The previous two posts covered the threat model and the seccomp sandbox. This one is about going further: a WebAssembly execution environment where the security properties come from the compilation target, not from OS-level filters.

Last week we shipped sandbox_exec — a 224-line C program using seccomp-bpf to isolate student code in AWS Lambda. The honest answer at the time was: "WASM would be cleaner, but the Python ecosystem isn't there yet."

This week we measured exactly what "the Python ecosystem isn't there yet" costs in milliseconds. The answer is more nuanced than expected.

Today my MSc project officially kicked off. The premise sounds simple: run student code safely inside AWS Lambda. The constraints make it interesting.

The Problem

Lambda Feedback is a platform where students submit code and get it evaluated in real time. The backend uses serverless functions — AWS Lambda spins up a container, runs the code, returns the result.