Learn Italian: Ogni utente deve avere una password sicura. - Every user must have a secure password.

Both are grammatically correct, but they have slightly different focuses. Ogni means "every" or "each" and must always be followed by a singular noun without an article. Tutti means "all" and takes a plural noun with a definite article (tutti gli utenti).

Why is the English word password treated as feminine here?

Foreign loanwords are assigned a gender in Italian. Password is feminine (la password), most likely because the traditional Italian translation is la parola d'ordine (the password/watchword). Because it is feminine, the article and adjective must agree, giving us una password sicura.

Is the noun utente masculine or feminine?

It can be either! Nouns ending in -nte usually have the same spelling for both genders. You tell them apart by the article: un utente for a male user, and un'utente (with an apostrophe) for a female user. In this sentence, it is used in a general, gender-neutral sense.