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RE: SQL Injection
From: The Crocodile <tcroc () pasture com>
Date: Sat, 05 Jun 2004 09:19:59 -0400

Blindly encoding characters may still cause errors and issues if that
input is utilized prior to encoding, or if the input in encoded form
contains characters that will cause errors. You should always validate
the input regardless and then encode the output prior to presenting it
back to the end user.

A better question is are there any publically available "whitelist"
based libraries that are easy to use for input validation? In what
langauages are these libraries available? Is there a compiled list of
these libraries somewhere that I am unaware of?

It would be great to have an extensible regex style whitelist library
available where a simple function(s) can be called with the line of
input and the whitelist regex and a 1 or 0 are returned. (This has been
oversimplified for the sake of the post).

I did a brief google search "input validation library", but didn't see
anything at first glance.

Comments welcome.


--The Crocodile

On Sat, 2004-06-05 at 02:17, stevenr () mastek com wrote:

When I had mentioned whitelists in my post, I meant whitelists as put
forth by Crocodile. Its about creating rules about what characters can
be allowed and rejecting (or encoding) the rest. Another approach which
I think may help (as mentioned by Crocodile) is blindly encoding all the
input and then saving it in the db (or showing it on the page). This may
not always be right, specially if there are other tools/systems which
read the data but are unable to decode it back to original form. 

BTW, any opinions on if I just encode all input without checking for any
characters? Say converting all <script> to &lt;script&gt; Can anyone
still do XSS or SQL Injection in that case? 

Steven Rebello 

-----Original Message-----
From: The Crocodile [mailto:tcroc () pasture com] 
Sent: Friday, June 04, 2004 5:55 PM
To: Steven M. Christey
Cc: webappsec () securityfocus com
Subject: Re: SQL Injection

I think I'm confused about your use of the term "whitelist" in the
scenario below. White lists IMHO aren't going to be vulnerability
specific at all. They are going to be specific to the particular input
parameter in question. I don't see how you would have differing
"whitelists" for XSS and for SQL Injection.

A white list is specifically what is going to be allowed for a
particular parameter. For example a phone number in the US might allow
1234567890()- and that's it. White listing the particular input field to
allow only those characters (and escape them if neccesary) should stop
both XSS and SQL injection attack characters. Encoding the output
presented to the user is an additional step that can be done to double
check for display type attacks (again XSS).

Maybe I misunderstood your post, but I just wanted to make sure these
subtle differences were clear to the list.


--The Crocodile

And yes I know "whitelisting" won't catch input data that is valid that
additional business logic should be catching. IE. Access control
violations. That is a different thread all together.

On Thu, 2004-06-03 at 20:35, Steven M. Christey wrote:
The best way would be creating a white list, allowing only defined 
characters and rejecting everything else. Saves you headaches in the 
long run. Use Regexs for this.

While white lists are far better than black lists, the correct "white 
list" will vary depending on which type of vulnerability you are 
protecting against.  For example, restricting inputs to alphanumeric, 
spaces, and hyphens will still open you up to certain argument 
injection vulnerabilities.  So, you may need to apply different white 
lists to the data, depending on where (and how) the data is being 
used, and which types of vulnerabilities may be present at that point.
You may want to use a "SQL injection" white list on data input, with 
an "XSS white list" on data output (though "XSS white list" is almost 
an oxymoron these days, with all the custom browser behaviors).

It would be interesting to know if anybody's tried to implement 
"context-sensitive taint checks" that know which filters have been 
applied to data elements, and when.

- Steve

"Making a valuable difference"
Mastek in NASSCOM's 'India Top 20' Software Service Exporters List.
In the US, we're called MAJESCO

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