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Scastie: use any Scala compiler and Scala library in the browser (scala-lang.org)
170 points by heathermiller 6 days ago | hide | past | web | 33 comments | favorite

(naturally, the very first thing I tried to evaluate is scala.io.Source.fromFile("/etc/passwd").getLines.mkString("\n") . Spoiler alert: it works!)

It does and you are root. You are evaluating inside a docker container. It's not a bulletproof method but it will stop a few. The instances evaluating your code is also on a network not accessible from the internet. I'm not an expert in security, if you have any advice on how we can improve our defence please tell us.

I am not a pentesting expert. My first reaction is to leave everything as is, as it is a very cool to play with root access to docker containers (I managed to reboot one, but a new one immediately appeared on page reload).

My worst concern now would be network security. With root access, it is trivial to e.g. install spambots in all your containers (just checked, command execution works, and external network access is enabled). I think it is a good idea to at least disable networking. (Update: and use a minimal Docker image like Alpine Linux).



I agree with this. What's to stop me from opening a bunch of the containers and using them to DDOS someone or to send out spam emails? I'm already playing around with system commands and they seem to be entirely unrestricted.

Basically I can run any bash script, as is, with

    import sys.process._
And I seem to be able to at least cause the containers to endlessly restart quite simply.

Yeah disabling networking was an idea. I prefer to leave it open so you can try http client/libraries that access the web. To limit spam, if it becomes an issue we could throttle the connection.

The general rule is that you want as many layers of security as you can get away with without making things impractically inconvenient. In this case, the first step is probably not letting the user's code run as root in the container. Gaining container-root is going to be the first step in many, many exploits and by letting code just run that way, you are giving a potential attacker that step for free.

Disclaimer: Absolutely not a security expert, just someone who is somewhat on the hook for security!

I put together a list of security tips:


Probably the biggest one is to use Virtualbox or another virtual machine so that Docker isn't your only line of defence.

> I put together a list of security tips: [...]

and 63 contributors. ;p

Check the blame log. That section's all me.


Please don't misunderstand this. I know from reading your postings on /r/scala that you are a humble person. Originally I actually didn't even knew if there was a contributor to the security section besides you and only wanted to tease. :D

Fair enough -- I was actually thinking of


but that's a fair counterexample.

Thumbs up for the honest upfront response!

Maybe Jessica McKellar's "Building and Breaking a Python Sandbox" talk can bring some ideas. (But maybe not! It might be too Python-specific or too language-level whereas you want to remain at a higher level with just Docker)

Video: https://www.youtube.com/watch?v=sL_syMmRkoU

Slides: https://speakerdeck.com/pycon2014/building-and-breaking-a-py...

Congrats Guillaume! How much of ScalaKata code is in there? Sorry if I missed it in the post, but is Scastie open source? I'd love to finally fix scalatutorials.com :) p.s. did you guys consider using a "serverless" architecture to isolate runners instead of docker? Also, are you running it with java -Djava.security.manager? If not then why?

The security manager can be disabled using reflection. If you don't allow you loose some libraries like Akka. The main thing that is missing from ScalaKata is autocompletion and it will be implemented in the summer during the GSOC. AWS lambda could be worth investigating. We don't expose an API for evaluating but others have expressed this need. Can you create an issue on GitHub for that?

You can use an agent to disable the security manager disabling. :-)


Gladly, sorry for the silly question, but on which repo? Scala Lang main website? Scalakata?

This is the main repo: https://github.com/scalacenter/scastie. It's not at silly question at all.

I recommend to at least wrap all the containers in a VM between the docker containers and your server-side orchestration code.

SELinux also helps, from what I've read.


/etc/shadow works as well :S But there are not encrypted bits in it.

Hi Heather!

Looks like you've got a Hacker News effect on your shoulders. :) Servers seem to be overloaded.

Nah, that's probably just Hacker Effect of people rebooting the container ;).

Working for me (TM).

Seems like the Scala and SBT equivalent of https://npm.runkit.com/ (which allows in-browser use of Node and NPM packages)

This looks cool, I just experimented with it. I am curious: what is the business model for this? Server costs are probably fairly expensive.

We are the [Scala Center](https://scala.epfl.ch). We are a non-profit organization. Our revenue is from donations. This service will be forever free.

Its great to see so much good stuff going on in Scala land!

I'm a Scala developer but I don't understand what is Scastie. How does it benefit from the perspective of developers like me?

It's an interactive playground for Scala so you can share code and send it to other people.


Will this allow running a play app? (with debug etc) ??

This looks so cool!!! Can't wait to try it out!

How am I supposed to dismiss this modal window? http://i.imgur.com/atx6KsX.png

hum this look like a but in safari https://github.com/scalacenter/scastie/issues/. You can visit this link: https://scastie.scala-lang.org/MasseGuillaume/kHn9lemPTayxoY... and it will clear the modal.

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