Due to the fact that we do not know who will create hardware Trojans (HTs), and when and where they would be inserted, it is very difficult to correctly and completely detect all the real HTs in untrusted ICs, and thus it is desired to incorporate in-situ HT invalidating functions into untrusted ICs as a countermeasure against HTs. This paper proposes an in-situ Trojan authentication technique for gate-level netlists to avoid security leakage. In the proposed approach, an untrusted IC operates in authentication mode and normal mode. In the authentication mode, an embedded Trojan authentication circuit monitors the bit-flipping count of a suspicious Trojan net within the pre-defined constant clock cycles and identify whether it is a real Trojan or not. If the authentication condition is satisfied, the suspicious Trojan net is validated. Otherwise, it is invalidated and HT functions are masked. By doing this, even untrusted netlists with HTs can still be used in the normal mode without security leakage. By setting the appropriate authentication condition using training sets from Trust-HUB gate-level benchmarks, the proposed technique invalidates successfully only HTs in the training sets. Furthermore, by embedding the in-situ Trojan authentication circuit into a Trojan-inserted AES crypto netlist, it can run securely and correctly even if HTs exist where its area overhead is just 1.5% with no delay overhead.