Users are particularly vulnerable when they view Portable Document Format, or PDF, files, which give attackers an opportunity to infect the devices with malicious software, giving them administrative rights to the device, the German Federal Office for Information Security said this week.

Once the device is infected, cyber criminals could read confidential information such as passwords, online banking data, calendars, emails and other information, as well as intercept telephone conversations and the location of the user. The security hole is present in several versions of Apple's iOS software on its iPhone, iPad and iPod Touch products, the agency said.

"Apple takes security very seriously, we're aware of this reported issue and developing a fix that will be available to customers in an upcoming software update," the company said in a statement.

Germany's information security agency said it hasn't yet observed any attacks on Apple products, but warned that until the hole is closed, users should avoid opening PDF documents from unknown sources, and surf only trusted websites. It added that it was in contact with Apple about the problem.

The German Federal Office for Information Security detected a similar security hole in Apple's software last year, which the company promptly fixed.