This is generally known as "device fingerprinting"--there are many ways to do it but they all involve probing for unique properties of a client via JS / Flash (listing installed fonts, drawing invisible characters and measuring via JS, etc.), then hashing them together to generate a unique ID for that user.
Some people think this practice violates users' privacy, and I'm one of them. This technology can be used to uniquely identify a user across multiple logins on the same site, or even multiple sites. It's quite widespread.
This paper[0] is mostly a survey of prominent DF providers and sites using this technology, and it's also a good primer on device fingerprinting techniques.
Some people think this practice violates users' privacy, and I'm one of them. This technology can be used to uniquely identify a user across multiple logins on the same site, or even multiple sites. It's quite widespread.
This paper[0] is mostly a survey of prominent DF providers and sites using this technology, and it's also a good primer on device fingerprinting techniques.
[0] http://www.cosic.esat.kuleuven.be/publications/article-2334....