The Product Owner has complete authority over the ordering of the Product Backlog. However, it is strongly recommended that the Product Owner put all known defects at the top of the Product Backlog so that the Team fixes them in the very next Sprint. By defects is meant features of functions of the system that have been built by the Team in previous Sprints where those features or functions do not behave according to the expectations of the Product Owner and where such mis-behavior is exposed to users of the system. There may be other quality problems with a system which are not categorized as defects (e.g. duplicated code). This prioritization of defects generally results in extremely high levels of quality in a system and a long-term reduction in costs for the system (total cost of the system) by making future features easier to add, and reducing effort spent on maintenance and support. Failing to put defects on the top of the Product Backlog generally leads to decreasing overall quality and in particular can severely damage the morale of a Scrum Team thus preventing them from getting into a high-performance state.
To learn more about Product Owners, visit the Scrum Team Assessment.