RE: Web Forms filtered with SQL constraints

[RSnake <rsnake () shocking com>]

        These are two seperate issues.  Your original comment was about
        protecting yourself using JScript.  That's what my email was
        responding to.  In regards to writing error conditions to trap
        user input after the fact, that's different - and good security.
        The main point is that it cannot be edited or modified by the
        user.  JScript relies on the browser interpreting the script in
        the first place.  I can turn off any rendering simply by
        telneting to port 80 and downloading the source manually,
        creating the query by hand, etc... and in effect bypassing any
        client side filtering you put in place.  Writing ASP that
        protects itself on the server side is much better.  I cannot
        bypass a well written filter because I don't have access to it
        without another out of band attack against the webserver (but if
        that's possible then the server probably has bigger problems).  

        If you simply want to obfuscate your source there are some easy
        techniques, like MD5ing your source in JScript and then
        outputting the text as it renders.  That'll stop 95% of people,
        but it's just obfuscation.  Also, you can employ other tricks
        like keeping your JScript in a remote location and watching for
        referrer headers, etc...  Also obfuscation.  These are additive
        security measures, and they help, but it's not really security.
        Honestly, just give up on the idea of hiding your source, start
        thinking of it as client side input validation - not security,
        and focus on your server side security instead.  You'll get a
        lot more bang for your buck.

On Fri, 8 Oct 2004, [iso-8859-1] B?noni MARTIN wrote:

| Date: Fri, 8 Oct 2004 17:09:21 +0100
| From: "[iso-8859-1] B?noni MARTIN" <Benoni.MARTIN () libertis ga>
| To: RSnake <rsnake () shocking com>, webappsec () securityfocus com
| Subject: RE: Web Forms filtered with SQL constraints
| 
| Hi !
| 
| Thanks for the reply, it's as I was thinking about ! I went on the web to get some more infos about that, and I found 
this article:
| http://www.developerfusion.com/show/4325/
| 
| So some tell this is a good idea, others say it's not, so I am lost :( :)
|  
| 
| -----Message d'origine-----
| De : RSnake [mailto:rsnake () shocking com] 
| Envoy? : vendredi 8 octobre 2004 01:42
| ? : webappsec () securityfocus com
| Cc : B?noni MARTIN
| Objet : Re: Web Forms filtered with SQL constraints
| 
| 
|       Nothing you do at the client side can be hidden.  I can write a
|       client that downloads the source, or watch it via a proxy, or
|       look at the cache, etc....  don't even bother trying.  You
|       should consider anything client side as protection from
|       inadvertant mistakes by users only, and you should always have a
|       fall back filter in place to catch the errors before they do any
|       damage.
| 
| On Wed, 6 Oct 2004, Ian wrote:
| 
| | Date: Wed, 06 Oct 2004 09:52:03 +0100
| | From: Ian <webappsec2 () fishnet co uk>
| | Reply-To: webappsec () securityfocus com
| | To: "[ISO-8859-1] B?noni MARTIN" <Benoni.MARTIN () libertis ga>,
| |     webappsec () securityfocus com
| | Subject: Re: Web Forms filtered with SQL constraints
| | 
| | On 5 Oct 2004 at 13:25, B?noni MARTIN wrote:  
| | 
| | > Hi list ! 
| | > 
| | > I was wondering how to solve the 2 following problems: I have ASP 
| | > (not
| | > ASP.NET) formulaires people have to fill in. To avoid SQ injection 
| | > attacks and other tricks, I have set up some Jscript filtering on each field (i.e.
| | > for instance a name can just be alphabet's characters and no figures 
| | > :) ), and I am planning to do the same on my Database (setting up constraints).
| | > 
| | > 
| | > But I have 2 questions:   - How can I hide my Jscript filtering from the
| | > user ? When I want to see the source, everything is diaplayed, quite 
| | > normal :( ... Maybe it's not so good to tell people what I have done 
| | > to filter them :) I saw some sites where it is impossible to see the 
| | > source, impossible to "hoover the site", impossible even to print 
| | > ... But I have not been able to find on the net how to do this :(
| | > 
| | >   - How can I deal with possible SQL errors within an ASP page ? I 
| | > mean, if a field has been filled in, bypass my Jscript filtering (no 
| | > matter how), and gets to the database but is then "stopped" by an 
| | > SQL onstraint, how do I raise this error on an ASP page without 
| | > diplaying an explicit error (giving the user the name of my database for instance) ?
| | > 
| | > Cheers for any clue, I am lost on this topic :(
| | 
| | Hi,
| | 
| | Using classic ASP with vbscript you would add this to the top of the page:
| | 
| | <% on error resume next %>
| | 
| | Then after every SQL query:
| | 
| | <%
| | if err then
| |     Response.write "There was a database error"
| |     ' Log to error to file
| | end if
| | %>
| | 
| | I think the equivalent in JScript is the Try, Catch, Finally:
| | 
| | http://msdn.microsoft.com/library/default.asp?url=/library/en-
| | us/script56/html/js56jslrfjscripterrorstoc.asp
| | 
| | Hope this helps
| | 
| | Ian
| | --
| | 
| | 
| | 
| | 
| | 
| 
| -R
| 
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| 
| 

-R

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