EDIT 2: It seems that I have confused the "encryption" with normal hashing. So this comment is invalid if they are doing encryption properly.

I wonder how hard it is to brute-force the credit card numbers. Given the last 4 digits, the Luhn algorithm and the starting digit of common cards (e.g. 4 for Visa), there are essentially only 10 digits left to crack. That's just 10 billion combinations.

As part of the source code is exposed, most likely the encryption algorithm/sequence is already known. If the passphrase is shared among all credit cards and the encryption algorithm is a fast one, there's a fair chance that eventually these credit cards are going to be exposed all at once.

Or am I missing anything?

EDIT: Even if the encryption algorithm is as slow as 10,000 tries per second, exhaustively attempting every single credit card number possible is going to take 277 CPU-hours, or about a day in three quad-core machines.