Some software for sale in the Mac App Store can easily be downloaded for free, according to reports.
Copies of popular apps - notably Angry Birds - are already circulating the web, a day after the launch of Apple's desktop app store.
The trick apparently only works on some apps, which don't validate receipts from the store, letting multiple users run the same version of the software.
"Unfortunately, many of the applications in the App Store can be pirated without payment," said Sophos researcher Chester Wisniewski on the company's blog. "Developers of applications like Angry Birds appear to have ignored Apple's advice on validating App Store receipts before launching."
"What does this mean? It allows people to reconfigure a paid application to run on other people's Apple IDs without requiring them to purchase the app," he said.
That could lead to those too "thrifty" to pay for apps to head over to Google to search for free, pirated versions, Wisniewski said, warning that pirated apps could easily be tweaked to include malware.
"Some applications downloaded from the App Store can easily be modified to include any sort of executable code you wish. It wouldn't surprise me to see a surge in markets for pirated applications that might just be booby-trapped to include unexpected surprises.
Apple has yet to return request for comment.