The Reuters agency revealed that Microsoft suffered a major security breach back in 2013. According to five former employees, hackers broke into the company vulnerabilities database, but the news was never disclosed. The database contained details of unfixed vulnerabilities in some of the most popular software and operating systems in the world. Microsoft addressed all the vulnerabilities listed in the compromised database within months so that the vulnerabilities would have limited exploitation against Microsoft systems. The employees attributed the attack to a highly sophisticated hacking group, likely they were referring a nation-state actor.”]
Source: https://securityaffairs.co/wordpress/64403/data-breach/microsoft-vulnerabilities-db-hack.html