An informant working for the F.B.I. coordinated a 2012 campaign of hundreds of cyberattacks on foreign websites, including some operated by the governments of Iran, Syria, Brazil and Pakistan, according to documents and interviews with people involved in the attacks.

Exploiting a vulnerability in a popular web hosting software, the informant directed at least one hacker to extract vast amounts of data — from bank records to login information — from the government servers of a number of countries and upload it to a server monitored by the F.B.I., according to court statements....