Building Use Rights (Hak Guna Bangunan or HGB) in Indonesia allow citizens to own, transfer, and use land as collateral. In civil cases, HGB land can be subject to a security seizure. When a court imposes such a seizure, a third party claiming ownership can file a legal opposition (derden verzet). Derden verzet is an extraordinary remedy for third parties not involved in the original trial but whose ownership rights are affected by the court's decision. This study explores the legal certainty for HGB land buyers from creditors involved with previous landowners and the legal protection for buyers of auctioned HGB land, referencing case number 72/Pdt.Plw/2017/PN Tng. Using normative research, the study analyzes legal materials to identify applicable rules, principles, and doctrines. Findings reveal that the transfer of land ownership must be executed in writing through a deed by a Land Deed Official (PPAT) and registered with the National Land Agency to ensure legality. In disputes, the Panel of Judges relies on valid evidence, such as the Deed of Sale and Purchase and the HGB Certificate, to confirm ownership. The derden verzet process aims to validate ownership claims in court and can suspend execution if successful. An example from the Tangerang District Court illustrates that the Panel of Judges can provide legal protection by lifting security seizures, thereby ensuring justice and legal certainty for claimants. This case demonstrates that derden verzet upholds the rights of all parties affected by court decisions, ensuring comprehensive legal protection and fairness.