PAN Discovery Browser by Sure QC

This extension allows you to prevent payment card numbers from being displayed and to get a notification that the page potentially contains such information. All detection is done in-browser and no information is sent to any other location.

It's useful for preventing accidental forwarding of emails with card numbers in online email systems. It can allow the identification, reorting and instigation of Incident Response where unexpected credit card or debit card numbers are located. See https://www.pcisecuritystandards.org/ for details of the PCI DSS standard.