The 'silent' tracking SMS you didn't see arrive

Tracking by “silent” or “hidden” SMS is used most often by adversaries that have means of obtaining the so called “Call Data Records” from a mobile operator. In this type of record for each activity of a phone on the mobile network a database entry is created that among other (mostly billing relevant) data contains the identification number of the mobile network cell (CellID) and signal strength data. This data can then be used to track a phones location.

Typically, Silent SMS are used to follow a mobile phones position during longer periods of time by sending the SMS in regular intervals. If you see Silent SMS arriving at intervals, one by one over a period of time you have to assume that you are being tracked.

The Cryptophone Baseband Firewall can not prevent the phone from receiving these SMS and can therefore not prevent the tracking, but make you aware of it. If you are the victim of silent SMS tracking consider getting a new prepaid SIM-card (if possible not under your name) with a new phone number as this is the only good method available to get rid of the tracking.

There are several different types of “silent” or “hidden” SMS. All of them share the property that they are not shown to the user but a delivery report is generated by the phone and sent back to the network.

Some types of Silent SMS are normally meant for internal purposes of the phone network, like re-configuring the phone when roaming. This also means that there are legitimate “silent” SMS, most often when joining a new network when roaming. Typically these arrive in batches of three to five SMS.

So if you see multiple Silent SMS tracking alerts in short sequence while on roaming, it is most likely harmless, especially when it occurred right after joining a new network.

For more information on this and other cellular vulnerabilities contact support@cryptophoneaustralia.com