家賊難防

家贼难防
jiāzéinánfáng

it is difficult to guard against a thief within one's house; bad people from within are the most difficult to guard against

Wiktionary

Entry Composition

1 家賊 jiāzéi
  1. (Northeastern Mandarin) sparrow
  2. sparrow
  3. thief within a house or family
  4. undiscovered traitor
=2 難 nán
  1. difficult (to...)
  2. problem
  3. difficulty
  4. difficult
  5. not good
=2 難 nàn
  1. disaster
  2. distress
  3. to scold
3 防 fáng
  1. to protect
  2. to defend
  3. to guard against
  4. to prevent