Authentication-based permission groups in Sputnik

Sputnik is a novel document-storage-based wiki written in Lua which has been previously featured on LuaNova. Today I’d like to share my experiences adding authentication-based permission groups to my running Sputnik installation at Over the course of the past two months I have had two spam posts on our forums which were fixed by removing those nodes from the document store, but this left me with a question of what to do with the user account that posted the spam message.

First, I should state that we have had very few instances of spam on our site. Any user accounts must be verified by email and require the user to input a captcha from the reCaptcha project. Once this is done, the user is free to make edits to the API documentation and to post new topics and replies on the discussion forums. However, the process of posting spam on the site is very difficult to automate due to the honeypots, field hashing, and post tokens that a default Sputnik installation employs. That’s not to say that the software is bullet-proof, but in practice it seems to work for my needs on a real site.

The first few accounts, I simply removed from my authentication system via a simple mysql query (although removing it from a default installation would be a easier, a simple edit of the sputnik/passwords node.) But then I realised I was giving up information about the spammers, including the email addresses they were using to register, something I could share with some of the spam registration databases that exist. It was clear that I needed an easier way to stop those user accounts from posting on the site without simply nuking their user accounts from the system. Really what I wanted was a way to ‘ban’ a user account and prevent them from making edits on the site.

Permissions in Sputnik

Each node in a Sputnik document store is a Lua script that is compiled and assembled into a Lua object at runtime. Consider the following simple node:

title = "Some Page"
content = [[This is a simple test node for a Sputnik installation.

Hello World!]]

Sputnik nodes can be more than just a symbolic representation of text data. For example, you can restrict the permissions of a node by adding a permissions field. Consider the following definition:

permissions    = [[
  deny(all_users, all_actions)
  allow(all_users, show)  -- show, show_content, cancel
  allow(Authenticated, edit_and_save) -- edit, save, preview
  allow(all_users, "post")  -- needed for login
  allow(all_users, "rss")
  allow(Admin, all_actions)

When this field is loaded, it’s obviously a string that we can then further process. Specifically, we load it in an environment where a few helpers functions are defined. The all_users function returns true for all users, all_actions does the same for all action types. Authenticated is a special function that returns true if the current user is authenticated, while Anonymous can be used when the user is not logged in. There’s also a special function called Admin that queries the authentication metadata for the current user and checks if the is_admin flag is set.

When I started looking for a way to ban users, I started looking in this section of the code and found the following gem that Yuri sneaked in after the definition for the Admin group:

local groups = self.saci.permission_groups -- just a local alias
-- Define group "Admin" as any user that has is_admin set to true
groups.Admin = function(user)
   return user and self.auth:get_metadata(user, "is_admin") == "true"

-- Define any group that starts with "is.<group>" as including all users
-- for whom "is_<group>" is set to true. For example, if is_clown is set
-- to true than user is a member of group "is.clown" and we can set:
--     allow(is.clown, "show")
local groups_mt = {
   __index = function(table, key)
      return function (user)
                return user and self.auth:get_metadata(user, "is_"..key) == "true"
} = setmetatable({}, groups_mt)

Bingo! With this bit of code, without making any changes to the Sputnik core, I had a way to ban users in my system, by setting the is_banned flag in their authentication metadata. For a simple authentication system (the default) this is just a matter of setting the given flag in the user’s authentication entry, whereas in a mysql authentication system I just add a row to the database. This was only part of the puzzle, since Sputnik doesn’t know anything about the new user group. I had to make a few edits to the @Root prototype to add the following at the end:

deny(is.banned, edit_and_save)

Since earlier in the permissions script the edit_and_save permissions are granted to all authenticated users, I needed to make sure the denial runs after that point, since the final ‘permission’ is what really matters. Since I have custom forums in my system, I needed to make the same one-line change in a few other places.

It took a total of about 6 minutes to familiarize myself with the code and make the necessary changes, and now I have an easy way to ban a user from being able to post anywhere on my website.

Down with the spammers!

Jim Whitehead

Jim Whitehead

Published on 19 May 2010