Researchers have discovered "chatty apps" which collude to share information on android without a users' permission.
The number of apps colluding is relatively low, at 54, but it's clear that there's a hole in the system to allow data sharing. The actual sharing of datas could be significant, of course, so if it's only one that's on thousands of android machines it could be significant.
The researchers will present their research at the Asia Conference on Computer and Communications Security this month.

“The bad news is that we found that apps can pass information around recklessly. The good news is that the amount of collusion is still quite low,” says Yao. In many cases, it wasn’t clear whether an app was designed to collude with others for malicious purposes, or if it was just a mistake. "Chatty Apps" Sharing Data on Android Without User Permission [newscientist.com]