New Delhi | Bengaluru: Cyber security professionals worked round the clock over the weekend, trying to protect corporate clients against what experts are calling the biggest ransomware attack ever, before they open for business Monday morning.

Cyber security firms told ET that though the impact on India from the Wanna-Cry onslaught has been relatively minimal so far, the real test will be on Monday when scores of professionals switch on their work systems.

A malicious piece of software that blocks access to computers until money is paid, ransomware WannaCry is said to have swamped machines in over 150 countries, including India. The hackers, who go by the name Shadow Brokers, are demanding $300 in virtual currency Bitcoin to unblock access to a user’s files and had reportedly received over $25,000 till Saturday morning.

India is among the most vulnerable because a large number of organisations and individuals still rely on older, outdated versions of the Windows operating system. The country also has the highest number of pirated software users.