webcontainer-core VS quickjs-emscripten

How does it work? There is no backend whatsoever. The API Security Academy leverages WebContainers, a new technology that allows running full-blown node instances directly in the browser. Each WebContainer contains a live GraphQL application, so you'll not only understand why a vulnerability is risky, but also how to exploit it and, most importantly, how to fix it.

StackBlitz made the claim "... run Node.js, entirely inside your browser" #658, then had to revise it's claim to "We currently do not expose a way to use WebContainer outside of StackBlitz.com,".

> Wasm though seems like the likely general heir, and will have many different offerings for how to do that (Deno being one!).

I was recently blown away by some ideas that StackBlitz [0] apply based on WebContainers. The idea of a "server in the browser", they allow you to run Node-based environment like that via Wasm.

[0] https://stackblitz.com/

[1] https://webcontainers.io/

See https://webcontainers.io.

This very simple fact is well known flaw, which was already often criticized and asked for solutions by users. It doesn't only affect this kind of very exotic bootstrap applications but also significantly limits rusts usefulness in many other areas. Pure browser based scientific code documentation and example notebooks (e.g. jupyterLite) and sandboxed CI and IDE solutions (e.g. web containers) as available for many other languages are simply not available for rust because of this very fundamental issue.

Looks like they plan to open it. From the FAQ section of README (https://github.com/stackblitz/webcontainer-core#faqs):

> Is there a developer API?

I'm skeptical about stackblitz claims. The last time I checked that is closed source code https://github.com/stackblitz/webcontainer-core/issues/658.

StackBlitz claimed https://blog.stackblitz.com/posts/introducing-webcontainers/ to have implemented Node.js in the browser, though I have not observed any evidence of that being true and correct https://github.com/stackblitz/webcontainer-core/issues/658.

The other honorable mention is StackBlitz, a web-based web editor for making containers that run Javascript applications. Interestingly, StackBlitz is championing WebContainers, a technology that allows developers to start NodeJS servers within the browser. The downside is, this technology only works with Javascript-based projects (NodeJS, NextJS, etc). I include it here because WebContainers could easily be extended to support other languages via WASM, like Ruby. I’ll also add in Buildah, a tool for building OCI images. I won’t say much about this tool because it’s designed for building images; you still need another service, like Podman, to actually create the containers.

Based on your comment below I think you figured out the difference - but if you're looking to execute JS, you can pick between ShadowRealm (where available, or using a polyfill) or my library quickjs-emscripten.

Pros of quickjs-emscripten over ShadowRealm:

- You can use quickjs today in any browser with WASM. ShadowRealm isn't available yet, and polyfills have had security issues in the past. See https://www.figma.com/blog/an-update-on-plugin-security/

- In ShadowRealm eval, untrusted code can consume arbitrary CPU cycles. With QuickJS, you can control the CPU time used during an `eval` using an [interrupt handler] that's called periodically during the eval.

- In ShadowRealm eval, untrusted code can allocate arbitrary amounts of memory. With QuickJS, you can control both the [stack size] and the [heap size] available inside the runtime.

- quickjs-emscripten can do interesting things with custom module loaders and facades that allow synchronous code inside the runtime to call async code on the host.

Pros of ShadowRealm over QuickJS:

- ShadowRealm will (presumably?) execute code using your native runtime, probably v8, JavaScriptCore, or SpiderMonkey. Quickjs is orders of magnitude slower than JIT'd javascript performance of v8 etc. It's also slower than v8/JSC's interpreters, although not by a huge amount. See [benchmarks] from 2019.

- You can easily call and pass values to ShadowRealm imported functions. Talking to quickjs-emscripten guest code requires a lot of fiddly and manual object building.

- Overall the quickjs(-emscripten) API is verbose, and requires manual memory management of references to values inside the quickjs runtime.

[interrupt handler]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[stack size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[heap size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[benchmarks]: https://bellard.org/quickjs/bench.html

The thing I want to achieve with WebAssembly is still proving a lot harder than I had anticipated.

I want to be able to take strings of untrusted code provided by users and execute them in a safe sandbox.

I have all sorts of things I want this for - think custom templates for a web application, custom workflow automation scripts (Zapier-style), running transformations against JSON data.

When you're dealing with untrusted code you need a really robust sandbox. WebAssembly really should be that sandbox.

I'd like to support Python, JavaScript and maybe other languages too. I want to take a user-provided string of code in one of those languages and execute that in a sandbox with a strict limit on both memory usage and time taken (so I can't be crashed by a "while True" loop). If memory or time limit are exceeded, I want to get an exception which I can catch and return an error message to the user.

I've been exploring options for this for quite a while now. The furthest I've got was running Pyodide inside of Deno: https://til.simonwillison.net/deno/pyodide-sandbox

Surprisingly I've not found a good pattern for running a JavaScript interpreter in a WASM sandbox yet. https://github.com/justjake/quickjs-emscripten looks promising but I've not found the right recipe to call it from server-side Python or Deno yet.

Can Extism help with this? I'm confident I'm not the only person who's looking for a solution here!

"Safely execute untrusted Javascript in your Javascript, and execute synchronous code that uses async functions" quickjs-emscripten, NPM

This maybe, as a start?

https://github.com/justjake/quickjs-emscripten

QuickJS Running in WebAssembly\ (17 comments)

The library was inspired by Figma’s blog posts about their plug-in system: https://github.com/justjake/quickjs-emscripten#background

If you need to call into user-generated Javascript synchronously or have greater control over the sandbox environment, you can use WebAssembly to run a Javascript interpreter: https://github.com/justjake/quickjs-emscripten#quickjs-emscr...

QuickJS in WebAssembly is much slower than your browser's native Javascript runtime, but possibly faster than async calls using postMessage. As an added bonus, it can make async functions in the host appear to be synchronous inside the sandbox using asyncify: https://emscripten.org/docs/porting/asyncify.html.

You can use One Weird Trick with generator functions to make your code "generic" over synchronicity. I use this technique to avoid needing to implement both sync and async versions of some functions in my quickjs-emscripten library.

The great part about this technique as a library author is that unlike choosing to use a Promise return type, this technique is invisible in my public API. I can write a function like `export function coolAlgorithm(getData: (request: I) => O | Promise): R | Promise`, and we get automatic performance improvement if the user's function happens to return synchronously, without mystery generator stuff showing up in the function signature.

Helper to make a function that can be either sync or async: https://github.com/justjake/quickjs-emscripten/blob/ff211447...

Uses: https://cs.github.com/justjake/quickjs-emscripten?q=yield*+l...

This seems like a pretty nice, recently enabled way of getting a sandboxed js environment: QuickJS compiled to WASM: https://github.com/justjake/quickjs-emscripten.