In one of our earlier posts, we took a look at Ultrasonic Cross-Device Tracking, a new technique has been developed where user tracking takes place across devices like smartphone, tablet, TV, PC or any device connected to the Internet, with the help of ultrasonic sounds Also, we had suggested several precautions one could take to mitigate this threat – one of which was making use of the Silverdog browser extension for Google Chrome.

Silverdog Chrome browser extension

Silverdog browser extension functions as a personal audio firewall by selectively filtering ultrasonic beacons and thus prevents Ultrasonic Tracking. It mitigates the risk of ultrasound tracking for desktop users by attenuating frequency above 18000 Hz. The extension filters all audio sources and removes all Ubeacons while leaving all audible frequencies intact. This prevents ultrasound tracking when you use it.

Silverdog also features an audio processing module with high shelf filter. Sounds, if you are not aware are generally composed of multiple frequency components. Sometimes it’s desirable to increase the level of some frequencies or decrease others. To deal with frequencies or bands of frequencies, selectively, developers of the app that employ ultrasonic tracking have to separate them out. This is done using filters that function as frequency processing tools.

There are multiple ways to categorize filters based on the frequencies they attenuate, these include:

High-shelf filter – boosts or attenuates high frequencies High-pass filter – retains only frequencies above a given threshold Low-pass filter – retains only frequencies below a given threshold Low-shelf filter – boosts or attenuates low frequencies Bandstop filter – eliminates frequencies within a given frequency band Shelving filters Bandpass filter – retains only frequencies within a given frequency band Comb filter – attenuates frequencies in a manner that, when graphed in the frequency domain, has a “comb” shape. Peaking filter – boosts or attenuates frequencies in a band

Mitigate the risk of Ultrasonic Tracking

Silverdog extension for Chrome works cleverly. It attenuates the sound frequency above 18000Hz by default and a certain filter type, gain and Q. This helps in removing all uBeacons while leaving all audible frequencies intact. Also, the app starts working automatically once you have configured it.

You can switch the firewall on or off with just a click of a mouse on the extension icon in the Chrome address bar. When enabled, the application turns green and red, when disabled. This color code makes the distinction between ‘on/off’ mode simple.

While the extension works great, it has certain limitations. For instance, Silverdog extension for Chrome browser doesn’t work with Flash content. It only supports HTML5 content.

In addition to this, the extension offers no explanation for the filters it supports. You alone have to read up about the various filters it supports. To read about the basic difference between various types of filters refer to the part above as the extension itself offers no explanation on the differences between those filters.

All said, it is important to remember that tracking happens only with your cooperation, whether you realize it or not. If you refuse to cooperate with those who are trying to track you outright, you can greatly reduce the amount and kind of information that app developers or others readily collect from you, including information about your behavior, and other possessions. These method work for Desktop users.

For mobile users, there’s a different method to mitigate this threat. Avoid installing any apps that request microphone permission as part of their installation. This is because the microphone on a cell phone picks up the ultrasonic sound. An app on the phone that’s particularly built to listen that kind of tone decodes it and sends it on to the company doing the tracking. So, preventing apps from accessing the microphone on your Smartphone nullifies this avenue of tracking entirely.

You can check the permissions each app has on your mobile phone by simply going to the Setup menu, selecting ‘Apps’, and then looking down the list of each app you have installed; there’s a section at the bottom for permissions in Android. Likewise, on iOS or Windows Phone, you can check these via the Settings menu, and check ‘Privacy’ option.

Lastly, blocking malvertising on your computer would prevent them from playing audio over your computer’s speakers. Still, if you do not mind online advertising and want to allow advertisements display you can put on a headphone or keep a computer’s sound off or mute the volume when you’re not specifically using it. Following this practice would not only allow the ads to be displayed but also prevent the tracking because, if the speakers are off, they can’t transmit ultrasonic information. TVs can be a little harder to mitigate, but muting the sound when ads come on would certainly help.

Both the Chrome extension, Silverdog and the AOSP patches are available for download from the GitHub source. SilverDog Chrome extension is your sound firewall, and you can get it here. You can also download the set of AOSP patches to implement a new permission to filter ultrasound spectrum from here.