2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188067We consider quantum backreaction of the quantized scalar field with an arbitrary mass and curvature coupling on ultraextremal horizons. The problem is distinguished in that (in contrast to non-extremal or extremal black holes) the WKB approximation remains valid near $r_{+}$ (which is the radius of the horizon) even in the massless limit. We examine the behavior of the stress-energy tensor of the quantized field near $r_{+}$ and show that quantum-corrected objects under discussion do exist. In the limit of the large mass our results agree with previous ones known in literature.revtex4, 9 pagesGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext