First of all:
- you do not report this to the government/external party/visa/mastercard
External parties serve to complicate matters via conflict of interest. It is not their business, it is the business of the company who has the vulnerable api. It is not for them to decide how to handle this, but the company.
- Tell no one in your circle of friends, family or anyone about the details.
Here you can get yourself into trouble very easily. If it is discovered later that someone you knew used or attempted to use the information, you could be held accountable for reckless action, or at the very least investigated heavily. Don't put yourself in this position and tell no one anything that would allow them to repeat the vulnerability.
The best thing you can do right now is calmly make a plan and execute it. Anyone who is in the position to exploit this likely already knows about it. Anyone who is not is likely not going to discover this within the next few days.
Now, on to: Responsible Disclosure
Step 1 - Prepare
Before anything we need to make sure that the vulnerable API is well documented and has a set of instructions. For example, a small report, doesn't have to be long, just easy to follow. The idea is, the intern within the office should be able to follow the steps, reproduce the vulnerability and successfully identify the security risks. A small PDF or WORD document will do, just make sure you have something ready in case things move quickly.
Step 2 - First Contact
Next comes first contact with the company. First contact is a delicate thing. On the one hand you would expect that the company has the interest of their customers as a first priority. On the other hand some companies respond rather poorly (and in my opinion, irresponsibly) to people disclosing vulnerabilities for the benefit of everyone involved. For this reason you want to withhold the exact details until an agreement is made that no action will be taken against you.
Me, I'm the paranoid sort, and when I encounter this sort of thing ill go to a coffee shop, use TOR and use an email (one I've set up by using a similar process) to initiate first contact. It is however up to you what level of anonymity you wish to use.
Some things you can check. If the company has a bug bounty, or has a disclosure policy. You should be fine. You can call the company and ask if they have a department responsible for security disclosures. Most companies of this size have an
email@example.com email. All of these would have information that would lead you to exactly where first contact should be made.
Step 3 - Processing
Once you are talking to the right person, it should work itself out from there, except as I've previously noted, if there is no bugbounty or disclosure policy make sure you get them to agree to a terms of disclosure that involves your safety first. At some point you will hand them the document you made in step 1.
Step 4 - Closure
The last step to do is public disclosure. This step is entirely optional, and will be heavily tied into the agreed upon disclosure terms in step 3. But usually both parties agree to an amount of time that the company has to fix this vulnerability before a public announcement can be made. It's entirely normal for the researcher to desire to post it on their blog, or use it as reference material for other projects or qualifications. This time period makes sure that the company is not put under undue pressure and gives them time to act.
Congratulations, you just had your first responsible disclosure. You may choose to not announce it ever, and that's fine. In that case the time period from step 3 would be, for you, when you can safely tell friends and family exact details.
I wish I could give rep for desiring to disclose responsibly, alas, in stead I'll wish you the best of luck. Try to be discreet, keep yourself safe, and try to work with them. Good Luck.