The Organization for Security and Co-operation in Europe (OSCE) refused to speculate on reports that Russian hackers were behind the attack, the BBC reported.
It has observers in several of Europe's hot spots, including eastern Ukraine, where it monitors ceasefire violations.
"We became aware of an incident at the start of November," said a spokeswoman.
"I don't know how long it went on for but it was serious. The integrity of the ICT system was compromised and put in danger," the spokeswoman told the BBC from the group's headquarters in Vienna.
French newspaper Le Monde quoted an unnamed Western intelligence agency as saying the cyber attack was likely to be down to Russian hacking group APT28, also known as Fancy Bear, Pawn Storm or Sofacy.
Read alsoRussian hackers tracked Ukrainian artillery units using Android implant: reportAPT28 has already been linked to an attack on French global network TV5 and the leaks of leading athletes' medical data, while senior US officials have also blamed Russian hackers for an attack on the Democratic Party's emails.
However, there has been no independent confirmation of who carried out the latest attack.
The OSCE said while it knew how the attacker had got into its ICT system, "as for speculating on who it was, we don't, we cannot (say)".
Russia, the US and Ukraine are all members of the OSCE and the body has provided a means of dialogue during times of tension.
Some 1,000 OSCE staff are currently working in Ukraine, and the special monitoring mission has often been caught up in the conflict in eastern Ukraine.
In its most recent report, filed late on Tuesday, the monitors said they had had to evacuate a forward patrol base at Svitlodarsk because of heavy artillery fire, including one shell that came within 70m of the base.
A renewed ceasefire came into force on December 24 but there are still frequent violations.