Is a child born from an unregistered 'urfi marriage, which was later formalized by a شرعي (Shar'i) contract after the birth, considered legitimate or not? And is that considered zina (adultery/fornication), and what is its شرعي (Shar'i) ruling?

Customary Marriage (Nikah Urfi) has two forms:

1. One that fulfills all conditions and pillars, including the guardian's (wali's) consent and witnessing, but is not officially documented or publicized. This is a valid marriage, and the child born from it is attributed to his father.

2. One that is secret and without the guardian's consent. This is a void marriage, and separation is obligatory in such a case. Sheikh Ibn Baz issued a fatwa stating its impermissibility because it contradicts Islamic law.

As for the child: He is attributed to his father if the couple believed the marriage to be valid, as it falls under the category of "intercourse due to misconception" (wat' shubha). However, if they believed it to be void, he is not attributed to him but rather to his mother, as it is considered zina (illicit sexual intercourse). Muslims have agreed that for every marriage whose validity the husband believed in, his child is attributed to him, and they inherit from each other.