Facebook Fundraisers Lacks Rate Limiting Protection. Malicious actors can bruteforce this by sending different random credit or debit card numbers. This could result to large scale fraud.
The Expected Behaviour is Once You tried to Repeatedly Link or Use a Declined or Invalid Credit/Debit Card the System Should block you from doing additional Transactions in Facebook as Part of its Anti Fraud Protections.
Proof of Concept
May 28, 2019 – Report Sent
May 31, 2019 – First Response from Facebook
June 12, 2019 – Triaged
August 13, 2019 – Fixed
August 23, 2019 – Bounty Issued