Twitter locked down thousands of verified accounts, including the accounts of Joe Biden, Bill Gates, Elon Musk, Apple, Uber and others, after it became clear that hackers had been able to compromise them. With limited confirmed information, a raft of theories and circumstantial evidence has come to light as to who was behind the attack and how they carried it out. Twitter posted a 900-word summary of the attack outlining what it knows. It stated that the company was hit with a social engineering scheme targeting a small number of employees.
Source: https://threatpost.com/the-great-twitter-hack-what-we-know-what-we-dont/157538/