only be the tip of the iceberg. the department of homeland security this week warned retailers nationwide to be on high alert as they investigate what experts call the largest hack against retailers. and experts say the attacks are not over. >> when we're seeing these types of attacks before and they create their attack methods and all the associated tools, they don't just target one or two companies. they say how many companies are susceptible to this? let's go after all of them. >> reporter: john walters company is helping them sophisticated malicious software that is used by credit scammers. >> what makes this executed and orchestrate, get access to the point sale system, get access to the credit card before it is encrypted, get that information out of your environment before it is detected. >> reporter: many retailers may not even know their credit card scanners are infected.