of three wallets to receive instructions to decrypt their computer. At the same time, the software exploits the Server Message Block protocol in Windows to infect local computers on the same network and any remote computers it can find. Additionally, the NotPetya software was found to use a variant of Mimikatz, a proof-of-concept exploit found in 2011 that demonstrated that user passwords had been retained in computer memory within Windows, exploiting these passwords to help spread across networks. The EternalBlue exploit had been previously identified, and Microsoft issued patches in March 2017 to shut down the exploit for Windows Vista, Windows 7, Windows 8.1, Windows 10, Windows Server 2008, Windows Server 2012, and Windows Server 2016. However, the WannaCry attack progressed through many computer systems that still used older Windows operating systems or older versions of the newer ones, which still had the exploit, or that users had not taken the steps to download the patches. Microsoft issued new patches for Windows XP, Windows Server 2003 and Windows 8 the day after the WannaCry attack. Security expert Lesley Carhart stated that "Every method of exploitation that the attack used to spread was preventable by well-documented means." Security experts found that the version of Petya used in the Ukraine cyberattacks had been modified, and consequently was renamed NotPetya or Nyetna to distinguish it from the original malware. NotPetya encrypted all of the files on the infected computers, not just the Master File Table, and in some cases the computer's files were completely wiped or rewritten in a manner that could not be undone through decryption. Some security experts saw that the software could intercept passwords and perform administrator-level actions that could further ruin computer files. They also noted that the software could identify specific computer systems and bypass infection of those systems, suggesting the attack was more surgical in its goal. Unlike the WannaCry software, a "kill switch" was never found in NotPetya, which could have been used to immediately stop its spread. According to Nicholas Weaver of the University of California the hackers had previously compromised MeDoc "made it into a remote-control Trojan, and then they were willing to burn this asset to launch this attack." Attack During the attack the radiation monitoring system at Ukraine's Chernobyl Nuclear Power Plant went offline. Several Ukrainian ministries, banks, metro systems and state-owned enterprises (Boryspil International Airport, Ukrtelecom, Ukrposhta, State Savings Bank of