ctf-collab

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Collaborative programming environment inside GitHub Actions – like Google Docs for hacking

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// repository documentation

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# Introduction

Create an ephemeral, collaborative programming environment inside GitHub
Actions; conveniently save data with git. Particularly useful for solving
Capture The Flag
([CTF](https://en.wikipedia.org/wiki/Capture_the_flag#Computer_security))
problems.

Less technically: this is kind of like Google Docs for working on hacking
competitions collaboratively.

# Quick start

[Create a repo from the
template](https://github.com/jstrieb/ctf-collab-template/generate). Do the rest
of the steps from within your copy of the repository. The link to create a copy
of the repository will only work for users logged into GitHub.

In all cases, the username is `runner` and the password is `ctf`.

## Connect With Tor

Connecting over Tor is the most secure way to use the server, and doesn't
require signing up for any service. It does, however, require downloading the
Tor Browser Bundle. It also runs with noticeably higher latency than using the
alternative, non-Tor connection method.

0. [Create a repo from the
   template](https://github.com/jstrieb/ctf-collab-template/generate). Many of
   the following links will only work from within your copy of the repository.
   Creating a copy only works for users logged into GitHub.
1. Download and run the [Tor Browser
   Bundle](https://www.torproject.org/download/).
2. Navigate to the ["Run Collaborative CTF Environment"
   workflow](../../actions?query=workflow%3A"Run+Collaborative+CTF+Environment").
3. Start the server using the button in the top right ("Run workflow").
4. View the output; wait for it to print the connection information.
5. SSH in or connect from the Web using the connection information printed
   during the Action run. Note: whether connection to the `.onion` address
   using the browser or SSH, Tor Browser must be running.
6. Tor browser has a default setting that causes text to be illegible in `ttyd`
   and/or its dependency `xterm.js`. To fix this, go to `about:config` in the
   address bar, and set `privacy.resistFingerprinting` to `false`.

## Connect With ngrok (Without Tor)

Connecting over ngrok instead of Tor is a less secure, but lower-latency way to
connect to the server running on GitHub Actions. This method of connecting
doesn't require downloading Tor, but it does require signing up for ngrok.

0. [Create a repo from the
   template](https://github.com/jstrieb/ctf-collab-template/generate). Many of
   the following links will only work from within your copy of the repository.
   Creating a copy only works for users logged into GitHub.
1. [Sign up for ngrok](https://dashboard.ngrok.com/get-started/setup).
2. [Copy your ngrok
   Authtoken](https://dashboard.ngrok.com/auth/your-authtoken).
3. Navigate to the ["Run Collaborative CTF Environment"
   workflow](../../actions?query=workflow%3A"Run+Collaborative+CTF+Environment").
4. Paste in your ngrok Authtoken when you start the server using the button in
   the top right ("Run workflow").
   - To avoid pasting the Authtoken every time, paste it into a secret called
     `NGROK_TOKEN` under the [Secrets](../../settings/secrets/actions) settings
     area. It will be saved here and used automatically.
5. View the output; wait for it to print the connection information.
6. SSH in or connect from the Web using the connection information printed
   during the Action run.

