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In response to this series of questions: What unique device fingerprinting information can an iOS9 app collect? and as an update to this question: What are the best practices for maintaining privacy on a non-jailbroken iOS device? What practices should privacy-conscious users adopt to minimize the ability for web sites in the browser AND in apps (by the app makers or by 3rd-party services used within apps), to use device fingerprinting to uniquely identify and track an iOS device across web pages or across apps?

I'll also assume that as a first step to avoid device fingerprinting, basic privacy hygiene must be observed: aggressive cookie management, using only a minimal set of trusted apps, etc.

Starter list of basic privacy hygiene practices:

  • In Safari Settings: Block Pop-ups ON, Do Not Track ON, Block Cookies = Allow from Current Website Only, Clear History and Website Data regularly. Don't reuse private browsing tabs for more than one site.

  • Don't allow apps permission to access unique personal data sets like Location, Contacts, Calendar (Settings|Privacy). If you use Location Services, disallow it for Weather - there's no "While using the app" choice, and weather.com has a pretty nasty privacy policy.

  • Don't add any accounts for app makers that have "integrated" status (e.g., Facebook, Twitter), since it's not clear if those apps enjoy privileged access to non-public APIs. Best to avoid any apps from companies that are ubiquitous trackers around the web (e.g., Google, and the others above).

  • Turn off Background App Refresh so that apps only access the internet when you are actively using them (Settings | General).

Starter list of device fingerprinting counter-surveillance practices:

  • In Settings|Privacy|Advertising, turn ON Limit Ad Tracking, and Reset Advertising Identifier OFTEN.

  • The other persistent identifier is the Vendor ID, which can be reset by (temporarily) deleting all apps from a given vendor, then reinstalling.

  • It may be necessary to completely wipe the device occasionally to clear out any persistent iOS Keychain items or iCloud Key-Value Stores, especially for apps that have been deleted.

  • New in iOS 9, apps are prevented/discouraged from determining what other apps are running/installed on your device. Prior to iOS 9, minimizing installed apps (to a small set of common apps) and frequent force-quitting apps was about the only defense against this.

  • Also new in iOS 9 will be the ability to block trackers. Rather than minimizing the fingerprinting attack surface, this addresses the ability of (3rd-party) trackers to detect that surface. It will be interesting to see how this develops.

  • Use a common browser to avoid a rare UA. Presumably for now this is still iOS 8 Safari (use in Private Browsing mode).

  • Change your IP address often, or use Tor or another IP-obfuscating tactic (e.g., an always-on VPN). Preferably use an IP address that many other users/devices are using as an exit IP address.

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