What Is “Localhost Tracking”?
Localhost tracking is a recently exposed technique that allowed companies—most notably Meta (the parent company of Facebook and Instagram)—to covertly link users’ mobile web browsing activity to their real identities, even if users never logged into their accounts through their browsers or used privacy features like incognito mode or cookie clearing.
This method exploited the “localhost” (or loopback address, 127.0.0.1), which is a networking feature allowing a device to communicate with itself. On Android devices, both web browsers and native apps (like Facebook or Instagram) can access this address. Meta’s tracking worked by having Meta’s apps (Facebook, Instagram) run in the background on your phone, listening on specific localhost ports. When you visited a website with the Meta Pixel tracking script in your mobile browser, the script would send data (like cookies and browsing metadata) to the localhost address. The Meta app on the same device would receive this data, linking your browsing session to your app identity.
This process allowed Meta to bypass conventional privacy measures, including cookie deletion, incognito mode, and even Android’s permission system, because the data never left the device—it was shared internally between browser and app.
How Was Meta Involved?
Meta’s involvement centered on its use of the Meta Pixel script, which is embedded on millions of websites for analytics and advertising purposes. When users visited these sites on Android devices, the script would attempt to communicate with the Meta app via localhost, sending identifying information that could tie web activity directly to a user’s Facebook or Instagram account—even if the user was not logged in via the browser.
Researchers found that this tracking method was active from September 2024 until June 2025, when public disclosure prompted Meta to halt the practice. Over 17,000 popular U.S. websites were found to attempt localhost connections via Meta Pixel, with the majority doing so without user consent.
The technique allowed Meta to de-anonymize users’ web browsing activity, directly linking it to real identities without consent and regardless of privacy settings. It worked even if users cleared cookies, used incognito mode, or reset advertising IDs, undermining standard privacy tools.
