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Runtime

How to run Deno code in a sandbox

Install Deno with its one-line installer in an isolated Linux microVM. Then run the file with deno run and grant only the permissions it needs.

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On Runtime, Deno's permission flags have a second layer behind them: a Firecracker microVM with its own kernel. For untrusted code, Deno's security guide recommends several layers. One of them is "a sandboxed environment like a VM or MicroVM (gVisor, Firecracker, etc)". Each Runtime sandbox provides that layer. The host enforces the network switch. A 2 vCPU, 4 GiB sandbox costs $0.03125 an hour while it waits (pricing).

What Deno's permissions do and do not cover

A program run with Deno has no access to files, the network, environment variables or subprocesses. You grant access with flags such as --allow-net, --allow-read and --allow-write. Each flag can be scoped (--allow-net=example.com, --allow-read=./data). Deno also "sets no limits on the execution of code at the same privilege level". The -A flag grants everything. For untrusted code, the Deno guide recommends all of these: limited permissions, a Web Worker with fewer permissions, OS sandboxing such as seccomp, and a VM or microVM (Deno security).

Layer What it stops Where it is enforced
Deno permissions File, network, env and subprocess access the flags did not grant Inside the Deno process
Firecracker microVM Reaching your host through a kernel or runtime bug Hardware virtualization
Runtime network rules Any outbound connection, or all but named hosts On the host, outside the VM
Command and sandbox time limits Loops that never end; a sandbox left running Runtime's API and host

Root inside the sandbox cannot change the last two (security).

Install Deno and run a file

Deno is not in the default image. Its Linux installer is a single command. It puts the binary in $HOME/.deno/bin/deno (Deno installation). In a sandbox, that path is /workspace/.deno/bin/deno. The installer needs unzip, and the image includes it. v2.9.8 was the latest release on 26 October 2026.

TypeScriptimport { Sandbox } from "withruntime";const program = `const text = await Deno.readTextFile("./input.txt");console.log(text.trim().split(/\\s+/).length, "words");`;await using sbx = await Sandbox.create({  network: { internet: true, allow: ["deno.land", "*.deno.land"] },  timeoutSeconds: 600,  onLeaseEnd: "stop",});await sbx.exec("curl -fsSL https://deno.land/install.sh | sh -s v2.9.8", {  check: true,  timeoutMs: 180_000,});await sbx.network.set({ internet: false });await sbx.files.write("/workspace/input.txt", "one two three four");await sbx.files.write("/workspace/count.ts", program);const deno = "/workspace/.deno/bin/deno";const run = await sbx.exec([deno, "run", "--allow-read=./input.txt", "count.ts"], {  timeoutMs: 30_000,});console.log(run.stdout); // 4 words
Pythonfrom withruntime import Sandboxprogram = """const text = await Deno.readTextFile("./input.txt");console.log(text.trim().split(/\\s+/).length, "words");"""with Sandbox.create(    network={"internet": True, "allow": ["deno.land", "*.deno.land"]},    timeout_seconds=600,    on_lease_end="stop",) as sbx:    sbx.exec("curl -fsSL https://deno.land/install.sh | sh -s v2.9.8", check=True, timeout_ms=180_000)    sbx.network.set(internet=False)    sbx.files.write("/workspace/input.txt", "one two three four")    sbx.files.write("/workspace/count.ts", program)    deno = "/workspace/.deno/bin/deno"    run = sbx.exec([deno, "run", "--allow-read=./input.txt", "count.ts"], timeout_ms=30_000)    print(run.stdout)  # 4 words

The installer script and the Deno download both come from deno.land hosts, so the allow list has two entries. The version argument pins the release. Without it, the script fetches the latest release.

Type-check before running

deno run strips types and does not check them. To type-check first, use deno check, or deno run --check. Both run the TypeScript compiler first (Deno TypeScript):

Terminal/workspace/.deno/bin/deno check count.ts/workspace/.deno/bin/deno run --check --allow-read=./input.txt count.ts

Deno in every sandbox

Install Deno once in a custom image. The installer reads DENO_INSTALL. Set it to /usr/local to put deno in /usr/local/bin, which is on the sandbox user's PATH:

TypeScriptimport { Runtime } from "withruntime";const runtime = new Runtime();await runtime.images.build({  name: "deno",  recipe: {    commands: ["curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/usr/local sh -s v2.9.8"],  },});await using sbx = await runtime.sandboxes.create({ image: "deno", network: { internet: false } });await sbx.files.write("/workspace/hi.ts", "console.log(`deno ${Deno.version.deno}`);");console.log((await sbx.exec(["deno", "run", "hi.ts"])).stdout);
Pythonfrom withruntime import Runtimeruntime = Runtime()runtime.images.build(name="deno", recipe={    "commands": ["curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/usr/local sh -s v2.9.8"],})with runtime.sandboxes.create(image="deno", network={"internet": False}) as sbx:    sbx.files.write("/workspace/hi.ts", "console.log(`deno ${Deno.version.deno}`);")    print(sbx.exec(["deno", "run", "hi.ts"]).stdout)

Recipe commands run as root in /workspace. Building the image is free. A stored image is billed by its size.

Deno or Node in a sandbox?

  • Deno when the code should run with no permissions by default, or when a project already uses deno.json and JSR packages.
  • Node.js or Bun when you want no install at all: both are in every sandbox (Node.js, Bun).

Deno can also be installed from npm with npm install -g deno. Its docs say this slows Deno's startup, and they recommend the shell installer.

A new account gets 100 free hours of a 1 GB machine in its first 30 days. No payment card is needed:

Terminalnpx withruntime sandboxes run -- bash -c 'curl -fsSL https://deno.land/install.sh | sh && ~/.deno/bin/deno --version'

Sources

Facts on this page were checked on 26 October 2026.

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