There was also an inquiry about whether to allow Dmitry O. Rogozin, a former Russian deputy foreign minister who had called for the annexation of Crimea, to travel to Austria for an international meeting on “the peaceful uses of outer space.” At the time, Mr. Rogozin was under European financial sanctions.
There was much analysis in the cables of foreign policy and of Europe’s strategies on issues of trade, counterterrorism, migration and enlargement that could be picked apart by China and other countries looking for an advantage.
Asked on Tuesday about the hack, the National Security Agency said it was still examining the discovery of the European trove. But the former senior intelligence official said that the European Union had been warned, repeatedly, that its aging communications system was highly vulnerable to hacking by China, Russia, Iran and other states.
The official said the warnings were usually received with a shrug.
European officials said they are now trying to overhaul their outdated and vulnerable networks — an expensive process in which technological improvements usually cannot protect against flawed human judgment. They insisted that confidential, secret and “tres secret” material is handled differently than the cables seized by the hackers and noted that a new system, known as EC3IS, is being developed to handle the more sensitive documents that are shared among the diplomats.
For communications in capitals like Moscow and Beijing, yet another network, known as Zeus, is being installed for delegations of member states.
The Europeans appear, belatedly, to be waking up to the threat. Its senior staff members increasingly use encrypted telephones, and isolated “speech rooms” of Lucite are being installed in key posts. One such room is already used for a daily 8:30 a.m. meeting of senior staff members, and another is in use in the European Council building in Brussels for intelligence briefings.
“Of course no security system is foolproof, and they must constantly be upgraded,” one senior E.U. official said.