Checks if a user is using a family-shared copy of Garry's Mod from an account which is banned and then kicks them

account-sharing.lua

-- This goes in:
--      garrysmod/garrysmod/lua/autorun/account-sharing.lua

-- Or, if you want to keep your custom code separate from the default GMod server files:
--        garrysmod/garrysmod/addons/accountsharing/lua/autorun/account-sharing.lua
-- You will need to make some of these folders

-- Original post http://facepunch.com/showthread.php?t=1341204&p=43469693&viewfull=1#post43469693
-- Credit to McSimp
local APIKey = "APIKEYGOESHERE" -- See http://steamcommunity.com/dev/apikey

local function HandleSharedPlayer(ply, lenderSteamID)
    local chatMessage = string.format("FamilySharing: %s (%s) has been lent Garry's Mod by %s", ply:Nick(), ply:SteamID(), lenderSteamID)
    print(chatMessage)

    if not (ULib and ULib.bans) then return end

    local playerIsBanned = ULib.bans[lenderSteamID]
    if playerIsBanned then
        local banMessage = string.format("%s (%s) has been lent Garry's Mod by a banned account- %s", ply:Nick(), ply:SteamID(), lenderSteamID)
        for i, ply in ipairs(player.GetHumans()) do
            ply:ChatPrint(banMessage)
        end

        ply:Kick("The account that lent you Garry's Mod is banned on this server")
    end
end

local function CheckFamilySharing(ply)
    http.Fetch(
        string.format("http://api.steampowered.com/IPlayerService/IsPlayingSharedGame/v0001/?key=%s&format=json&steamid=%s&appid_playing=4000",
            APIKey,
            ply:SteamID64()
        ),
        
        function(rawJSONBody, len, headers, status)
            body = util.JSONToTable(rawJSONBody)

            if status ~= 200 or not body or not body.response then
                error(string.format("FamilySharing: Invalid Steam API response (%s) for %s | %s: \nJSON: \"%s\"\n", status, ply:Nick(), ply:SteamID(), rawJSONBody))
            end

            local lender = body.response.lender_steamid or nil
            if lender and lender ~= '' and lender ~= "0" then
                HandleSharedPlayer(ply, util.SteamIDFrom64(lender))
            end
        end,
        
        function(code)
            error(string.format("FamilySharing: Failed API call for %s | %s (Error: %s)\n", ply:Nick(), ply:SteamID(), code))
        end
    )
end

hook.Remove("PlayerAuthed", "CheckFamilySharing")
hook.Add("PlayerAuthed", "CheckFamilySharing", CheckFamilySharing)

I've updated the code to have some better logging and error handling.
Could you update your copy of the code @isaac309 ?

I've updated the code to have some better logging and error handling. Could you update your copy of the code @isaac309 ?

Yes! The new code was a success erased the error. Thought it maybe could've been the firewall off the localhost, but it cleared right up! Thank you!

This no longer works as Valve have removed the API endpoint
https://developer.valvesoftware.com/w/index.php?title=Steam_Web_API&type=revision&diff=238804&oldid=230181

IsPlayingSharedGame

This could be re-written to use the newer, in-game method to retrieve this information
https://wiki.facepunch.com/gmod/Player:OwnerSteamID64