SAN FRANCISCO » Some hackers are exploiting Microsoft’s offer of free upgrades to its new Windows 10 operating system.

Security researchers are warning about a wave of bogus spam emails with malicious attachments, labeled as if they’re legitimate copies of the new program.

The attachments contain a "ransomware" program that, when opened, locks all the data on a computer and demands payment to release them.

Researchers at Cisco Systems say the emails are designed to look like an official upgrade notice from Microsoft Corp., but several words have random, out-of-place letters and punctuation.

Another important clue: Microsoft says its update mechanism provides computer owners with a notice on their screens — not via email — when a direct Internet download is available. Experts warn against clicking on files that come with unsolicited emails.