Beware, Sudoku fans: You may receive an email that promises you can get free, unlimited puzzles with a simple click on a keyword. In fact it's the latest way to install identity-stealing "malware" on your computer — but using a decades-old trick.

A new scam email lures players with the promise of free puzzles, then installs malware after asking players to enable macros in Excel. — Getty Images

These emails, discovered recently by online security firm Sophos, promise to automatically generate the popular puzzles if you open an attached Microsoft Excel spreadsheet. If you do that, you then get a message that you need to "enable macros." Macros are little programs that, in their benign form, let you tell your computer to carry out a lengthy sequence of keystrokes or mouse actions.

But if you follow the prompt, warn Sophos experts and the Better Business Bureau, you'll also unknowingly disable your security software and install malware that collects sensitive information from your computer — including passwords — to send to a remotely based scammer.

Back in the 1990s, scammers frequently used macros to stealthily install malware. But Microsoft largely neutralized that threat by changing all its software so that the default setting was to disable macros. The Sudoku email, under the guise of being helpful, gives you instructions for how to restore your computer's ability to run macros.

Today, certain legitimate files may ask you to enable macros, but that should be done only if you fully trust the source. The Sudoku email obviously doesn't meet that standard (nor do other attachments in unsolicited emails or questionable websites making such requests).

So delete the Sudoku email without opening the spreadsheet, and instead feed your habit for the number puzzles at this AARP page.

Prefer other types of online games and puzzles? Beware of any that require you to provide your cellphone number under the guise of texting you results. That's another old trick to secretly charge you for new services or hit you with bothersome "robocalls."

Sid Kirchheimer is the author of Scam-Proof Your Life (AARP Books/Sterling). He writes the Scam Alert column for AARP.