Many of you may have heard about the recent Dexter Malware that has infiltrated POS systems worldwide. In case you haven't, here is a link for more information about it.
The good news is it cannot infect your Mac as it is only on other platforms. Nor can it intervene in MacPOS credit card processing as it employs Triple DES encryption of sensitive card information that prevents thieves from skimming that from your POS system.
An excerpt from securitywatch.pcmag.com :