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Re: IBM Websphere Commerce Server 5.5 XSS detect mode
From: Paul Johnston <paul () westpoint ltd uk>
Date: Wed, 12 May 2004 12:19:15 +0100

Hi Crocodile,

While I agree with you in principle, I feel it's hard to get this "whitelist" right. In the example you give, your validation routing would reject +44-(0)161-237-1028 which is a widely accepted way of displaying an international phone number.

Another issue with this approach is that it's easy to then think "ok this has been sanitised, so now it's trusted" which is a fallacy. For example, you take user_id from the client. It's all digits so it passes sanitisation. However, if you now trust this variable then a user could tamper with the parameters and impersonate another.

I'm not against sanitisation, but this idea of "only allow good chars and all will be fine" is overly optimistic.

Best wishes,


The Crocodile wrote:

While I'm sure this is a great technique to do, and certainly a step in
the right direction for many applications, the better way to validate
data supplied by the client side would be to compare the input against a
known set of GOOD data and if there are characters that are not in this
set of known good, then reject the request.
For example: An input field of a phone number should only accept numbers
and dashes, it should not accept any other characters and should reject
on any input that contains anything other than numbers or dashes. (or at
least give an error to the user). Validation routines ideally should be
done on a per field basis.

Rejecting data based on it containing certain known bad characters is
like firewalls listing all the things that need to be dropped and
accepting everything else. It's not really best practice.

I hope this makes sense.


On Mon, 2004-05-10 at 22:37, Jim+Lisa Weiler wrote:
IBM Websphere Commerce server 5.5 has a switch that causes the server to
examine all fields in POSTs and all variables in GETs and check the input
against a set of strings and characters that are not allowed, and return one
of 3 customs web pages if non allowed strings or characters are found. Does
anyone have experience with this feature in Websphere Commerce Server?

Thanks, Jim

Paul Johnston
Internet Security Specialist
Westpoint Limited
Albion Wharf, 19 Albion Street,
Manchester, M1 5LN
Tel: +44 (0)161 237 1028
Fax: +44 (0)161 237 1031
email: paul () westpoint ltd uk
web: www.westpoint.ltd.uk

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