乘虚而入

Explanation: Virtual: the gap. Taking advantage of the gap or no access to prevention and 　　Usage: Partial official; as predicate; used for military 　　Source: Song Zhangjun room "to seven Quan RB" 120 back: "approaching the home, since after the wall take advantage of the trails and the courtyard." 　　Examples: If the views of the deposit, not harmony, is his first solitary, others Ande Bu ~. (Qing Li Ruzhen "Flowers in the Mirror," the seventy-fourth back) 　　Riddle: Loophole Thesaurus:  Swoop, Take advantage of a loophole Infiltration