Computer servers at Mexican state oil firm Pemex were hit by a ransomware attack, crippling sensitive operations. Pemex was targeted by "Ryuk," a strain of ransomware that targets billion-dollar global companies, Reuters reported on Tuesday.

"We are taking measures at the national level to fight RYUK ransomware, which is affecting various Pemex servers in the country," a company official told the agency. The state oil company said the cyber attack was neutralized and that it had affected less than 5% of its computers.

Pemex, which has been creaking under a massive debt load, said operations were normal and that oil production and storage were unaffected, the news agency added. It said the computer center in Mexico detected the ransomware attack that aimed to "block a computer screen or encode important, predetermined files with a password."

What is a ransomware attack?

A ransomware attack is a cyber attack on an individual or organization wherein the malware infects the computers through attachments or links in phishing emails. Once a computer or network is infected with ransomware, the malware blocks access to the system, or encrypts the data on that system, Kaspersky explains. "Cybercriminals demand that the victims pay a ransom in order to regain access to their computer or data."

Locky and WannaCry are some of the dreaded ransomware that posed threat to global organisations in the last few years. While Locky was first released in a 2016 attack by an organized group of hackers, WannaCry, which exploits a vulnerability in Windows, spread across 150 countries in 2017.