on.

For example, a URL that will view the contents of the boot.ini file, which
is in the root directory of an NT system is:

http://www.someserver.com/msadc/Samples/SELECTOR/showcode.asp?source=/msadc/Samples/../../../../../bSamples/../../../../../boot.ini

This URL requires that IIS 4.0 was installed in its default location.


III. Solution

For production servers, sample files should never be installed so delete
the entire /msadc/samples directory.  If you must have the showcode.asp
capability on development servers the showcode.asp file should be modified
to test for URLs with '..' in them and deny those requests.


For specific questions about this advisory, please contact
weld () l0pht com

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