Cyber-thieves are preparing malware and spam campaigns in a bid to catch out retailers and shoppers during the run-up to Christmas, experts say.
One gang had updated the sophisticated malware it used to target tills in stores, security company iSight said.
There had also been an increase in spam and phishing emails crafted to catch out people seeking bargains.
And some crime groups had made fake copies of popular shopping apps in a bid to steal payment-card data.
The warnings are being given just prior to Black Friday and Cyber Monday, which bracket the weekend following the US Thanksgiving holiday, when many online and offline stores offer special deals.
The 50 biggest retail brands in the US were now hunting through their internal corporate networks to see if they had been infected by the "highly sophisticated" Modpos malware, said iSight senior director Stephen Ward.
The modular malware could lurk unseen on point-of-sale equipment, said Mr Ward, and sought to scoop up payment-card data during the few moments this information was passed around unencrypted in the memory of computerised tills.
"It's a Swiss-army knife of sorts that can be used for any type of nefarious activity," he said.
The Retail Cyber Intelligence Sharing Center, a US government-backed organisation set up to pass on information about threats aimed at retailers, has sent out advice about the "2015 hacking season".
"Downtime is expensive, but especially so at this time of year," it said.
"Retail staff is motivated and focused on sales, at the risk of possibly allowing fraudulent transactions or other types of breaches."
Reacting quickly to threats could be tricky at this time of year, it said, because systems were often "frozen" to limit downtime.
Mr Ward said iSight had been tracking the gang behind Modpos for some time, but it had now been revamped for the run-up to Christmas.
And traditional anti-virus systems were unlikely to catch the stealthy malware because of the clever way it was built.
"The guys behind this seem to have an incredible level of knowledge related to the way that security technology works and the industry operates," said Mr Ward.
But iSight had passed on information about telltale signs that would reveal a retailer had been compromised by Modpos.
"We are stealing Christmas from these bad guys," he said. "Here, we are being the Grinch."
Anti-fraud company ThreatMetrix said online retailers were also coming under sustained assault from many different hi-tech crime groups.
It said it had seen signs of an increase in fraud campaigns before the main shopping season got under way and expected a "major spike" in such activity in the run-up to Christmas.
In a report, it said attacks against online retailers had already jumped 25% over earlier in the year and it expected the trend to continue.
"Generally, the third quarter is a slower time for businesses as consumers anticipate spending money during the Christmas and New Year shopping season, but this year it yielded record numbers in attack attempts," said Vanita Pandey, strategy director at ThreatMetrix.
The vast majority of the attacks were attempts to defraud companies by using fake logins or stolen credentials, said Ms Pandey.
And ThreatMetrix had seen evidence of crime groups using botnets, networks of hijacked computers, to batter away at login screens searching for loopholes and bugs.
Michael Owen, head of security at payments processor Sage Pay, said criminals could also target online stores in other ways during busy shopping days.
"Criminals know that merchants need their websites to be live and accessible this weekend," he said, "which makes them an appealing target for blackmail."
"If a criminal can take down a website and demand funds to let the merchant get back to trading, they'll want to do it this weekend."
Paul Ducklin, a senior security advisor at Sophos, said spammers were gearing up for the Christmas shopping season too.
He urged people to be vigilant and exercise common sense when browsing offers sent via email or other messaging services.
No-one should ever buy anything offered via unsolicited email, he said.
"If it sounds too good to be true, it is too good to be true," he said.
"Even if you think that the crooks will take every care with your payment details and your identity, and even if the goods you are buying turn out to be the genuine article, why give these guys your business?
"Instead, ask yourself, 'Do I consider a spam campaign to be the basis of a business relationship founded on mutual trust?'"
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