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The title compound, C8H8N2O4, is almost planar (r.m.s. deviation = 0.037 Å) and an intramolecular N—H...O hydrogen bond generates an S(6) ring. In the crystal, inversion dimers linked by pairs of O—H...O hydrogen bonds generate R22(8) loops. Intermolecular N—H...O hydrogen bonds (involving the same H atom that forms the intramolecular hydrogen bond) link the dimers into infinite sheets lying parallel to (102).