乘虚而入
chéng​xū'ér​rù

to take advantage of a weakness

  1. 乘 chéng
    to take advantage of
    To avail oneself of an opportunity, circumstances, or someone's weakness.
  2. 虚 虛 xū
    weak point
    a gap; a weak point; a vulnerability
  3. 而 ér particleconjunction
    [adverbial linker]
    Connects an adverb-like description to the following verb to show the manner, cause, or basis of an action.
  4. 入 rù verbparticle
    to enter
    to enter; to go into; to move from outside to inside