description = [[
Attempts to enumerate the users on a SMTP server by issuing the VRFY, EXPN or RCPT TO
commands. The goal of this script is to discover all the user accounts in the remote
system.

The script will output the list of user names that were found. The script will stop
querying the SMTP server if authentication is enforced. If an error occurrs while testing
the target host, the error will be printed with the list of any combinations that were
found prior to the error.

The user can specify which methods to use and in which order. The script will ignore
repeated methods. If not specified the script will use the RCPT first, then VRFY and EXPN.
An example of how to specify the methods to use and the order is the following:

<code>smtp-enum-users.methods={EXPN,RCPT,VRFY}</code>
]]

---
-- @usage
-- nmap --script smtp-user-enum.nse [--script-args smtp-enum-users.methods={EXPN,...},...] -p 25,465,587 <host>
--
-- @output
-- Host script results:
-- | smtp-enum-users:
-- |_  RCPT, root
--
-- @args smtp-enum-users.domain Define the domain to be used in the SMTP commands
-- @args smtp-enum-users.methods Define the methods and order to be used by the script (EXPN, VRFY, RCPT)

-- changelog
-- 2010-03-07 Duarte Silva <duarte.silva@myf00.net>
--   * First version ;)
-- 2010-03-14 Duarte Silva <duarte.silva@myf00.net>
--   * Credits to David Fifield and Ron Bowes for the following changes
--   * Changed the way the user defines which method is used
--   + Script now handles 252 and 550 SMTP status codes
--   + Added the method that was used by the script to discover the users if verbosity is
--     enabled
-----------------------------------------------------------------------

author = "Duarte Silva <duarte.silva@myf00.net>"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"discovery","external","intrusive"}

require "shortport"
require "comm"
require "unpwdb"

portrule = shortport.port_or_service({ 25, 465, 587 }, { "smtp", "smtps", "submission" })

ERROR_MESSAGES = {
	["EOF"] = "connection closed",
	["TIMEOUT"] = "connection timeout",
	["ERROR"] = "failed to receive data"
}

STATUS_CODES = {
	ERROR = 1,
	NOTPERMITTED = 2,
	VALID = 3,
	INVALID = 4
}

---Counts the number of occurrences in a table. Helper function from LUA documentation
-- http://lua-users.org/wiki/TableUtils.
--
-- @param from Source table
-- @param what What element to count
-- @return Number of occurrences
function table_count(from, what)
	local result = 0

	for index, item in ipairs(from) do
		if item == what then
			result = result + 1
		end
	end

	return result
end

---Creates a new table from a source without the duplicates. Helper function from LUA
-- documentation http://lua-users.org/wiki/TableUtils.
--
-- @param from Source table
-- @return New table without the duplicates
function table_unique(from)
	local result = {}

	for index, item in ipairs(from) do
		if (table_count(result, item) == 0) then
			result[#result + 1] = item
		end
	end

	return result
end

---Send a command and read the response (this function does exception handling, and if an
-- exception occurs, it will close the socket).
--
-- @param socket Socket used to send the command
-- @param request Command to be sent
-- @return False in case of failure, true and the response in case of success
function do_request(socket, request)
	-- Exception handler.
	local catch = function()
		socket:close()
	end

	local try = nmap.new_try(catch)

	-- Lets send the command.
	try(socket:send(request))

	-- Receive server response.
	local status, response = socket:receive_lines(1)

	if not status then
		-- Close the socket (the call to receive_lines doesn't use try)
		socket:close()

		return false, (ERROR_MESSAGES[response] or "unspecified error")
	end

	return true, response
end

---Send a SMTP quit command before closing the socket.
--
-- @param socket Socket used to send the command
function quit(socket)
	do_request(socket, "QUIT\r\n")
	socket:close()
end

---Get a domain to be used in the SMTP commands that need it. If the user specified one
-- through a script argument this function will return it. Otherwise it will try to find
-- the domain from the typed hostname and from the rDNS name. If it still can't find one
-- it will use the nmap.scanme.org by default.
--
-- @param host Current scanned host
-- @return The hostname to be used
function get_domain(host)
	local result = "nmap.scanme.org"

