Apple has patched a hole in security that circumvented restrictions on what some applications can run on iPhones, iPads and the iPod Touch.

The JailbreakMe application used the same vulnerability to unlock the devices to run non-approved applications.

The bug came from the way Apple's mobile Safari browser managed PDF document files.

Security experts warned that it left open some users to be vulnerable to attack via booby-trapped sites.

The risk was all theoretical, since no criminals have been thought to exploit the security hole.

It is widely used to run applications, utilities and add-ons that are not approved by Apple.

The company has a tight watch on the apps available in the store and a lot of people turn to alternatives like Cydia for unofficial applications.

The lapse in security was first used for the JailbreakMe program that allows used to visit a cleverly crafted webpage to trigger the process.

The newest patch will keep JailbreakMe from working, but will seal up a vulnerable spot that, if exploited, could give attackers access to the mobile devices.

The updates will be given to owners when they connected the device to a computer for updates.

The company and security experts recommended that owners download and install the update immediately.

However, the update is not required, and a lot of people may avoid downloading and installing it to make sure they can still run unofficial applications.

"It remains to be seen, of course, how many iPhone and iPad users decide to install this security patch," said Sophos senior security analyst Graham Cluley on his blog.

"Some may be delighting in their newly-jailbroken gadget."

Mr Cluley said leaving the vulnerability unpatched would be a "foolish move".

"It would be relatively trivial for a malicious hacker to exploit it, and cause a problem on your shiny Apple gear," he warned.

In its update, Apple said that it applies to second generation and later iPods and the iPod Touch. A security package that fixes the bug on first generations devices is being developed by a progammer named Jay Freeman.