The UN has revealed that its offices were hit by an "apparently well-resourced" cyber attack that exposed lists of user accounts.
It said the attack happened last year at its offices in Geneva and Vienna, but the damage had been contained.
Geneva is home to several UN agencies, including the Human Rights Council, the High Commissioner for Human Rights, the High Commissioner for Refugees, the World Health Organisation and the World Trade Organisation.
Agencies in Vienna include the International Atomic Energy Agency and the Office on Drugs and Crime.
"The attribution of any attack is very uncertain and fuzzy, but this was apparently a well-resourced attack," UN spokesman Stephane Dujarric said.
"The attack resulted in a compromise of core infrastructure components at both cities and was determined to be serious.
"The damage related to this specific attack has been contained, and additional mitigation measures implemented.
"Nevertheless, the threat of future attacks continues, and the UN Secretariat detects and responds to attacks of various levels of sophistication often."
The UN Human Rights Office said the servers accessed by the hackers "did not hold any sensitive data or confidential information".
"The hackers did manage to access our Active User Directory, which contains the user IDs for our staff and devices," it said.
"However, they did not succeed in accessing passwords. Nor did they gain access to other parts of the system."
The UN has its headquarters in New York, where Secretary-General Antonio Guterres, the Security Council and the General Assembly are based.
A UN spokesman said last week that the world body directed officials in June not to use WhatsApp to communicate because "it's not supported as a secure mechanism".
But some UN staff have said WhatsApp is regularly used by many.
