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I have received this notification on Tor Browser (Firefox). What is it?

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Canvas is a feature that allows a website to generate graphics dynamically, at client-side, right in front of your eyes.

It is harmless, but there is a process named Canvas Fingerprinting where some Canvas features can be used to obtain detailed information about your Browser, like what your browser is and what exact version, whats your OS, what fonts you have installed etc.

Nothing implicitly harmful, but theorically this information can be used to route you to a potentially dangerous page specifically designed to attack the browser you are using.

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