$\mathrm{CN}^{-}$is strong field ligand. This is due to the fact that :

Cyanide ion is strong field ligand because it is a pseudohalide ion. Pseudohalide ions are stronger coordinating ligand \& they have the ability to form $\sigma$ bond (from the pseudohalide to the metal) and $\pi$ bond (from the metal to pseudohalide). Related Theory A pseudohalide is an anion which has similar properties to a halogen, but not a real halogen. Pseudohalogens are polyatomic analogues of halogens, or also called 'fake' halogens, whose properties resemble that of true halogens.