The Outer Space Treaty is the 1967 international agreement that sets the basic legal rules for military activity in space. Its most important restriction is that states may not place nuclear weapons or other weapons of mass destruction in orbit, and they may not station such weapons on celestial bodies. It also limits military activity on the Moon and other celestial bodies, while still allowing many non-WMD space activities.
In cyber security, the treaty matters because it shapes how space systems are built, described, and defended. Most modern space threats do not involve violating the treaty directly; they target satellites through jamming, spoofing, malware, supply-chain compromise, or attacks on ground control networks. The treaty does not prohibit those cyber or electronic warfare methods, so defenders must treat space assets as part of a wider cyber-physical system. In practice, legal compliance and operational security are separate problems: a system can be treaty-compliant and still be highly vulnerable to cyber disruption.