Exploit/Stagefright.J detects media files that attempt to exploit a "division-by-zero" vulnerability in the Android media library Stagefright.
Indication of Infection
Suspicious MMS messages or instant messages with attached videos from unknown contacts would cause phone crash.
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.