The ransomware virus — which infects a computer and then encrypts its files, promising to decrypt them if a ransom is paid — crippled state-run infrastructure and major companies.

Cybersecurity researchers have said that the virus uses an exploit developed by the NSA known as EternalBlue.

Valentyn Ogirenko / Reuters

The exploit, dubbed ExPetr, takes advantage of a vulnerability in Microsoft’s Windows XP through 2008, and was also used for the WannaCry attack that happened in May. (North Korea has been identified as the likely source of WannaCry.)

EternalBlue was developed by the NSA over five years ago and remained secret until April, when a trove of NSA secrets were disclosed by a group known as the Shadow Brokers. NSA alerted Microsoft to the vulnerability in March and Microsoft has released a patch, however the patch depends on end-users making software updates, leaving those who are slow to update vulnerable to attack.

Ukraine was the hardest hit, but companies in Russia, the United States, UK, Denmark, France, and others were also hit. Initially Symantec and Kaspersky Labs had identified the attack as a strain of Petya, a virus available for distribution on the dark web, but Kaspersky Labs released a statement later in the day saying that this is “new ransomware that has not been seen before,” and that they were renaming it NotPetya.