引狼入室（yǐn láng rù shì）

引狼入室（yǐn láng rù shì；invite a wolf to enter your house）

Once upon a time, there was a shepherd who was tending to sheep. One day he realized there was a wolf following his flock. At first, he was very cautious, afraid the wolf would attack the sheep. The wolf, however, did not hurt the sheep. The next day, when the shepherd drove his sheep back into the mountains, the same wolf appeared. Again, he did not attack the sheep but just followed quietly. After a month, the shepherd came to believe that the wolf was quite friendly, and so he began treating the wolf as if he were a shepherd dog. One evening, when it was time to herd the sheep home, he allowed the wolf to return together with his sheep, and stay overnight in the sheepfold. The next day, the shepherd discovered to his horror that the wolf had eaten a lamb and wounded several sheep!

We use to describe the situation in which one trusted an enemy and as result hurts oneself.