According to a post on the Russian website Habrahabr.ru, there is a serious vulnerability in iOS6 which allows nefarious users to remotely crash apps on iOS 6, or destroy them.
The vulnerability is due to a bug in Apple's CoreText font rendering framework. It also effects OS X Mountain Lion.
What makes this dangerous is that it is not a difficult hack - simply exposing various iOS or OS X apps to one of several possible strings of text is enough to cause a crash.
Apple's browser is also vulnerable to the bug. Naming a wi-fi network with one of the strings of text can cause an error while an Apple device is scanning for networks.
What might worry some users is Apple has known about this bug for six months and has not done anything about it. It has changed the beta versions of iOS 7 and of OS X Mavericks but it seems that it is not going to bother helping out its older users.
After all if you want an incentive to force users to upgrade, what's better than refusing to update a previously buggy product?