Remember OAuth1 session fixation? No? Read writeup from Eran Hammer (the guy who hates OAuth2 as much as I do).
Guess what - there's exact same vulnerability in Paypal Express Checkout flow (they will not fix it). Furthermore, tons of other payment-related providers can be vulnerable to the same attack. How does it work?
OAuth1-like flows are based on request_token/invoice id (for example https://bitpay.com/invoice?id=INVOICE). Before using %PROVIDER_NAME% your CLIENT is supposed to make API call with all parameters (client_id, client_secret, redirect_uri, amount, currency, etc). Provider will respond with request_token=TOKEN1. This is your token and it's tied to your Client account.
Wow, secure! - no URL parameters tampering like we have in OAuth2 (browse my blog a bit to find kilobytes of rants about it). But OAuth2 returns "code" which is not guessable. OAuth1 returns the same request_token it received in initial URL. Voila, fixation.
Thing with Paypal is: no matter who pays this invoice - you will only need to visit Client-Return-URL?token=TOKEN1 to add funds someone paid.
How can we trick a victim into paying our TOKEN1?
1. John, you must buy this book! (x=window.open('BOOK STORE'))
2. John clicks Pay with Paypal and gets redirected to Paypal?token=TOKEN2
3. our opener-page should somehow detect that John is about to pay. Hash-based cross domain URL detection can help. We fixate x.location to Paypal?token=TOKEN1
4. Payment is done, user is redirected to Client-Callback?token=TOKEN1 and doesn't get the book. TOKEN1 was issued for other Client session and only attacker can use it on callback.
5. Attacker uses TOKEN1 on return URL, Client does API call to make sure TOKEN1 is paid = Attacker gets the book.
Successfully tested this hack on Namecheap. This is what victim sees on first step (valid and trustworthy page)
Error after paying for TOKEN1
Attacker visits Callback?token=TOKEN1 to get victim's funds
Provider should issue another oauth_verifier random nonce to make sure the user using callback is the payer.
Is it really severe bug? I don't think so, but it should be fixed for sure.
P.S. researchers complain a lot about paypal's bounty program. E.g. they refused to fix obvious headers injection https://www.paypal.com/.y%0dSet-Cookie:x=1