Browsers let websites know how much data they can store locally. But the formula used to calculate this "quota" is often based on a percentage of your total free disk space. This leaks the exact size of your hard drive and how much space is left — a highly unique identifier.
Your exact disk capacity and free space can identify you across sites.
When you visit two different websites, they both see the exact same byte-for-byte storage quota.
Because this number fluctuates as you download or delete files, it creates a continuously updating fingerprint that is unique to your physical device. No cookies required.
Estimated Free Disk Space
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This precise byte count is broadcast to any website that asks.
Data Used by This Site
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▶ how does this work?
The navigator.storage.estimate() API is designed to let web applications know if they have enough space to save files for offline use.
However, different browsers calculate the quota differently. Chrome and Edge typically set the quota to a percentage (e.g., 60%) of the total available disk space on the volume where the browser profile is stored.
By simply doing the math backwards, a website can determine the exact free space on your computer's hard drive down to the byte. Since free space changes over time, tracking these changes across different websites allows third parties to correlate your browsing history.