It looks like Google needed to patch some security holes on Android recently. They just started pushing out the latest security patch for Android 6.0/Marshmallow. Google promised to release a new security patch every month to stay ahead of the black hats. April's update came toward the end of the month instead of earlier, but at least it's here.
The new update fixes 13 high priority issues and 6 critical issues. The update will begin pushing out to the following devices: Nexus 5, Nexus 5X, Nexus 6, Nexus 6P, Nexus 7 (2013 WiFi and LTE), Nexus 9 (WiFi and LTE), Nexus 10, the Nexus Player, and the Pixel C.
Google also released the Factory Images for a few of these devices already. Here are some links: