Samsung has officially acknowledged that there is an exploit on it’s Exsynos processors – used in Samsung phones including Galaxy SII,Galaxy SIII and Galaxy Note II ,that leads the devices get infected by malware and possibly leak personal data out of them. I have witnessed for strange behaviours on many Galaxy SII and similar Samsung devices . Finally the hard reset had given the solution .
Finally Samsung accepted the loophole it had in the Exynos processor and promised an immediate patch for the devices as software update. The patch will be available soon to the infected models using Exsynos processors.
How to protect your Galaxy Device from Attacks
It is too late for an advice for SII users as it has been used for more than an year . But in general ,uninstall all recently installed software developed by less reputed companies (rely on Google like giants only for the time being )postpone installing freeware until you get the patch from Samsung ,that want take more time .