It is possible to de-anonymise any Tor user if they have JS enabled and you have...

nanoscopic on July 2, 2014 | parent | context | favorite | on: Things You Should Know About Tor

It is possible to de-anonymise any Tor user if they have JS enabled and you have passive listeners at their ISP. See http://webcache.googleusercontent.com/search?q=cache:kVKMeKx...

The described attack on Tor may not be well known, but at the very least I told the FBI how to do it myself, so they certainly know about it.

maerF0x0 on July 3, 2014 [–]

IIRC Tails helps you by encouraging you to turn off the JS to avoid the exploit.