How can I make sure my cellphone has no virus?

[Galinho13]

Hi, guys!
My phone's screen broke a few weeks ago. I took it straight to the assistance, in order to replace it.
However, I'm a little bit worried that they could have installed any virus or malware.
How can I assure, with certainty, that there's no viruses in my cellphone?
I already formatted my cellphone and wiped the cache. Is this enough? Or could any malware be installed in a hidden place (like bios - making an analogy with the computer)? If yes, how could I remove it?
My cellphone is not rooted.

 

[Hadron]

Firstly, a virus is a self-propagating malware, so not what you are worried about (and actually there aren't any true viruses for android: it's trojans you have to watch for in practice).

But that's not the point. The answer is that yes, it would be possible for someone sufficiently knowledgeable to insert malware to the system partition (i.e. in the ROM), where factory resets won't do anything to it. Of course that doesn't make it likely, I'm just saying that it's technically possible.

What I'd suggest is checking your Knox status: if that shows that the phone has been interfered with then you have reason to be suspicious. I don't know whether it's even possible to reset the Knox flags, but if it is it's certainly difficult, so personally I'd say that if your Knox status shows the phone has not been tampered with then you are probably good.

If you really want to be certain, download the latest firmware for your model (precise model, not just S7E) from Sammobile.com and reflash the phone. That will overwrite everything, including the system partition (and the lower-level stuff as it happens, but the system partition is really the only place where you could hide malware). No malware can survive that. But personally I'd say that the Knox flag should be a good indication.

 

[Mikestony]

Firstly, a virus is a self-propagating malware, so not what you are worried about (and actually there aren't any true viruses for android: it's trojans you have to watch for in practice).

But that's not the point. The answer is that yes, it would be possible for someone sufficiently knowledgeable to insert malware to the system partition (i.e. in the ROM), where factory resets won't do anything to it. Of course that doesn't make it likely, I'm just saying that it's technically possible.

What I'd suggest is checking your Knox status: if that shows that the phone has been interfered with then you have reason to be suspicious. I don't know whether it's even possible to reset the Knox flags, but if it is it's certainly difficult, so personally I'd say that if your Knox status shows the phone has not been tampered with then you are probably good.

If you really want to be certain, download the latest firmware for your model (precise model, not just S7E) and reflash the phone. That will overwrite everything, including the system partition (and the lower-level stuff as it happens, but the system partition is really the only place where you could hide malware). No malware can survive that. But personally I'd say that the Knox flag should be a good indication.

^^^^this^^^^
Couldn't possibly say it any better.

 

[Galinho13]

Firstly, a virus is a self-propagating malware, so not what you are worried about (and actually there aren't any true viruses for android: it's trojans you have to watch for in practice).

But that's not the point. The answer is that yes, it would be possible for someone sufficiently knowledgeable to insert malware to the system partition (i.e. in the ROM), where factory resets won't do anything to it. Of course that doesn't make it likely, I'm just saying that it's technically possible.

What I'd suggest is checking your Knox status: if that shows that the phone has been interfered with then you have reason to be suspicious. I don't know whether it's even possible to reset the Knox flags, but if it is it's certainly difficult, so personally I'd say that if your Knox status shows the phone has not been tampered with then you are probably good.

If you really want to be certain, download the latest firmware for your model (precise model, not just S7E) from Sammobile.com and reflash the phone. That will overwrite everything, including the system partition (and the lower-level stuff as it happens, but the system partition is really the only place where you could hide malware). No malware can survive that. But personally I'd say that the Knox flag should be a good indication.

That was a really complete answer. Thank you very much, Hadron.
I just checked my Knox Status (did it by acessing the Downloading Mode, is that right?) and it said the Warranty was not Voided (0x0000). So, apparently, there's no reasons for worry, right?
Not sure if I should reflash the phone... I did a quick search and it appeared to be a quite complicated task (specially for me, a newbie).
Anyway, really appreciate your help!

 

[Hadron]

If the flags are all zero I'd say that the reset you have already done would suffice to be sure.