Millions of people may have been left vulnerable to hackers while surfing the web on Apple and Google devices, thanks to a newly discovered security flaw known as "FREAK attack".
There is no evidence so far that any hackers have exploited the weakness, which companies are now moving to repair.
Researchers blame the problem on an old US government policy, abandoned over a decade ago, which required American software makers to use weaker security in encryption programs sold overseas due to national security concerns.
Many popular websites and some internet browsers continued to accept the weaker software, or can be tricked into using it, according to experts at several research institutions who reported their findings on Tuesday.
They said that could make it easier for hackers to break the encryption supposed to prevent digital eavesdropping when a visitor types sensitive information into a website.
About a third of all encrypted websites were vulnerable as of Tuesday, including sites operated by American Express, Groupon, Kohl's, Marriott and some US government agencies, the researchers said.
University of Michigan computer scientist Zakir Durumeric said the vulnerability affects Apple web browsers and the browser built into Google's Android software, but not Google's Chrome browser or current browsers from Microsoft or Firefox-maker Mozilla.
Apple and Google both said they have created software updates to fix the "FREAK attack" flaw, which derives its name from an acronym of technical terms.
Apple said its fix will be available next week and Google said it has provided an update to device makers and wireless carriers.
A number of commercial website operators are also taking corrective action after being notified privately in recent weeks, said Matthew Green, a computer security researcher at Johns Hopkins University.
But some experts said the problem shows the danger of government policies that require any weakening of encryption code, even to help fight crime or threats to national security.
They warned those policies could inadvertently provide access to hackers.
"This was a policy decision made 20 years ago and it's now coming back to bite us," said Edward Felten, a professor of computer science and public affairs at Princeton, referring to the old restrictions on exporting encryption code.