Security researchers have found more malicious Android apps on Google's official download site and being spread through Chinese app stores.

Lookout Security spotted four apps on the Android Market that were infected with a variant of the "DroidDream Light" malware which has now plagued the e-store three times this year. Researchers at North Carolina State University have also announced they had found new malware that forced Android smartphones into texting a premium number.

Lookout's find was the third instance of DroidDream-infected applications making it into Google's e-store, following an original run in March and a second in early June. Those two campaigns forced Google to pull over 80 poisoned apps from its store.

According to Lookout, Google quickly yanked the four new applications from the Android Market.

"Fortunately the malware was available in the Android Market for a short period of time so the number of downloads was limited to 1,000-5,000," Lookout reported in an alert.

The mobile security company said that, like the June campaign, last Friday's DroidDream Light malware launched itself without user interaction after it was downloaded. DroidDream Light is a stripped-down version of the March attack code. Once on an Android smartphone, DroidDream Light can prompt owners to download other apps from the market, bait users with a malicious URL or even automatically download more apps to the device.

But Lookout's report was not the only one making the rounds.

On Monday, Xuxian Jiang, an assistant professor in computer science at North Carolina State University, chimed in with a warning of a new Android threat, which he dubbed "HippoSMS."

Jiang has published several Android alerts in the last six weeks, including one last month after he discovered 10 infected apps on the Android Market. HippoSMS was only published to unauthorised Chinese app stores, however.

Like almost all Android malware, HippoSMS piggybacks on a host app and is installed when that app is downloaded and approved by the user.

Its makers are monetising the malware by forcing an infected smartphone to text a premium number. Attackers are paid a portion of the revenues earned by such numbers, but they're also trying to hide that behaviour from users.

"It will delete any SMS message if it starts with the number '10,'" said Jiang, noting that numbers such as "10086" and "10010" are used by Chinese mobile service providers to notify customers about ordered services and their current bill balances. "We believe the removal of the related SMS messages is used to hide the additional charges caused from the malware," Jiang said.

Both Lookout and Jiang said one way that Android users can avoid malware is to carefully examine the access permissions an app asks for.

"Use common sense to ensure that the permissions an app requests match the features the app provides," said Lookout.