## Once Connected

- By default, the collaborative environment uses `tmux` with <kbd>Ctrl</kbd> +
  <kbd>A</kbd> as the "prefix," and a few other custom keys that can be viewed
  in [.tmux.conf](dotfiles/.tmux.conf)
  - Read a [good introduction to
    `tmux`](https://www.hamvocke.com/blog/a-quick-and-easy-guide-to-tmux/)
  - Press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>c</kbd> to open a new
    window
  - Press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>n</kbd> to go to the
    next window and <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>p</kbd> to go
    to the previous window
  - Press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>|</kbd> to split the
    current pane vertically (this is a vertical bar "|" – not the letter "L" or
    "i")
  - Press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>S</kbd> to split the
    current pane horizontally (this is a *capitalized* "S" – `tmux` cares about
    capitalization)
  - Press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then one of the arrow keys to move
    to the pane in that direction
- By default, `tmux` adjusts the size of a window to accommodate the smallest
  viewer
  - Every window is the same size for all viewers, at all times
  - To make the window adjust to the largest client instead of the smallest,
    press <kbd>Ctrl</kbd> + <kbd>A</kbd> and then <kbd>G</kbd> (must be capital
    "G")
  - Be careful! This will mean that some of the window is difficult or
    impossible to see for the viewer with the smaller client
- Use the `quit` command to terminate the server for all connected users
  - Alternatively, the workflow can be canceled from within the GitHub web
    interface
- Create folders in the repo (`~/ctf`) for each CTF to stay organized
  - Save any changes by committing using `git` within the `~/ctf` folder
  - The default identity associated with commits is anonymous
- Theoretically, unlimited users can connect at once via Tor, and up to 40 can
  connect using ngrok for free
  - Practically, the real upper-limit is probably far lower
  - Every new user connects using the same link/connection information
  - Additional users do not need GitHub accounts to connect or commit to the
    repository from within the environment
  - Each user can create their own windows independently, or switch to windows
    created by other users
  - All connected users have passwordless `sudo` capabilities
- If you regularly install packages, add them to the [GitHub Actions
  workflow](./.github/workflows/run-server.yml#L35) so they are installed every
  time the environment starts up
  - The default packages that are installed in the environment can be viewed
    [here](./.github/workflows/run-server.yml#L35)

<summary><b>Graphical Server</b></summary>

**Don't use the graphical workflow unless absolutely necessary!**

0. [Create a repo from the
   template](https://github.com/jstrieb/ctf-collab-template/generate). Many of
   the following links will only work from within your copy of the repository.
   Creating a copy only works for users logged into GitHub.
1. [Sign up for ngrok](https://dashboard.ngrok.com/get-started/setup).
2. [Copy your ngrok
   Authtoken](https://dashboard.ngrok.com/auth/your-authtoken).
3. Navigate to the ["Run Graphical RDP CTF Environment (Ubuntu GNOME)"
   workflow](../../actions?query=workflow%3A"Run+Graphical+RDP+CTF+Environment+(Ubuntu+GNOME)").
4. Paste in your ngrok Authtoken when you start the server using the button in
   the top right ("Run workflow").
   - To avoid pasting the Authtoken every time, paste it into a secret called
     `NGROK_TOKEN` under the [Secrets](../../settings/secrets/actions) settings
     area. It will be saved here and used automatically.
5. View the output; wait for it to print the connection information.
6. Connect to the HTTP link in your browser or paste the connection link into
   your [remote desktop (RDP)
   client](https://en.wikipedia.org/wiki/Remote_Desktop_Protocol).
   - On Windows, press the start button and search for "Remote Desktop
     Connection" – the client should be installed by default.
   - On Linux [Remmina](https://remmina.org/how-to-install-remmina/) is a
     well-regarded RDP client.

</details>

# Who This Project is For

This project is for those with limited access to compute resources – students
in particular. By making it possible to connect from the web and over Tor, the
project is designed to be accessible to those who are behind a firewall and/or
are unable to install software on the computer they are using. For example,
someone who wants to participate in a CTF competition, but is using
shared computers in a library or a school computer lab.

## Be a Good Citizen

This project uses generously-offered, free resources in a way that was likely
not intended by GitHub. Please do not abuse them.

