RE: Securing file access

["Shields, Larry" <Larry.Shields () FMR COM>]

        Be aware that the acrobat reader if using the IE browser plug-in
viewer has been known to have problems when using this method to send
PDF files.  We have seen common problems where users ended up with blank
documents or received errors about the PDF file not being correct.  From
what I've seen, the two solutions if you can't have this possible
failure are to either a) make customers all change their acrobat reader
configuration to start in "helper" mode (meaning kick off acrobat
external to the browser plug-in) or b) store things on the file system
and redirect the user to the PDF file.  Option b does have issues to
make sure it is done securely of course, such as clean-up of old files,
making sure that file names cannot be guessed or predicted by other
users, and that there is no potential for file naming collisions between
users.

-Larry

 
-----Original Message-----
From: Ian [mailto:webappsec2 () fishnet co uk] 
Sent: Tuesday, September 28, 2004 5:05 AM
To: John M. L.; webappsec () securityfocus com
Subject: Re: Securing file access

On 27 Sep 2004 at 11:57, John M. L. wrote:

> I have a project that involves a members only area on web page on IIS.
> The members' only area is secured by a database (MS Access) so users 
> are authenticated by their name and some MD5 hash etc.  I need to 
> allow files (mostly PDFs) for download to authenticated users only.  
> In my opinion this means that the files can not be stored in any www 
> accessible folder (regardless of any renaming convention etc, I 
> absolutely cannot have someone guess a file name to download).  In 
> order to access the files, the database would link a file to a unique 
> id, so a page that validates the user would then give access to the 
> file stored outside of the www on the server.  Now, this is where the 
> real question lies.  How is this possible since the files are not in a
www accessible path, since a mere link to a file won't due.
> Any thoughts would be welcome.  If I'm going about this completely 
> wrong that would be nice to no too :)  Forgive me if the answer is 
> simple, I'm a Linux fan and haven't used IIS etc for years.
> One more note: IIS, MS Access and VBScript are not my technologies of 
> choice, but merely what I was given to work with.  I also have very 
> limited control over administering IIS.
>
> John
> www.recaffeinated.com
Hi John,

You are going about this the right way.

Store the PDFs outside the www root, but still give the user
ISR_<computer name> permission to read the files ( or whatever user your
site is running under).  Once the web user has authenticated, your
script can read the PDF into memory then stream the file to the user.  A
simple example is below:

<%

set fs=server.createobject("Scripting.filesystem")

set PDFin=fs.opentextfile(pathtoPDF,1,false)
PDF=PDFin.readall
PDFin.close
set PDFin=nothing

Response.contenttype="application/pdf"

resonse.binarywrite StrToBin(PDF)

response.end

function StrToBin(str)
        for x=1 to Len(str)
                StrToBin=StrToBin & ChrB(Asc(Mid(str,x,1)))
        next
end function

%>

You may not need to use the StrToBin function - I can't remember off the
top of my 
head ;)

Regards

Ian
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