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The deepest privacy threat from mobile phones—yet one that is often completely invisible—is the way that they announce your whereabouts all day (and all night) long through the signals they broadcast. There are at least four ways that an individual phone's location can be tracked by others.
- Mobile Signal Tracking from Towers
- Mobile Signal Tracking from Cell Site Simulators
- Wi-Fi and Bluetooth Tracking
- Location Information Leaks from Apps and Web Browsing
Mobile Signal Tracking — Towers
In all modern mobile networks, the operator can calculate where a particular subscriber's phone is located whenever the phone is powered on and registered with the network. The ability to do this results from the way the mobile network is built, and is commonly called triangulation.
One way the operator can do this is to observe the signal strength that different towers observe from a particular subscriber's mobile phone, and then calculate where that phone must be located in order to account for these observations. This is done with Angle of Arrival measurements or AoA. The accuracy with which the operator can figure out a subscriber's location varies depending on many factors, including the technology the operator uses and how many cell towers they have in an area. Usually, with at least 3 cell towers the operator can get down to ¾ of a mile or 1km. For modern cell phones and networks trilateration is also used. In particular, it is used where the “locationInfo-r10” feature is supported. This feature returns a report that contains the phone’s exact GPS coordinates.
There is no way to hide from this kind of tracking as long as your mobile phone is powered on, with a registered SIM card, and transmitting signals to an operator's network. Although normally only the mobile operator itself can perform this kind of tracking, a government could force the operator to turn over location data about a user (in real-time or as a matter of historical record). In 2010, a German privacy advocate named Malte Spitz used privacy laws to get his mobile operator to turn over the records that it had about his records; he chose to publish them as an educational resource so that other people could understand how mobile operators can monitor users this way. (You can visit here to see what the operator knew about him.) The possibility of government access to this sort of data is not theoretical: it is already being widely used by law enforcement agencies in countries like the United States.
Another related kind of government request is called a tower dump; in this case, a government asks a mobile operator for a list of all of the mobile devices that were present in a certain area at a certain time. This could be used to investigate a crime, or to find out who was present at a particular protest.
- Reportedly, the Ukrainian government used a tower dump for this purpose in 2014, to make a list of all of the people whose mobile phones were present at an anti-government protest.
- In Carpenter v. United States, the Supreme Court ruled that obtaining historical cell site location information (CSLI) containing the physical locations of cellphones without a search warrant violates the Fourth Amendment.
Carriers also exchange data with one another about the location from which a device is currently connecting. This data is frequently somewhat less precise than tracking data that aggregates multiple towers' observations, but it can still be used as the basis for services that track an individual device—including commercial services that query these records to find where an individual phone is currently connecting to the mobile network, and make the results available to governmental or private customers. (The Washington Post reported on how readily available this tracking information has become.) Unlike the previous tracking methods, this tracking does not involve forcing carriers to turn over user data; instead, this technique uses location data that has been made available on a commercial basis.
Mobile Signal Tracking — Cell Site Simulator
A government or another technically sophisticated organization can also collect location data directly, such as with a cell site simulator (a portable fake cell phone tower that pretends to be a real one, in order to “catch” particular users' mobile phones and detect their physical presence and/or spy on their communications, also sometimes called an IMSI Catcher or Stingray). IMSI refers to the International Mobile Subscriber Identity number that identifies a particular subscriber's SIM card, though an IMSI catcher may target a device using other properties of the device as well.
The IMSI catcher needs to be taken to a particular location in order to find or monitor devices at that location. It should be noted that IMSI traffic interception by law enforcement would meet the parameters for a warrant. However, a “rogue” CSS, (not set up by law enforcement) would be operating outside of those legal parameters.
Currently there is no reliable defense against all IMSI catchers. (Some apps claim to detect their presence, but this detection is imperfect.) On devices that permit it, it could be helpful to disable 2G support (so that the device can connect only to 3G and 4G networks) and to disable roaming if you don't expect to be traveling outside of your home carrier's service area. Additionally, it could be helpful to use encrypted messaging such as Signal, WhatsApp, or iMessage to ensure the content of your communications can’t be intercepted. These measures may protect against certain kinds of IMSI catchers.
Wi-Fi and Bluetooth Tracking
Modern smartphones have other radio transmitters in addition to the mobile network interface. They usually also have Wi-Fi and Bluetooth support. These signals are transmitted with less power than a mobile signal and can normally be received only within a short range (such as within the same room or the same building), although someone using a sophisticated antenna could detect these signals from unexpectedly long distances; in a 2007 demonstration, an expert in Venezuela received a Wi-Fi signal at a distance of 382 km or 237 mi, under rural conditions with little radio interference. However, this scenario of such a wide range is unlikely. Both of these kinds of wireless signals include a unique serial number for the device, called a MAC address, which can be seen by anybody who can receive the signal.
