There is a bug in Apple's iOS and OS X operating systems that will cause the current application to crash if it attempts to render a specific string of Arabic characters.
The bug, found in the CoreText framework Apple uses in iOS and OS X, was apparently mentioned on Twitter as far back as February but recently gained attention among the coding and hacking communities. Apple so far has not acknowledged the issue or offered a fix, though researchers claim that while it affects prior versions of OS X and iOS, the latest developer previews of OS X Mavericks and iOS 7 are unaffected.
This suggests that Apple may be aware of the issue, and a fix may be in the works for prior versions of OS X, but it could also simply mean that changes Apple has made in the development of CoreText for the upcoming operating systems have fixed the issue on their own.
Being that this bug affects any service or program that uses CoreText, it can, unfortunately, be used in a denial-of-service attack, where someone can send the string of characters as a text message, e-mail, iMessage, Web page, or even incorporate it in a Wi-Fi SSID or computer name on a network, and result in OS X systems and handling applications that interact with it crashing.
So far this has not been noted as happening, but is a possibility given the nature of this bug.
This situation in OS X comes a few months a similar one that occurred in OS X Mountain Lion in February of this year, where programs that attempted to render file address URLs would crash. Apple quickly fixed this issue, but the mishandling of Arabic characters, which has been around since the same time frame, has gone unfixed.
Hopefully Apple will address this problem soon, to prevent mischievous individuals from causing problems for users by sending them the string of characters.