When is it safe (and when might it not be safe) to automatically de-duplicate contacts submitted from a public facing form?
How might a malicious user leverage automatic contact merges to their advantage?
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As soon as you automatically "merge", it will never be 100% safe. If you want to be sure, never dedupe (ie put dedupe rules that are not possible to match) and ask your users to login.
Obviously, it means that you will end up with tons of duplicates, so in practice, you will need to relax the rules and accept that you might have two different persons that end up merged in the same contact. This isn't as bad as it sounds, and it happens with every shared list of items that several people can update, and it doesn't create too much problems.
So your choice is what type of error you want to avoid more: having more duplicates or having more incorrect merges. For what I've experienced, the most relaxed rule is to consider that everyone sharing an address is the same person, and the strictest want the first and last name and email and country.
you might want to add an extra layer of protection if you send emails with the hash token (so your contacts don't have to login and are automatically identified) so they can't change their name by accident with this extension https://civicrm.org/extensions/no-overwrite
And you could enable the detailed logging http://book.civicrm.org/user/advanced-configuration/logging/ so you keep the history of all the changes if you haven't done it already
fill email address of someone else (so it will match with that person), then use different name/phone number/employer/membership/event sign up etc...
It would mess up your system, and your communication to original contact. That could be advantage enough for them.
Above is a theoretical possibility. In real life I have never seen it happen. So in my opinion it is safe to use it and do an automated merge only by e-mail.
I do have a customer who for this reason chose to not merge new contacts with existing ones and de-duplicate only manually.