乘虚蹈隙[chéng xū dǎo xì]
词典take advantage of sb.'s undefended state to attack：乘虚蹈隙。
词典乘虚蹈隙
词典attack an unguarded point：乘虚蹈隙。
take advantage of sb.'s undefended state to attack; attack an unguarded point