I claim no responsibility for how you use this project. Based on my reading of
GitHub's:

- [Terms of
  Service](https://docs.github.com/en/free-pro-team@latest/github/site-policy/github-terms-of-service#the-github-terms-of-service)
- [Acceptable Use
  Policy](https://docs.github.com/en/free-pro-team@latest/github/site-policy/github-acceptable-use-policies)
- and [Actions Usage
  Policy](https://docs.github.com/en/free-pro-team@latest/github/site-policy/github-additional-product-terms)

I have concluded that using this Actions workflow to do productive work,
particularly if you don't have the ability to run a Linux computer of your own,
is probably permissible if done in good faith, and with an effort made to
reduce excessive resource consumption. For example, don't use the graphical
workflows unless absolutely necessary, because they are considerably more
resource intensive to set up and run, and don't leave the workflows running
when not using them.

That being said, I may be reading the policies wrong, and GitHub has the right
to change them at any time. In particular, if this use of GitHub Actions is
abused, it is likely that future ability to do things like this will be limited
by GitHub. **If you like it, don't ruin it for others.**

Note: typical users get [3,000 minutes of private Actions
time](https://docs.github.com/en/free-pro-team@latest/actions/reference/usage-limits-billing-and-administration#usage-limits)
per month per user. This project can quickly run up that time if you are not
careful. Check your own usage [here](https://github.com/settings/billing) to
see how close to the limit you are. Also note that actions stop automatically
after 6 hours.

# Known Issues

- **Problem:** Tor browser loads the hidden service indefinitely, never
  connecting

**Solution:** Restart Tor browser
- **Problem:** The web interface text is illegible when using Tor browser or
  Firefox

**Solution:** Disable the `privacy.resistFingerprinting` setting in
  `about:config`
- **Problem:** The "Display connection info and sleep" workflow step fails when
  trying to display ngrok connection information

**Solution:** The ngrok authtoken supplied either as input to the workflow,
  or as a repository secret is invalid or corrupted
- **Problem:** Connecting to the server using SSH over Tor fails

**Solution:** Make sure Tor browser is running while the command is run. If
  it is still not working, try the command on port `9050` instead of `9150`
- **Problem:** Tor fails to bind to the port when running the workflow

**Solution:** Run the action again. If it continues to fail, [open an
  issue](https://github.com/jstrieb/ctf-collab-template/issues/new)
- **Problem:** The web interface does not work with tor2web sites like
  [tor2web.io](https://tor2web.io) or [onion.sh](https://onion.sh)

**Solution:** Use Tor browser and do not use tor2web sites

# How to Read This Code

- The vast majority of the code is contained in a single, [primary GitHub
  Actions workflow](./.github/workflows/run-server.yml), which sets up the
  environment and displays connection details to the user. This file is fairly
  well-commented, and most design decisions should have included justification.
  For readability, I have
  tried to use long command-line options where possible. There may be more
  information in the commit history, for example [this
  commit](https://github.com/jstrieb/ctf-collab-template/commit/47148f0ecc78c755cd69e5f09d76a4fc94530df9)
  justifies switching from [tmate.io](https://tmate.io) to ngrok and Tor.
- The [`dotfiles`](./dotfiles) directory contains configuration files for many
  programs running in the collaborative environment.
  - [`install.sh`](./dotfiles/install.sh) copies dotfiles to the proper
    locations
  - [`.tmux.conf`](./dotfiles/.tmux.conf) contains custom shortcuts for `tmux`
  - [`.bashrc`](./dotfiles/.bashrc) is pretty minimal, but contains a few
    useful command aliases
  - [`ttyd_run.sh`](./dotfiles/ttyd_run.sh) is run every time a new user
    connects via SSH or the Web interface and is responsible for opening a new
    `tmux` session that uses one common set of windows for all clients

# Acknowledgments

Thanks to my good friend Logan Snow ([@lsnow99](https://github.com/lsnow99))
for testing, fixing bugs, and other contributions!

This project makes use of several great software packages that have made their
services available for free:

- [Tor](https://www.torproject.org/)
- [ngrok](https://ngrok.com)
- [ttyd](https://github.com/tsl0922/ttyd)
- [tmux](https://en.wikipedia.org/wiki/Tmux)
- [GitHub Actions](https://github.com/features/actions)

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