I found this on the guardian today, it states that a particular string of non-latin characters such as arabic or Chinese sent via sms or iMessage can crash an iOS device.
I hope that doesn't put iPhone 4s at risk.
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please note. this is a bug, not a virus. don't make it sound worse then it is. it's easily fixed by a user if they get this happening to them, and the apple teams are working on this as we speak. it is, how ever, not a virus, which is a program that does things. just a bug in a program.