Paypal Security Flaw?

问题

I have a the following form at the end of a booking process (simplified):

<form action="https://www.paypal.com/cgi-bin/webscr" name="paypalForm" method="post">
<input type="hidden" name="amount" value="<?=$price;?>">
<input type="hidden" name="business" value="business@email.co.uk">
<input type="hidden" name="notify_url" value="http://website.co.uk/ipn">
</form>

I have only left out things like address name etc. So when they pay via Paypal, I am using paypal IPN to mark them in the database as paid. However..

I have gone to the end of my booking system and viewed source of the webpage, modified the business email address and amount. I haven't tried a full transaction yet, but surely with the 'notify_url' in there Paypal with send an IPN message to my server and will mark the person off as paid? Isn't this terrible security? Surely this not how all paypal payments work, I must be missing something.

There are two things I can think of that might prevent this:

1. If I remove the "notify_url", will the IPN URL that I have set in paypal work instead? What value does paypal place on the hidden var notify_url, does it override the settings in the back end of paypal?
2. In my IPN code I could check for business and Amount. I don't currently, as I didn't read any where in the documentation that I should. But now, I am thinking that maybe it would be a terribly good idea.

回答1:

There isn't any check that can be done by PayPal to know what the correct amount, or email address should be that was used, or that the IPN URL should only be used with a particular PayPal account. Your options would to be write in the additional checks like you have already mentioned. In additional to what you already stated about your 2 workarounds, a 3rd option would be to create a hosted or encrypted button on the fly using PayPal's BMCreateButton API. Then the buyer would only see the encrypted button code, they would not be able to view your HTML button code. Therefore they would not be able to modify any of the variables, or see what they are currently set to.

来源：https://stackoverflow.com/questions/16143548/paypal-security-flaw