How can a downloaded module for CMS ruin a site?

Question

CMS

Hello!
Many of the novice developers do not hesitate to download free plugins, modules and extensions for their sites with an unverified reputation of sites that integrate into the site's file system itself.
How can the presence of hidden code affect the site?
I know about the presence of hidden links in a similar one, but what else could there be?
Why is it said that the hidden code does not allow to promote the site and the most important question: how to deal with it? (it’s clear that it’s worth downloading from the developer’s offsite sites, but there are also those who have completed project support or, for one reason or another, this is impossible)

Answer 1

you have some kind of mess in your head - the hidden code implied is unknown what is the fate of the site and modules.
Of course, do not download from anywhere. but the problem of modules is that they can be crookedly written and errors will simply appear on the site