The messages contain common subjects like "please print", "documents", "photo", "Images", "scans" and "pictures", the government said in its advisory. However the subject texts may change in some cases, it added.
The messages contain "zip" attachments that trick the computer into downloading the ransomware which demands a ransom of half bitcoin, currently equivalent to over Rs. 1.5 lakh, for unscrambling the user's files.
"Users are advised to exercise caution while opening e-mails and organisations are advised to deploy anti-spam solutions and update spam block lists," the alert said.
The ransomware, which targeted outdated copies of Microsoft's Windows operating system, also struck thousands of systems worldwide.