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It is most likely to be a defective app, yes.
Make sure that your Android version is not subject to the StageFright exploit, also... that could cause system problems. For details on how to test for it, see:
If your phone has (presumably!) the Android OS on it, then if that OS version is not sufficiently updated, then it may contain a significant 'security hole' that allows almost anyone to gain access and control over your phone.
To test as to that security hole, download and use this app:
to tell whether there is such a security hole in your phone's Android operating system.
To avoid the problem in the first place, not just using the Stagefright detector, but also completely updating your Android OS ab initio when you first start using it, is a good idea.
(Some older versions of Android OS cannot be patched, by merely updating that version... but a higher version, if it can run on the older Android OS -compatible phone, can cure that problem.)