Rustbox vs Docker, Firecracker, gVisor & Wasm: Code Isolation Compared

Rustbox runs untrusted code inside a hardened shared-kernel sandbox built from Linux namespaces, cgroups v2, and a seccomp-BPF deny-list. Docker and gVisor are general-purpose container runtimes, Firecracker is a hardware-virtualized microVM, and WebAssembly is an in-process capability sandbox. They compare across the four axes that matter when you execute code from users, AI agents, or LLM tool calls: cold-start latency, per-sandbox memory overhead, security boundary layer, and pipeline setup friction.

Every Rustbox number is measured on Rustbox infrastructure across the full sandbox lifecycle (creation, execution, and teardown) with no warm pooling. See the Rustbox benchmarks for the per-language breakdown.

WebAssembly has the lowest cold start, instantiating a precompiled module in under 1 ms because there is no process, no kernel, and no guest OS to start. Among methods that run unmodified native code, Rustbox (about 36 ms) and a warm Docker container (about 50 ms) are comparable, and both are well ahead of Firecracker microVMs at roughly 125 ms.

Two caveats keep this honest. First, Rustbox builds a brand-new sandbox for every submission, with no container reuse or pre-warmed pool, so the 36 ms figure is the worst case rather than an amortized warm path. Second, the ~50 ms Docker number is a cached-image container start, not a hardened sandbox: the comparable Rustbox run already applies the full isolation stack below.

Rustbox adds almost no sandbox bookkeeping memory. A trivial program peaks at roughly 3-8 MB, and nearly all of that is the language runtime itself, not the isolation layer. There is no guest kernel and no user-space syscall emulator to host, so per-sandbox overhead stays close to the cost of the process you are already running.

In Rustbox, the memory limit is enforced by cgroups v2 per profile (256 MB on the Judge profile), and every run reports the cgroup's peak memory and any OOM-kill events as kernel-backed evidence. Memory accounting is observable, not estimated.

The boundary layer is the most important difference. Firecracker is the strongest because it runs a separate guest kernel behind hardware virtualization, so a kernel exploit in the workload cannot reach the host. Rustbox, Docker, and gVisor all share the host kernel, but they harden it very differently.

Rustbox does not wrap containers. It composes kernel primitives directly in a fixed order that the Rust type system enforces at compile time: skip a privilege-stripping step and the code does not compile. It deliberately omits user namespaces (a recurring source of privilege-escalation CVEs) and blocks the exploit-class syscalls attackers rely on, including ptrace, bpf, io_uring, mount, and clone(CLONE_NEWUSER). The isolation model is validated against 147 adversarial scenarios across 8 languages with zero escapes. See the Rustbox isolation model and seccomp filtering.

The honest caveat: a shared-kernel sandbox like Rustbox, Docker, or gVisor is not a hardware boundary. If your threat model requires defense against a host-kernel zero-day, a microVM like Firecracker (or a full VM) is the stronger choice. Rustbox closes the gap that matters for untrusted application code while keeping cold start near 36 ms.

Rustbox has the least setup friction of any option here. There is no image to build, no daemon to run, and no registry to manage. You send source code to one HTTPS endpoint and receive a verdict. Every other method requires you to package, host, or compile the workload before it can run.

Because Rustbox is a managed API, the same request runs all 8 languages. You change one language field, not your infrastructure.

• Use WebAssembly when you own the source and can compile it to Wasm/WASI, and you want sub-millisecond, in-process sandboxing for pure compute (plugins, edge functions, deterministic evaluation).
• Use Firecracker when you need hardware-level isolation between tenants and can absorb guest-image and VM lifecycle management. This is what powers AWS Lambda and Fargate.
• Use Docker or gVisor when you already run containerized services and the code is semi-trusted. gVisor adds a user-space kernel for a stronger boundary at a syscall-performance cost.
• Use Rustbox when you must run arbitrary, unmodified code in any of 8 languages with strong kernel-enforced isolation and per-run kernel evidence, and you do not want to operate a runner: competitive-programming judges, coding-assessment platforms, EdTech, and AI agent code interpreters.

Comparing Rustbox to other hosted code-execution engines rather than to isolation primitives? See Rustbox vs Judge0 vs E2B vs Piston.