$CN^-$ is a strong field ligand. This is due to the fact that :-

A strong field ligand produces a large crystal field splitting ($\Delta_o$).
$CN^-$ donates a lone pair from carbon to the metal (a $\sigma$ bond). In addition, it has empty $\pi^*$ antibonding orbitals that can accept electron density back from the filled metal $t_{2g}$ d orbitals (back bonding, $\pi$-acceptor behaviour).
This metal-to-ligand $\pi$ back donation lowers the energy of the $t_{2g}$ orbitals and increases $\Delta_o$. That is why $CN^-$ (like CO) is at the strong end of the spectrochemical series.
Being a pseudohalide or carrying a negative charge does not make a ligand strong field ($Cl^-$ and $F^-$ are negative but weak field). Strong field ligands form low spin complexes, not high spin ones.
So $CN^-$ is a strong field ligand because it can accept electrons from the metal species ($\pi$-acceptor).