CuS + 8 HNO2 → Cu(NO3)2 + S + 6 NO + 4 H2O
This is an oxidation-reduction (redox) reaction:
S-II - 2 e- → S0 (oxidation)
6 NIII + 6 e- → 6 NII (reduction)
2 NIII - 4 e- → 2 NV (oxidation)
CuS is a reducing agent, HNO2 is an oxidizing agent, HNO2 is a reducing agent.