Can a NAS (Network Attachment) be hack via guest account using CIFS protocol?

NASSMB used Ask the Experts™
I have a NAS (Linus based file system) and my log indicated a guy from ChinaNet successfully hacked to my NAS with a guest account and get all my username/password account. That person frequently visited back to my NAS with the accounts he got (Failed as I already know).  I don't have any personal file, just music and some movies.

I have SSH enabled and tested with guest account. (it failed), however, CIFS connection with guest account allowed to see the folders tree from my NAS. How is this guy do this ? Anyone can show me to understand his concept of hacking. I will appreciated your expertise. I know how to stop him. Is anyone here experience this problem ?
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Too late to contain, but good you learned a lesson exposing only non-valuable data.
Anyone concerned with security would disable guest access completely as
you never know how it may be exploited.  Too bad these products ship to
non-expert consumers with guest enabled by default.

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