I am not sure if it is my heightened sense of post-DefCon paranoia, but this just seems like a bad idea to me. If I were a hacker, wouldn’t I just love a way to insert myself into the payment process? With most security analysis processes, I start by examining trust relationships I can exploit. This tends to be fertile ground for logic flaws, and these trust points tend not to be closely inspected by users. If I can insert myself into an established trust relationship to launch my attack, I am far more likely to succeed, and this seems like an open window for me to do just that. Bogus image tags, XSS, XSRF, inline frames, or whatever attack du jour; it seems like a natural target for inserting myself between these two trusted entities. I am not saying that any particular merchant site is insecure at this time, but I am willing to bet that regardless of any vetting process third parties go through, their security is not uniformly as strong as eBay’s and PayPal’s.
In general, I have no relationship with any of the third party merchant software, so I have no reason to trust the sites or their security. I make purchases on eBay with PayPal because I have a basic trust in their sites, processes, and security teams. This trust does not fully extend to every one of their affiliated merchants and third party sites, now and in the future. Not only that, the third party site offers me, the buyer, no added value, only potentially decreased security.
From PayPal’s own “Top Ten Safety Tips”, which they provide with the Security Key, tip number nine is “Stay Safe on eBay: … Pay safely using PayPal, the secure payment method that enables you to shop without sharing your financial information with the seller”. But if the merchant has been linked into the process, and you have to go to a merchant site first, it is somewhat at the seller’s discretion. And if the merchant site has been hacked, all bets are off.
I sent the question over to eBay and PayPal security and have not received a response, so I wanted to know what the community at large felt about this.