Exploit/Stagefright.M detects media files that attempt to exploit a vulnerability (CVE-2015-1539) in the Android media library Stagefright.
Indication of Infection
Suspicious MMS messages with attached videos from unknown contacts unless the attacker is able to remove them once the exploit is automatically executed.
Methods of Infection
MMS messages but also any other way to make the device pre-process the crafted media file like an app that renders the file automatically or spam messages with links to websites that host the exploit. In order protect your device against Stagefright attacks turn off MMS messages or turn off auto-reading/rendering MMS media.