A Western intelligence source has told French newspaper Le Monde that Russian hackers APT28 were behind the breach. The OSCE has said that all systems are now safe.
The Organization for Security and Cooperation in Europe (OSCE) confirmed on Wednesday that it has been the target of a "major security information incident."
The international security and human rights watchdog became aware of the security breach in November. According to an OSCE spokesperson, the systems are now safe.
"We were given entirely new security systems and passwords," she added.
French newspaper Le Monde reported on Wednesday that an unnamed Western intelligence agency believes Russian hacker group APT28 - which is also known as Fancy Bear, Pawn Storm or Sofacy Group - to be behind the OSCE attack.
The same source told Le Monde that APT28, which is believed to have connections to the Russian intelligence services, carried out cyber attacks on the Democratic National Committee during this year's presidential campaign in the United States.
An investigation by US intelligence services concluded in December that the Russian government sought to interfere deliberately in the US elections.
Russian President Vladimir Putin has denied any involvement by the Kremlin in the election hacking.
The OSCE has said that it is aware of the "speculation" surrounding the potential perpetrators of the attack on its computers, but it lacks the "capacity" to investigate at this stage.
All of the OSCE's 57 member states - which include the US, Russia and Ukraine - were reportedly informed of the security breach.