Several NATO websites have been hit by cyber attacks while in Crimea being a referendum on annexation to Russia. The group of hacktivists known as Cyber Berkut targeted the NATO websites with significant DDoS (denial of service) attack The main NATO website www.NATO.int, the website of a NATO-affiliated cyber security center in Estonia was also affected. Fortunately the cyber attacks against NATO web services have had no impact on the military alliances operations, a spokeswoman said.”]
Source: http://securityaffairs.co/wordpress/23097/cyber-warfare-2/nato-websites-hit-ddos-attack.html