趁虚而入
      Explanation: When the invasion force to take advantage of weakness.
      Usage: Partial official; as predicate, object; containing derogatory
      Source: Qing Shi Yukun "Chivalric Heroes" 40 back: "Now all diseases will slip."
      Examples: Now riddled ~. (Qing Shi Yukun "Chivalric Heroes" forty-back)
  • Thesaurus:  to gain an advantage over something through his in attention, Take advantage of a loophole Infiltration