The same vulnerability used tocan be used by a hacker to take complete control of it, security firms have warned.
This rather worrying caution comes from Symantec, Lookout and Vupen, licking their lips at the prospect of Apple users buying their security software.
But the threat is real and simply needs an iPhone user to visit a Web site that has a fiddled PDF file. When your trusty iOS displays the PDF -- hey presto! -- the hacker has complete control of your private stuff.
Apple was investigating the problem, Reuters reported.
So far criminals haven't tried to use the flaw, but as Apple's market share grows this could change. A recent security conference showed Google's Android operating system vulnerable to attack as well.
"We shouldn't be surprised to see security bugs happen in very complex software," Kevin Mahaffey, of mobile security firm Lookout, said to Reuters.
Mobile malware has so far not been much of a problem, simply because there's not much money to be made from it. But malware has plagued the PC world for many years, while Apple's Mac OS X, previously thought to be untouchable, now regularly has to be patched for security flaws.
The iPhone has been generally thought of as well protected due to the nature of its OS, which only allows apps that have been approved through a strict process.
But as this problem shows, and it's not the first time it's happened, once you get through the 'jail' the system is housed in, it can suffer problems.
But we needn't panic yet. The security industry has tried to persuade us for a few years that mobile security software is worth buying, and as yet this hasn't been the case. But it's obviously worth being a little more careful as smart phones become more popular every day.