From Zimperium Mobile Security:
Zimperium zLabs VP of Platform Research and Exploitation, Joshua J. Drake (@jduck), dived into the deepest corners of Android code and discovered what we believe to be the worst Android vulnerabilities discovered to date. These issues in Stagefright code critically expose 95% of Android devices, an estimated 950 million devices. Drake’s research, to be presented at Black Hat USA on August 5 and DEF CON 23 on August 7 found multiple remote code execution vulnerabilities that can be exploited using various methods, the worst of which requires no user-interaction.
Attackers only need your mobile number, using which they can remotely execute code via a specially crafted media file delivered via MMS. A fully weaponized successful attack could even delete the message before you see it. You will only see the notification. These vulnerabilities are extremely dangerous because they do not require that the victim take any action to be exploited. Unlike spear-phishing, where the victim needs to open a PDF file or a link sent by the attacker, this vulnerability can be triggered while you sleep. Before you wake up, the attacker will remove any signs of the device being compromised and you will continue your day as usual – with a trojaned phone.
The Zimperium site has more information on which phones are at risk and what you can do to protect yourself.