Twitter reinstated an account on Sunday that was suspended last week after it leaked the personal contact details of Democratic politicians in the US.
"Here I am! They'll have to try much harder to block me!" the account tweeted.
This latest leak follows an incident in July, in which Guccifer leaked 19,000 emails from Democratic National Committee members and also took responsibility for releasing a treasure trove of documents about presidential candidate Hillary Clinton.
The hacker name Guccifer has previously been associated with leaks about the Bush family and the hacking of "Downton Abbey" writer Julian Fellowes.
Twitter did not immediately respond to a request for comment.