you should assume whatever environment you're in to be unsafe, and thus perform due diligence in validating your actions and the data you are getting and working with... in short making use of [strong] checksums, knowing the source of where you are downloading data [your iso] from, using virus/malware scanning as necessary, and having legitimate, trusted, software properly installed on your computer (i.e. you cd burning softare).
Can an OS iso (or software) downloaded in an unsafe environment still be safe for
use?
yes
Can other hidden sectors or metadata not being part in the crc process be used to dispatch malware or can I be 100% sure that the DVD content is safe for setup a new machine ?
yes I believe so, which is why for a CD/DVD use factory fresh new media not some blank disc coming from who knows where which could have been previously written to - which could then be described as having hidden sectors that could dispatch malware when the cd is used. The technicality of how that could happen, which I do not fully know enough to describe here, I believe is more prevalent in Microsoft Windows with their autorun feature and .exe style of executable; I don't think it's as likely in linux (to autorun and execute). But from a security perspective, it's not so much as executing some code as it would be just getting some code [data] onto the system, to create some sort of exploit or problem by some other means later on.