	-- Use the user provided options.
	if (nmap.registry.args["smtp-enum-users.domain"] ~= nil) then
		result = nmap.registry.args["smtp-enum-users.domain"]
	elseif type(host) == "table" then
		if host.targetname then
			result = host.targetname
		elseif (host.name ~= "" and host.name) then
			result = host.name
		end
	end

	return result
end

---Get the method or methods to be used. If the user didn't specify any methods, the default
-- order is RCPT, VRFY and then EXPN.
--
-- @return A table containing the methods to try
function get_method()
	local result = {}

	if (nmap.registry.args["smtp-enum-users.methods"] ~= nil) then
		local methods = nmap.registry.args["smtp-enum-users.methods"]

		if type(methods) == "table" then
			-- For each method specified.
			for index, method in ipairs(methods) do
				-- Are the elements of the argument valid methods.
				local upper = string.upper(method)
				
				if (upper == "RCPT") or (upper == "EXPN") or (upper == "VRFY") then
					table.insert(result, upper)
				else
					return false, method
				end
			end
		end
	end

	-- The methods weren't specified.
	if #result == 0 then
		result = { "RCPT", "VRFY", "EXPN" }
	else
		result = table_unique(result)
	end

	return true, result
end

---Generic function to perform user discovery.
--
-- @param socket Socket used to send the command
-- @param command Command to be used in the discovery
-- @param username User name to test
-- @param domain Domain to use in the command
-- @return Status and depending on the code, a error message
function do_gnrc(socket, command, username, domain)
	local combinations = {
		string.format("%s", username),
		string.format("%s@%s", username, domain)
	}

	for index, combination in ipairs(combinations) do
		-- Lets try to issue the command.
		local status, response = do_request(socket, string.format("%s %s\r\n", command, combination))

		-- If this command fails to be sent, then something went wrong with the connection.
		if not status then
			return STATUS_CODES.ERROR, string.format("Failed to issue %s %s command (%s)\n", command, combination, response)
		end

		if string.match(response, "^530") then
			-- If the command failed, check if authentication is needed because all the other attempts will fail.
			return STATUS_CODES.AUTHENTICATION
		elseif string.match(response, "^502") or string.match(response, "^252") or string.match(response, "^550") then
			-- The server doesn't implement the command or it is disallowed.
			return STATUS_CODES.NOTPERMITTED
		elseif string.match(response, "^250") then
			-- User accepted.
			if nmap.verbosity() > 1 then
				return STATUS_CODES.VALID, string.format("%s, %s", command, username)
			else
				return STATUS_CODES.VALID, username
			end
		end
	end

	return STATUS_CODES.INVALID
end

---Verify if a username is valid using the EXPN command (wrapper
-- function for do_gnrc).
--
-- @param socket Socket used to send the command
-- @param username User name to test
-- @param domain Domain to use in the command
-- @return Status and depending on the code, a error message
function do_expn(socket, username, domain)
	return do_gnrc(socket, "EXPN", username, domain)
end

---Verify if a username is valid using the VRFY command (wrapper
-- function for do_gnrc).
--
-- @param socket Socket used to send the command
-- @param username User name to test
-- @param domain Domain to use in the command
-- @return Status and depending on the code, a error message
function do_vrfy(socket, username, domain)
	return do_gnrc(socket, "VRFY", username, domain)
end

issued_from = false

---Verify if a username is valid using the RCPT method. It will only issue the MAIL FROM
-- command if the issued_from flag is false. The MAIL FROM command does not need to
-- be issued each time an RCPT TO is used. Otherwise it should also be issued a RSET
-- command, and if there are many RSET commands the server might disconnect.
--
-- @param socket Socket used to send the command
-- @param username User name to test
-- @param domain Domain to use in the command
-- @return Status and depending on the code, a error message
function do_rcpt(socket, username, domain)
	if not issued_from then
		-- Lets try to issue MAIL FROM command.
		status, response = do_request(socket, string.format("MAIL FROM:<usertest@%s>\r\n", domain))

