Blaming Russia – Not Hacktivists, Insiders or Incompetence – is easy for Yahoo to blame Russia. Security experts have long cautioned that attributions are political, not technical, in part because accurately tracing attacks back is notoriously tricky. The Wall Street Journal reports that Yahoo detected the attack – several weeks after it occurred – in the fall of 2014, believed that it had been launched from Russia and said it seemed to be seeking data relating to up to 40 specific users of its service.”]
Source: https://www.govinfosecurity.com/blogs/yahoo-breach-great-nation-state-cop-out-p-2260