Add security topic guide

Fixes #13

README.rst

@@ -28,6 +28,7 @@ Development Status
 ==================
 
 This project is currently in **pre-alpha**, and things will break all the time.
+There is no supported upgrade pathway, and security issues most likely exist.
 However, feedback at this time is extremely valuble, so you should still try
 it out!
 

docs/index.rst

@@ -5,6 +5,14 @@ The Littlest JupyterHub
 A simple `JupyterHub <https://github.com/jupyterhub/jupyterhub>`_ distribution for
 a small (0-50) number of users on a single server.
 
+Development Status
+==================
+
+This project is currently in **pre-alpha**, and things will break all the time.
+There is no supported upgrade pathway, and security issues most likely exist.
+However, feedback at this time is extremely valuble, so you should still try
+it out!
+
 Installation
 ============
 
@@ -37,16 +45,18 @@ How-To guides answer the question 'How do I...?' for a lot of topics.
    howto/user-environment
    howto/notebook-interfaces
 
-Guides
+Topic Guides
-======
+============
 
-Guides provide in-depth explanations of specific topics.
+Topic guides provide in-depth explanations of specific topics.
 
 .. toctree::
    :titlesonly:
 
    guides/requirements
    guides/admin
+   topic/security
+
 
 Troubleshooting
 ===============

docs/topic/security.rst

@@ -0,0 +1,67 @@
+=======================
+Security Considerations
+=======================
+
+The Littlest JupyterHub is in pre-alpha state & should not be used in
+security critical situations. We will try to keep things as secure as possible,
+but sometimes trade security for massive gains in convenience. This page contains
+information about the security model of The Littlest JupyterHub.
+
+System user accounts
+====================
+
+Each JupyterHub user gets their own unix user account created when they
+first start their server. This protects users from each other, gives them a
+home directory at a well known location, and allows sharing based on file system
+permissions.
+
+#. The unix user account created for a JupyterHub user named ``<username>`` is
+   ``jupyter-<username>``. This prefix helps prevent clashes with users that
+   already exist - otherwise a user named ``root`` can trivially gain full root
+   access to your server.
+
+#. A home directory is created for the user under ``/home/jupyter-<username>``.
+
+#. No password is set for this unix system user by default. The password used
+   to log in to JupyterHub (if using an authenticator that requires a password)
+   is not related to the unix user's password in any form.
+
+#. All users created by The Littlest JupyterHub are added to the user group
+   ``jupyterhub-users``.
+
+``sudo`` access for admins
+==========================
+
+JupyterHub admin users are added to the user group ``jupyterhub-admins``,
+which is granted complete root access to the whole server with the ``sudo``
+command on the terminal. No password required.
+
+This is a **lot** of power, and they can do pretty much anything they want to
+the server - look at other people's work, modify it, break the server in cool &
+funky ways, etc. This also means if an admin's credentials are compromised (
+easy to guess password, password re-use, etc) the entire JupyterHub is compromised.
+
+Off-boarding users securely
+===========================
+
+When you delete users from the JupyterHub admin console, their unix user accounts
+are **not** removed. This means they might continue to have access to the server
+even after you remove them from JupyterHub. Admins should manually remove the user
+from the server & archive their home directories as needed. If the user removed
+from the server is an admin, extra care must be taken since they could have
+modified the system earlier to continue giving them access.
+
+Per-user ``/tmp``
+=================
+
+``/tmp`` is shared by all users in most computing systems, and this has been
+a consistent source of security issues. The Littlest JupyterHub gives each
+user their own ephemeral ``/tmp`` using the `PrivateTmp <https://www.freedesktop.org/software/systemd/man/systemd.exec.html#PrivateTmp>`_
+feature of systemd.
+
+HTTPS
+=====
+
+The Littlest JupyterHub does not currently support HTTPS. Follow `this issue
+<https://github.com/yuvipanda/the-littlest-jupyterhub/issues/29>`_ for progress
+on HTTPS support.