Smartphones running Google's Android software have been hijacked by an illegal botnet, according to a Microsoft researcher.
Researcher Terry Zink said there was evidence of spam being sent from Yahoo mail servers by Android devices.
"We've all heard the rumours," Mr Zink wrote in a blog post."But this is the first time I have seen it - a spammer has control of a botnet that lives on Android devices.
Google told the BBC it did not respond to queries about specific apps but was working to improve security on the Android platform. "We are committed to providing a secure experience for consumers in Google Play, and in fact our data shows between the first and second halves of 2011, we saw a 40% decrease in the number of potentially malicious downloads from Google Play," a spokesman said.