Petya is a family of encrypting malware that was first discovered in 2016. The malware targets Microsoft Windows–based systems, infecting the master boot record to execute a payload that encrypts a hard drive's file system table and prevents Windows from booting. It subsequently demands that the user make a payment in Bitcoin in order to regain access to the system.

A series of powerful cyberattacks using the Petya malware began on 27 June 2017 that swamped websites of Ukrainian organizations, including banks, ministries, newspapers and electricity firms. Similar infections were reported in France, Germany, Italy, Poland, Russia, United Kingdom, the United States and Australia. ESET estimated on 28 June 2017 that 80% of all infections were in Ukraine, with Germany second hardest hit with about 9%. On 28 June 2017, the Ukrainian government stated that the attack was halted. On 30 June 2017, the Associated Press reported experts agreed that Petya was masquerading as ransomware, while it was actually designed to cause maximum damage, with Ukraine being the main target.