This is the most serious Android vulnerability discovered so far, said Joshua Drake, VP of platform research and exploitation at San Francisco-based security vendor Zimperium, Inc.
That's because the user doesn't have to do anything to get infected, and the attacker doesn't have to be in close proximity to the victim.
"Now you can send malware directly to any Android device if you know their phone number," he said.
In the prototype code that Drake put together, the only indication that there might be something wrong is an MMS notification from an unknown number.
The message itself could be anything, he said.
Plus, since the vulnerable code is executed before the notification happens, an attacker might be able to eliminate it, or delete the original message so that there's no trace of an attack.
Once the malware is installed, the attackers can execute code, access the Internet and local files, and listen to the microphone. On some devices, the attackers can go even further, with unfettered access to system privileges -- just one step below root access.
Or attackers can combine this exploit with another to escalate privileges, Drake added.
He said that he hasn't seen any examples of this exploit being used out in the wild.
Stagefright is a media library that processes several popular media formats.
How to stay safe
The first thing users need to do is update their phones to the latest software, Drake said.
"The patches are propagating as we speak," he said. "It's a long process though, and there are a lot of people involved."
Zimperium submitted the first round of patches to Google in early April, and a second set in May.
After each submission, Google accepted and applied the patches within 48 hours.
But, unlike the iOS ecosystem, where Apple can push updates simultaneously to all of their users, Android updates have to go through mobile carriers or device manufacturers.
Drake said that he decided to publish the vulnerability to ensure that everyone moved quickly.
"Nothing lights a fire under people's asses like full disclosure," he said. "I gave them a 90-day window. This follows Google's own Project Zero disclosure guidelines."
Right now, there's no way to tell whether a mobile device is vulnerable or not, Drake added. Users can contact their providers and ask, he suggested.
It might also be a good idea to fully shut down the phones before going into sensitive meetings, he added.
"I've seen some places where they collect the phones before you go into meetings," he said. 'I think that's a good idea. Meetings are a place where people should focus on one another."