Fingerprinting

Fingerprinting identifies a device by collecting the combination of its settings, like screen size, fonts, language, and browser version, rather than by storing anything on it.

Every browser leaks small details when it loads a page: the operating system, the time zone, the installed fonts, the graphics quirks. On their own each detail is common, but stacked together they often single out one device, and the site never had to set a cookie you could clear.

That is why it is hard to shake. You can delete your cookies and open a fresh browser session, but your screen resolution and font list stay the same, so the same fingerprint keeps forming. Google's own ad policy changed in February 2025 to permit advertisers to use these signals, including your IP address, for targeting and measurement.

Fingerprinting is far harder for a user to see or refuse than a cookie banner. There is no jar to empty and usually no consent prompt, which is exactly why privacy regulators treat it as a higher-risk practice than cookies.

When you cannot store the ID, you reconstruct it, and that is the whole trick.