RE: Securing file access

["Michael Silk" <michaels () phg com au>]

Hi Chad,

        Of course the problem isn't that it refers to the real file but that it could be used (if it isn't validated 
properly) to delete any file on the server.

                <input type="hidden" name="remove_file" value="..\..\..\autoexec.bat" />

        So the solution is to validate it for any path manipulation. Of course, it's far easier just to read in the 
file and write the bytes out like other posters have suggeted.

-- Michael 

-----Original Message-----
From: Beckner, Chad A [mailto:cbeckner () iupui edu] 
Sent: Friday, 1 October 2004 8:44 AM
To: Calderon, Juan Carlos (GE Commercial Finance, NonGE); Shields, Larry; webappsec () securityfocus com
Subject: RE: Securing file access

Remember, this would be only the temporary file that was sent to the user, NOT a link to the actual file.  Per the 
example I used, here is a variable that could be used:

<input type="hidden" name="remove_file" value="adobe.pdf">

Your processing script(s) would point to the file in the temporary file location, such as:

wwwroot\_common\_temp_file_cache\adobe.pdf

It would never "touch" the "real" file.

Chad

-----Original Message-----
From: Calderon, Juan Carlos (GE Commercial Finance, NonGE) [mailto:juan.calderon () ge com]
Sent: Thursday, September 30, 2004 5:15 PM
To: Beckner, Chad A; Shields, Larry; webappsec () securityfocus com
Subject: RE: Securing file access
Importance: High

woah!

Sending a path in a hidden field to remove a file in server doesn't sound good to me.  be carefull with this approach!


-----Original Message-----
From: Beckner, Chad A [mailto:cbeckner () iupui edu]
Sent: Miércoles, 29 de Septiembre de 2004 09:09 p.m.
To: Shields, Larry; webappsec () securityfocus com
Subject: RE: Securing file access


You could also copy the file to a temporary read-only directory, and run an administrative script to clean out the temp 
directory every night, hour, or even after the user has received the file (i.e. on the next page hit using a hidden 
variable, such as "remove_file", value="adobe.pdf").

- Chad

-----Original Message-----
From: Shields, Larry [mailto:Larry.Shields () FMR COM]
Sent: Wednesday, September 29, 2004 1:16 PM
To: webappsec () securityfocus com
Subject: RE: Securing file access

 
        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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