United Nations networks in Geneva and Vienna suffered a hack last year as part of an apparent spy operation, which was kept under wraps by top officials, according to a Wednesday report by the Associated Press. It's not clear who the hackers were or how much data they accessed.

Dozens of servers were reportedly compromised in places including the UN's human rights office, where sensitive data is collected, according to an internal confidential document leaked to The New Humanitarian.

"Staff at large, including me, were not informed," Ian Richards, president of the Staff Council at the United Nations, told the AP. "All we received was an email (on Sept. 26) informing us about infrastructure maintenance work."

The internal document reportedly says 42 servers were "compromised," while another 25 were "suspicious." It also says hackers exploited a flaw in Microsoft's SharePoint software to gain access to the networks, but it's not clear what kind of malware was used, according to the report.

A UN official, who asked to remain anonymous, reportedly told the AP that given the sophistication of the attack, it's possible a state-backed actor was responsible. They added that "systems have since been reinforced."

The UN didn't immediately respond to a request for comment.

The leaked report from Sept. 20 reportedly says logs that would have exposed what hackers did within UN networks were "cleared." Additionally, some of the accounts that were accesseed belong to domain administrators, who have access to all user accounts, according to the report.

The hack at the UN human rights office wasn't severe, spokesman Rupert Colville told the AP. "We face daily attempts to get into our computer systems," he reportedly told the publication. "This time, they managed, but it did not get very far. Nothing confidential was compromised."

The earliest hacking activity seems to have taken place in July and was noticed in August, UN spokesman Stephane Dujarric reportedly told the AP, adding that that "the methods and tools used in the attack indicate a high level of resource, capability and determination."

Security experts say the UN's decision to keep the hack a secret from staff was a "terrible decision," since it's important to alert people and let them know what to look out for.