Securing ChatService to prevent exploiters from executing?

Alright guys, mayor issue here;

For one of my announcement scripts I'm using a ChatService function to see if a user is a certain rank in my group, or if his name is on the whitelist. I'm unsure if my system is protected against exploiters who might abuse it, so I'm asking for any advice if there is any.

My script:

local prefix = "-ann "

game.Players.PlayerAdded:Connect(function(plr)
	plr.Chatted:Connect(function(msg)
		if plr:GetRankInGroup(123456) >= 100 or plr.Name == "RegisteredCode" then
			if string.sub(msg, 1, 5) == prefix then
				local ann = string.sub(msg, 6);
				for i,v in pairs (game.Players:GetPlayers()) do
					local gui = script.messageGui:Clone();
					gui.Parent = v.PlayerGui;
					gui.msg.TextLabel.Text = ann;
				end
					wait(6)
				for i,v in pairs (game.Players:GetPlayers()) do
					v.PlayerGui.messageGui:Destroy();
				end
			end
		end
	end)
end)

Some might see several issues already, perhaps by the way I format my code or using double for loops. Any suggestions are more than welcome.

Cheers!