		if not status then
			-- If this command fails to be sent, then something went wrong with the connection.
			return STATUS_CODES.ERROR, string.format("Failed to issue MAIL FROM:<usertest@%s> command (%s)", domain, response)
		elseif string.match(response, "^530") then
			-- If the command failed, check if authentication is needed because all the other attempts will fail.
			return STATUS_CODES.ERROR, "Couldn't perform user enumeration, authentication needed"
		elseif not string.match(response, "^250") then
			-- Only accept 250 code as success.
			return STATUS_CODES.NOTPERMITTED, "Server did not accept the MAIL FROM command"
		end
	end

	status, response = do_request(socket, string.format("RCPT TO:<%s@%s>\r\n", username, domain))

	if not status then
		return STATUS_CODES.ERROR, string.format("Failed to issue RCPT TO:<%s@%s> command (%s)", username, domain, response)
	elseif string.match(response, "^553") then
		-- 553 Relaying Denied
		return STATUS_CODES.NOTPERMITTED
	elseif string.match(response, "^530") then
		-- If the command failed, check if authentication is needed because all the other attempts will fail.
		return STATUS_CODES.AUTHENTICATION
	elseif string.match(response, "^250") then
		issued_from = true
		-- User is valid.
		if nmap.verbosity() > 1 then
			return STATUS_CODES.VALID, string.format("RCPT, %s", username)
		else
			return STATUS_CODES.VALID, username
		end
	end

	issued_from = true

	return STATUS_CODES.INVALID
end

---Script function that does all the work.
--
-- @param host Target host
-- @param port Target port
-- @return The user accounts or a error message.
function go(host, port)
	-- Get the current usernames list from the file.
	local status, nextuser = unpwdb.usernames()

	if not status then
		return false, "Failed to read the user names database"
	end

	local socket = nmap.new_socket()
	socket:set_timeout(5000)

	local options = {
		timeout = 10000,
		recv_before = true
	}
	local domain = get_domain(host)
	local methods

	status, methods = get_method()
	
	if not status then
		return false, string.format("Invalid method found, %s", methods)
	end

	-- Try to connect to server.
	local response

	socket, response = comm.tryssl(host, port, string.format("EHLO %s\r\n", domain), options)

	-- Failed connection attempt.
	if not socket then
		return false, string.format("Couldn't establish connection on port %i", port.number)
	end

	-- Close socket and return if EHLO command failed.
	if not string.match(response, "^250") then
		quit(socket)
		return false, "Failed to issue EHLO command"
	end

	local result = {}

	-- This function is used when something goes wrong with the connection. It makes sure that
	-- if it found users before the error occurred, they will be returned.
	local failure = function(message)
		if #result > 0 then
			table.insert(result, message)

			return true, result
		else
			return false, message
		end
	end

	-- Get the first user to be tested.
	local username = nextuser()

	for index, method in ipairs(methods) do
		while username do
			if method == "RCPT" then
				status, response = do_rcpt(socket, username, domain)
			elseif method == "VRFY" then
				status, response = do_vrfy(socket, username, domain)
			elseif method == "EXPN" then
				status, response = do_expn(socket, username, domain)
			end

			if status == STATUS_CODES.NOTPERMITTED then
				-- Invalid method. Don't test anymore users with the current method.
				break
			elseif status == STATUS_CODES.VALID then
				-- User found, lets save it.
				table.insert(result, response)
			elseif status == STATUS_CODES.ERROR then
				-- An error occurred with the connection.
				return failure(response)
			elseif status == STATUS_CODES.AUTHENTICATION then
				quit(socket)
				return false, "Couldn't perform user enumeration, authentication needed"
			else -- STATUS_CODES.INVALID
				table.insert(result, string.format("Method %s returned a unhandled status code.", method))
				break
			end

			username = nextuser()
		end
		
		if username == nil then
			-- No more users to test, don't test with other methods.
			break
		end
	end

	quit(socket)
	return true, result
end

action = function(host, port)
	local status, result = go(host, port)

	-- The go function returned true, lets check if it didn't found any accounts.
	if status and #result == 0 then
		return stdnse.format_output(true, "Couldn't find any accounts")
	end

	return stdnse.format_output(true, result)
end