Common Space Law is a piece or pieces of human legislation. It contains sections pertaining to rules of engagement and rendering aid to vessels in distress. This law, or group of laws may or may not include the five international treaties negotiated by the United Nations Committee on the Peaceful Uses of Outer Space in the 20th century. It may also be the successor to those agreements.
According to Codename: SURGEON, prior to the Insurrection, UNSC officers and merchant men had a near-religious reverence for Common Space Law. During the Callisto Incident, Preston Cole severely broke protocol by sending a fake distress signal in order to lure his opponent closer. SURGEON's remarks imply that as the Insurrection wore on, respect for Common Space Law lessened significantly. During the Human-Covenant war it was likely ignored altogether.