Don't claim a security boundary holds — demonstrate it

A system has to run a chunk of code you don't control —a plugin, a dependency, something generated— and you want to guarantee that code cannot touch the file system or spawn processes. Not that it "shouldn't": that it can't , mechanically. That's capability confinement, and it's one of the central problems of runtime security. Designing it and demonstrating it are two different things, and confusing them is expensive. A design is a claim You can write an impeccable document: "access to fs and to spawning processes is controlled like this, with these mechanisms, under this threat model". It's real and necessary work. But it's a claim . And the failure mode of a security boundary is that it looks like it holds until it doesn't —silent, invisible in tests, visible only when someone crosses it—. In security, an unverified claim has exactly the shape of a beautiful, wrong architecture. The design can assume that a module-loader hook fires at a point where it actually doesn't, and all the reasoning hanging off that is correct and worth nothing. The mechanisms exist, and there are several In Node, to name a concrete runtime, there are at least three layers, and they aren't interchangeable: The native permission model ( --permission , --allow-fs-read …), which cuts access to fs and to spawning processes at the whole-process level. SES / Hardened JavaScript (Compartments, lockdown() ), which confines what each module can import within the process. Module-loader interception , which controls what resolves when the code asks for something. Choosing well among them is the design. But choosing well doesn't prove the choice holds against the real dependency tree you're going to run. Demonstrate instead of claim The alternative to signing off a design is delivering a confinement harness : untrusted code that tries to reach the dangerous capability —open a file, spawn a process— against the real runtime and its real dependency tree, and a log that shows each attempt was blocked . O