The 29 year old Scot will be eligable for all game between this Saturday at Barnsley and the away trip to Birmingham on Jan 1st.
His parent club Norwich City will be able to recall him, if required, after 28 days.
Malky Mackay has moved to bring in reinforcements after losing both Andrew Taylor and Ben Turner to injury last Saturday.
Lappin can play at left back and in midfield. He will be squad no. 24 and will train with the club on Thursday morning before heading to Barnsley on Friday for City's Championship fixture at Oakwell on Saturday.
Source: Cardiff City Online
Source: FOOTYMAD
