Firstly, the rant – it’s not hacking! (well technically it is – but it’s not some fancy complicated technical attack requiring specialist knowledge and equipment).
A lot of people are under the impression that mobile phone voicemail is only accessible from the mobile phone itself and some may even believe that messages are stored on the phone. In fact, messages are recorded at the mobile network providers’ data centres and played back over the network when the user dials in to pick them up. It isn’t even necessary to have access to the mobile phone itself to get access to someone’s voicemail account – dialling their number while the phone is off or busy on another call results in call diversion so a message can be left, and this is where the “hack” can start. By pressing the right key sequence during the “please leave a message” welcome message, anyone can get to the menu which allows voicemail to be played back. It’s a feature designed to let users listen to their messages from anywhere in the world, whether their phone is working or not, and is genuinely useful – but it creates a backdoor through which messages can be accessed.
Of course, a PIN is required to gain access to the mailbox but many people leave the default PIN on their account, and these are very well known – most are published on the network providers’ websites or are available in the manuals available with any phone or SIM from the provider. In other cases, PINs can be guessed in the same way as passwords by doing a little bit of background research to find out things like birthdays of relatives, friends or pets, other significant dates or registration numbers of cars. Other methods, like social engineering – where carefully crafted questions and behaviour are used to get the target to reveal their PIN or even just “shoulder surfing” (watching someone enter their PIN while they listen to their messages) can be very successful too.
However the PIN is obtained, once the attacker has it, they have full control of the voicemail system and can listen to and delete messages at will.
For some users this could lead to personal data being disclosed, while for businesses it could be used to discover sensitive material.
If you don’t need voicemail, turn it off. If you do need it – don’t use the default PIN, use a number which isn’t associated with anything that is obviously connected to you – and change it regularly. Avoid obvious PINs like 1111, 1234, 9999 and so on – treat it like the PIN for your bank card, it could have similar value to someone who wants to spy on you. The same rules also apply to the answering machine on your land line – most of them have remote access capabilities so anyone who dials your number could listen to your messages if they can guess the access code.
Get into the habit of checking your voicemail. If you regularly seem to be receiving messages without the network telling you that they’re waiting, it could be an indication that someone else is listening to them. Don’t store sensitive messages on the server for too long either. Delete them as soon as you can.
If you’re going to leave a message for someone – don’t disclose any sensitive material, or better yet send a text message. SMS is far more difficult to intercept without legal authority.
Of course, there is another way to access voicemail – but that does require some technical skill and access to right equipment. It would be unprofessional of me to describe it here though. Suffice to say that OFCOM take an interest in anyone trying to offer the service commercially.
