Learn Italian: La password per aprire l'allegato è molto difficile. - The password to open the attachment is very difficult.

You are right that most English words used in Italian default to masculine (like il computer). However, password is feminine because its direct Italian translation, la parola d'ordine (or simply la parola, meaning the word), is feminine.

Why do we use per before aprire here?

In Italian, when you want to express purpose or say "in order to" do something, you use the preposition per followed by the infinitive form of the verb. Here, per aprire means "in order to open".

Is the word l'allegato related to a verb?

Yes, it comes from the verb allegare, which means to attach or to enclose. Allegato is the past participle used as a noun to mean the attached file or document in an email.

Since password is feminine, why doesn't molto change to molta?

Because here molto is used as an adverb meaning "very", describing the adjective difficile. When molto means "very", its ending never changes. It only changes to match gender and number when it means "a lot of" or "many" and is placed directly before a noun.