just bought an used phone. is it secured?

[luv2liv]

just bought an used phone from swappa. a Samsung s8 model
it arrived with the welcome screen as if it has been factory reset. i went ahead and did a factory reset again just to be sure.
how can i know if there are no back doors, keylogger, or malware waiting to siphon my personal data?

 

[s44]

If you're paranoid, just reflash a full image with Odin.

Make sure you match the exact device model.

 

[luv2liv]

it's been awhile since i mess with custom roms. totally forgot about that option. thanks!