Whenever Wi-Fi is turned on, a typical smartphone will transmit occasional “probe requests” that include the MAC address and will let others nearby recognize that this particular device is present. Bluetooth devices do something similar. These identifiers have traditionally been valuable tools for passive trackers in retail stores and coffee shops to gather data about how devices, and people, move around the world. However, on the latest updates on iOS and Android, the MAC address included in probe requests is randomized by default programmatically, which makes this kind of tracking much more difficult. Since MAC randomization is software based, it is fallible and the default MAC address has the potential to be leaked. Moreover, some Android devices may not implement MAC randomization properly (PDF download).
Although modern phones usually randomize the addresses they share in probe requests, many phones still share a stable MAC address with networks that they actually join, such as sharing a connection with wireless headphones. This means that network operators can recognize particular devices over time, and tell whether you are the same person who joined the network in the past (even if you don't type your name or e-mail address anywhere or sign in to any services).
A number of operating systems are moving towards having randomized MAC addresses on WiFi. This is a complex issue, as many systems have a legitimate need for a stable MAC address. For example, if you sign into a hotel network, it keeps track of your authorization via your MAC address; when you get a new MAC address, that network sees your device as a new device. iOS 14 has settings per-network, “Private MAC addresses.”
Location Information Leaks From Apps and Web Browsing
Modern smartphones provide ways for the phone to determine its own location, often using GPS and sometimes using other services provided by location companies (which usually ask the company to guess the phone's location based on a list of cell phone towers and/or Wi-Fi networks that the phone can see from where it is). This is packaged into a feature both Apple and Google call “Location Services”. Apps can ask the phone for this location information and use it to provide services that are based on location, such as maps that display your location on the map. The more recent permissions model has been updated for applications to ask to use location. However, some applications can be more aggressive than others asking to either use GPS or the combination of Location Services.
Some of these apps will then transmit your location over the network to a service provider, which, in turn, provides a way for the application and third parties they may share with to track you. (The app developers might not have been motivated by the desire to track users, but they might still end up with the ability to do that, and they might end up revealing location information about their users to governments or a data breach.) Some smartphones will give you some kind of control over whether apps can find out your physical location; a good privacy practice is to try to restrict which apps can see this information, and at a minimum to make sure that your location is only shared with apps that you trust and that have a good reason to know where you are.
In each case, location tracking is not only about finding where someone is right now, like in an exciting movie chase scene where agents are pursuing someone through the streets. It can also be about answering questions about people's historical activities and also about their beliefs, participation in events, and personal relationships. For example, location tracking could be used to find out whether certain people are in a romantic relationship, to find out who attended a particular meeting or who was at a particular protest, or to try to identify a journalist's confidential source.
The Washington Post reported in December 2013 on NSA location-tracking tools that collect massive amounts of information “on the whereabouts of cellphones around the world,” mainly by tapping phone companies' infrastructure to observe which towers particular phones connect to, and when those phones connect to those towers. A tool called CO-TRAVELER uses this data to find relationships between different people's movements (to figure out which people's devices seem to be traveling together, as well as whether one person appears to be following another).
Behavioral Data Collection and Mobile Advertising Identifiers
In addition to the location data collected by some apps and websites, many apps share information about more basic interactions, such as app installs, opens, usage, and other activity. This information is often shared with dozens of third-party companies throughout the advertising ecosystem enabled by real-time bidding (RTB). Despite the mundane nature of the individual data points, in aggregate this behavioral data can still be very revealing.
Advertising technology companies convince app developers to install pieces of code in software development kit (SDK) documentation in order to serve ads in their apps. These pieces of code collect data about how each user interacts with the app, then share that data with the third-party tracking company. The tracker may then re-share that information with dozens of other advertisers, advertising service providers, and data brokers in a milliseconds-long RTB auction.
This data becomes meaningful thanks to the mobile advertising identifier, or MAID, a unique random number that identifies a single device. Each packet of information shared during an RTB auction is usually associated with a MAID. Advertisers and data brokers can pool together data collected from many different apps using the MAID, and therefore build a profile of how each user identified by a MAID behaves. MAIDs do not themselves encode information about a user’s real identity. However, it’s often trivial for data brokers or advertisers to associate a MAID with a real identity, for example by collecting a name or email address from within an app.
Mobile ad IDs are built into both Android and iOS, as well as a number of other devices like game consoles, tablets, and TV set top boxes. On Android, every app, and every third-party installed in those apps, has access to the MAID by default. Furthermore, there is no way to turn off the MAID on an Android device at all: the best a user can do is to “reset” the identifier, replacing it with a new random number. In the latest version of iOS, apps finally need to ask permission before collecting and using the phone’s mobile ad ID. However, it’s still unclear whether users realize just how many third parties may be involved when they agree to let a seemingly-innocuous app access their information.
Behavioral data collected from mobile apps is used primarily by advertising companies and data brokers, usually to do behavioral targeting for commercial or political ads. But governments have been known to piggyback on the surveillance done by private companies.
Further reading on browser tracking: What Is Fingerprinting?