I found this to be an interesting question for two reasons,
- It turned the classic password brute force on its head by applying it to credit cards
- It attracted the attention from a large number of relatively new users
Jeff Ferland postulated that the website was too helpful with its error codes and recommended returning the same “Transaction Failed” message no matter the error.
User w.c suggested using some kind of additional verification like a CAPTCHA. Also mentioned was the notion of instituting time delays for multiple successive CAPTCHA or transaction failures.
A slightly different tack was discussed by GdD. Instead of suggesting specific mitigations, GdD pointed out the inevitability of the attackers adapting to whatever protections are put in place. The recommendation was to make sure that you keep adapting in turn and force the attackers into your cat and mouse game.
Ajacian81 felt that the attacker’s purpose may not be finding valid numbers at all and instead be performing a payment processing denial of service. The suggested fix was to randomize the name of the input fields in an effort to prevent scripting the site.
John Deters described a company that had previously had the same problem. They completely transferred the problem to their processor by having them automatically decline all charges below a certain threshold. He also pointed out that the FBI may be interested in the situation and should be notified. This, of course, depends on USA jurisdiction.
Both ddyer and Winston Ewert suggested different ways of instituting artificial delays into the processing. Winston discussed outright delaying all transactions while ddyer discussed automated detection of “suspicious” transactions and blocking further transactions from that host for some time period.
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