News of the bug first started on the MacRumors forum, where several iOS 5 users complained. The theft happened when phone owners were using the latest feature of iOS 5, called iMessage. iMessage was meant to be an easier way to communicate between iOS 5 gadgets, using the Wi-Fi or 3G connection.
The guys from theverge tested and managed to reproduce this bug.
How iMessage Bug works
1) Let’s assume we have three iPhone devices with iOS 5, which are all using the iMessage feature: iPhone #1 (the victim), iPhone #2 (the spy), and iPhone #3 (victim’s friend).
2) We get the sim card from the first iPhone (number#1), and we insert it into the #2 iPhone. After the phone verifies the new sim card, it will start to work using the number from the #1 iPhone. Now the #1 iPhone has no sim card inserted, but can use the iMessage feature. The #2 iPhone has the sim card, and the number from the #1 iPhone.
3) You can put the sim card from the #2 iPhone back into the #1 iPhone.
4) If you send a message from the #3 iPhone to the #1 iPhone, you will notice that the #2 iPhone receives the same message. In fact, all three iPhones see the whole conversation. So, three iPhones are communicating together even if initially only iPhone #1 and iPhone #3 are supposed to be able to see this private conversation.
How you can fix it?
Unfortunately, if you are the victim, the only solution is to replace your sim card with a new one. If you are now the owner of the first iPhone, you can fix this issue very simply: turn off and on iMessage several times. Or, insert your original (and personal) sim card.
Apple has not released any official statement about this bug, but a fix is expected to be released soon.