In the title compound, C(7)H(6)N(2)O(4), an intra-molecular O-H⋯O hydrogen bond generates an S(6) ring. In the crystal, inversion dimers linked by pairs of N-H⋯O hydrogen bonds occur. Weak C-H⋯O links consolidate the packing, leading to R(2) (1)(7) and R(2) (2)(10) loops within (100) polymeric sheets.