WhiteHat Security Advisory 1004
November 11, 2002

===================
Problem Description
===================
Vulnerable web shopping cart software passes prices between web pages 
using hidden form fields.  What this means is that every time a customer 
adds something to their shopping cart, the cart checks HTTP-POSTed data 
coming from the CUSTOMER computer to determine the price.  The problem is 
that the user can alter this data before sending it to your web server, 
allowing the user to set the price of his or her choice.

This hack is already widely known in the WhiteHat and BlackHat 
communities.  I hope to spread awareness to those site owners who are 
trusting their stores to faulty software.

===================
HOW THIS HACK WORKS
===================
Visit some vulnerable site and look at a set of expensive "FooBars". 
Install an simple IE plugin that allows you to edit HTTP POST data before 
submission and then change the hidden form field containing the price of 
the FooBars from $575 to $10.

Now, send the edited data and look at the confirmation page. 

======
Impact
======
Malicious users may set their own prices at any site using vulnerable 
cart software.  If prices are not hand-verified, vulnerable sites lose 
revenue.

=====================================
Mitigating Factors / Vendor Snake Oil
=====================================
1> Some vendors think it is sufficent to change from HTTP GET requests to 
HTTP POSTs. 
Insufficent.  Handcrafted-HTTP requests using PERL, C++, etc allow the 
user to fake a post.

2> Checking HTTP Referer (http://www.cart32.com/kbshow.asp?article=C051)
Insufficent.  HTTP Referer is a header sent FROM the client and thus 
should not be trusted.  User can either fake header or use a trivial IE 
plugin which allows on-the-fly POST editing.  Writing such a plugin took 
the author 5 hours.  The widely available test proxy known as Achilles 
can also execute this attack.

=======================================
Vendors Affected and Notification Dates
=======================================
JustAddCommerce			- Notified July 15
Cart32							- Notified July 8
Approximately 50% of the hand-coded carts tested	- Notified at 
assorted dates/times

Related note [1]: PayPal does not claim that its donations are secure, 
and thus I do not consider them vulnerable.  Prices are passed in URL.
https://www.paypal.com/cgi-bin/webscr?amount=9.99&return=http%
3A//www.thisistrue.com/thanks.html&item_name=Whatever

Related note [2]: A number of vendors have protected their item price 
data, but not their shipping charge data.  When submitting a shipping 
charge of -40, the user receives a $40 discount on their order.

=====================
Where to go from here
=====================
Find out if you are vulnerable.  Review your code or your HTTP traffic to 
determine where the prices are coming from.

If you find you are vulnerable:
1> Immediately begin verifying orders and prices.
2> Call your vendor and request a patch
3> Read the Web Security section of "Writing Secure Code" or similar to 
figure out how to fix this class of vulnerability.

===========================
How to prevent this problem
===========================
Cart software should NEVER trust ANY data coming from the client.  This 
includes HTTP Headers.  If the cart must rely on HTTP POSTed data, it 
should be delivered in a cryptographically secure manner.