If attackers have access to a user's network, such as by sharing the same unsecured wireless service offered by a restaurant, they could see or alter exchanges between the user and protected sites such as Gmail and Facebook, experts said.

"It's as bad as you could imagine, that's all I can say," said Johns Hopkins University cryptography professor Matthew Green.

Apple did not say when or how it learned about the flaw in the way iOS handles sessions in what are known as secure sockets layer or transport layer security, nor did it say whether the flaw was being exploited.

But a statement on its support website was blunt: The software "failed to validate the authenticity of the connection."

Apple released software patches and an update for the current version of iOS for iPhone 4 and later, 5th-generation iPod touches, and iPad 2 and later.

Without the fix, a hacker could impersonate a protected site and sit in the middle as email or financial data goes between the user and the real site, Green said.

Apple did not reply to requests for comment. The flaw appears to be in the way that well-understood protocols were implemented, an embarrassing lapse for a company of Apple's stature and technical prowess.

The company was recently stung by leaked intelligence documents claiming that authorities had 100 percent success rate in breaking into iPhones.

Friday's announcement suggests that enterprising hackers could have had great success as well if they knew of the flaw.