When you setup your paypal module, you will notice towards the bottom a label called Digest Key and set to PayPal_Shopping_Cart_IPN This SHOULD be changed, immediately to something around 12 alphanumeric bits.
(Example: MWU26S2S7jgZ)This is from
osCommerce Instant Payment Notification Contribution for PaypalShould I change the digest key default value.
* Yes, but remember what it is if uninstalling and reinstalling this contrib payment module in the admin since there might be existing pending orders which will require it in order to associate with their corresponding IPN(s).
DevOSC May 28 2004, 10:38 PM "PayPal_Shopping_Cart_IPN"
http://forums.oscommerce.com/index.php?showtopic=159436 Last Accessed July 16th 2009
I have submitted a feature request for both EOS and CRE at
http://smeagol.strikehawk.com/mantis/ to hook into the error stack when the module is activated to remind a store owner to change the default value.