The reports that South Korea has been granted special permission to build missiles that exceed the guidelines of the Missile Technology Control Regime are confusing. Reports in the New York Times and the Wall Street Journal said that South Korea and the US had agreed to extend the range of missiles that South Korea would build. However, the MTCR is not an arms control treaty that limits the range of South Korean missiles; it is a suppliers agreement that limits the US and other members, who agree not to supply Korea with missiles or technology or parts for missiles beyond the agreed range. Thus, it appears that the violator of the MTCR is the United States, not Korea.
I assume that the US has gotten the agreement of the other 33 supplier-country members of the MTCR to this extension of range and payload. It should not be a bilateral decision when the US is a member of a suppliers' group.