Collateral attack on land title

Section 48 of Presidential Decree (P.D.) No. 1529 or the Property Registration Decree proscribes a collateral attack to a certificate of title and requires that the judicial action needed to challenge the validity of a title is a direct attack, to wit:
Sec. 48. Certificate not subject to collateral attack. A certificate of title shall not be subject to collateral attack. It cannot be altered, modified, or canceled except in a direct proceeding in accordance with law.
Let's distinguish between a direct and a collateral attack:
The attack is considered direct when the object of an action is to annul or set aside such proceeding, or enjoin its enforcement. Conversely, an attack is indirect or collateral when, in an action to obtain a different relief, an attack on the proceeding is nevertheless made as an incident thereof. Such action to attack a certificate of title may be an original action or a counterclaim, in which a certificate of title is assailed as void. (Emphasis in the original; Arangote v. Maglunob, G.R. No. 178906, February 18, 2009)