Silent SMS
SMS is the protocol by which standard text messages are delivered to your cell phone; a “silent” SMS message would deliver a “message” to the phone without you being aware of it 1. In other words, you wouldn’t see a text message or notice anything at all on your phone.
However, this creates traceable data traffic for mobile operators, as the SMS (and later also its confirmation of receipt) is forwarded to its destination via all the necessary mobile phone cells. The path taken by the SMS can then be evaluated by the authorities, allowing locations to be determined with an accuracy of up to a few meters.
Don’t let your phone receive SMS, by either:
- taking out you SIM card
- turning on airplane mode
- shutting your phone off
There have been some apps floating around over time, that promised to detect silent SMS. The problem with those is, that:
- The mobile modems, e.g. the chips that actually receive the SMS, are technical black boxes to the public - we can’t tell what they are really doing.
- For most apps, your phone has to be rooted, which we strongly advice against. While it enables more user freedom, rooting breaks the fundamental security mechanisms of your mobile operating system.
Silent SMS - Application
How often law enforcement uses Silent SMS may very a lot, mostly depending on the country. As with many other threats we describe here, we don’t have a lot of concrete numbers about the usage of those techniques. What is sure is that it’s a very cheap technique, which can be easily used by most authorities without further ado.
In Germany for example, the federal authorities (which don’t include the regional ones) send between 200.000 - 400.000 silent SMS per year.
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