By Robert McMillan

Malicious software disguised as legitimate apps for Android smartphones and tablets has seized control of more than one million Google accounts since August, according to research from security firm Check Point Software Technologies Ltd.

The apps had innocent-sounding names, such as StopWatch, Perfect Cleaner and Wi-Fi Enhancer. But they exploited known flaws in older versions of the Android operating system to take control of devices and install other apps and ad-spewing software without permission. Some of the unauthorized apps alsoused the victim's username and password to post fake reviews.

The malicious Trojan-horse software known as Gooligan was found in 86 fraudulent apps and has been infecting about 13,000 Android devices a day, Check Point said. The Gooligan apps come from third-party app stores rather than Google's authorized Play store, but some apps downloaded without authorization can be found on Play, Check Point said.

Users whose devices have been infected see pop-up ads and unwanted software, said the Israel-based security firm.

Gooligan is a variant of malicious software known as Ghost Push, which has been giving Android users headaches for two years. Google, a unit of Alphabet Inc., last year tracked more than 40,000 Ghost Push apps.

"We appreciate Check Point's partnership as we've worked together to understand and take action on these issues," a Google spokesman said.

Check Point researchers shared their findings with Googleand worked closely with the company to develop techniques to fix infected devices, a Check Point spokeswoman said. "We continue to work with Google today to discover who or what group is responsible for the Gooligan campaign," she said.

Google said it has removed apps associated with Ghost Push from Google Play. It has also taken steps disrupt the servers used by the malware's creators and to secure Google accounts compromised by the malicious software.

Although the free apps offered by alternative stores can be enticing, they come with risks, Google said. In a Google+ post, the company urged users to download only from the Play store.

Devices at risk from the Gooligan software are those using Android 4 (the versions nicknamed Jelly Bean or KitKat), initially released in 2012, or Android 5 (Lollipop), released in 2014, Check Point said.

Users wondering if their devices have been compromised can visit Check Point's site for a mobile-phone checkup and to learn more.

Gooligan preys on an increasingly serious Android problem: Users don't update their operating systems, leaving their smartphones and tablets vulnerable to attacks that exploit known software bugs.

Android's overall security "hasn't measurably improved" since 2012, said Dave Aitel, chief executive of cybersecurity firm Immunity Inc. "It's been a long time since everybody has been telling Google that they have a serious problem with the ecosystem and lack of updates."

Because control over software updates lies in the hands of users, carriers, and phone manufacturers, there is no single entity that can mandate a widespread software update, he said.

According to Google, 73% of Android users are on the Jelly Bean, KitKat or Lollipop Android releases. Fewer than 25% are on newer Android versions, including Marshmallow, released last year, or Nougat, released this year.

Althoughthe malicious software has infected more than one million Google accounts, that's a small percentage of the more than one billion devices that run on Android software.

Write to Robert McMillan at Robert.Mcmillan@wsj.com

(END) Dow Jones Newswires

November 30, 2016 14:32 ET (19:32 GMT)

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