While only using official stores to download apps is good advice, it's not a guarantee of safety. Once again, a fake version of a popular app has been downloaded millions of times by unsuspecting users. On this occasion, it was a bogus version of WhatsApp on Google's Play Store.

The application in question was called "Update WhatsApp Messenger." Those responsible designed it to look like an official product of WhatsApp Inc. by using what appeared to be the same developer ID, but they added a Unicode character for a no-break space at the end to differentiate between the two URLs. Once installed, the app created a blank icon so users couldn't see it.

Update WhatsApp was downloaded over a million times before Google removed it. It generated revenue for its makers via ads, but the same technique could be used to insert malware onto users' phones.

Nikolaos Chrysaidos, a security researcher at anti-virus company Avast, has identified more fake WhatsApp apps on Google Play over the last month, as well as a bogus Facebook Messenger application that was downloaded 10 million times.

Checking an app's user reviews is often the best way of spotting a malicious duplicate, but Update WhatsApp had a four-star rating and over 6000 reviews (some of which did note it was a fake), so it's not always instantly obvious.

Last month, the Chrome Web Store was found to be hosting a fake Adblock Plus extension. It mimicked the name of the original app, changing only the "b" in the title to a capital "B". Downloaded over 37,000 times before being removed, it brought invasive ads and opened tabs without permission.