ssh2-enum-algos

NSE detection rule by Kris Katterjohn · Same as Nmap--See https://nmap.org/book/man-legal.html

Reports the number of algorithms (for encryption, compression, etc.) that the target SSH2 server offers. If verbosity is set, the offered algorithms are each listed by type. If the "client to server" and "server to client" algorithm lists are identical (order specifies preference) then the list is shown only once under a combined type.

local nmap = require "nmap"
local shortport = require "shortport"
local stdnse = require "stdnse"
local string = require "string"
local stringaux = require "stringaux"
local table = require "table"

local openssl = stdnse.silent_require "openssl"
local ssh2 = stdnse.silent_require "ssh2"

description = [[
Reports the number of algorithms (for encryption, compression, etc.) that
the target SSH2 server offers. If verbosity is set, the offered algorithms
are each listed by type.

If the "client to server" and "server to client" algorithm lists are identical
(order specifies preference) then the list is shown only once under a combined
type.
]]

---
-- @usage
-- nmap --script ssh2-enum-algos target
--
-- @output
-- PORT   STATE SERVICE
-- 22/tcp open  ssh
-- | ssh2-enum-algos:
-- |   kex_algorithms (4)
-- |       diffie-hellman-group-exchange-sha256
-- |       diffie-hellman-group-exchange-sha1
-- |       diffie-hellman-group14-sha1
-- |       diffie-hellman-group1-sha1
-- |   server_host_key_algorithms (2)
-- |       ssh-rsa
-- |       ssh-dss
-- |   encryption_algorithms (13)
-- |       aes128-ctr
-- |       aes192-ctr
-- |       aes256-ctr
-- |       arcfour256
-- |       arcfour128
-- |       aes128-cbc
-- |       3des-cbc
-- |       blowfish-cbc
-- |       cast128-cbc
-- |       aes192-cbc
-- |       aes256-cbc
-- |       arcfour
-- |       rijndael-cbc@lysator.liu.se
-- |   mac_algorithms (6)
-- |       hmac-md5
-- |       hmac-sha1
-- |       hmac-ripemd160
-- |       hmac-ripemd160@openssh.com
-- |       hmac-sha1-96
-- |       hmac-md5-96
-- |   compression_algorithms (2)
-- |       none
-- |_      zlib@openssh.com
--
-- @xmloutput
-- <table key="kex_algorithms">
--   <elem>ecdh-sha2-nistp256</elem>
--   <elem>ecdh-sha2-nistp384</elem>
--   <elem>ecdh-sha2-nistp521</elem>
--   <elem>diffie-hellman-group-exchange-sha256</elem>
--   <elem>diffie-hellman-group-exchange-sha1</elem>
--   <elem>diffie-hellman-group14-sha1</elem>
--   <elem>diffie-hellman-group1-sha1</elem>
-- </table>
-- <table key="server_host_key_algorithms">
--   <elem>ssh-rsa</elem>
--   <elem>ecdsa-sha2-nistp256</elem>
-- </table>
-- <table key="encryption_algorithms">
--   <elem>aes128-ctr</elem>
--   <elem>aes192-ctr</elem>
--   <elem>aes256-ctr</elem>
--   <elem>aes128-cbc</elem>
--   <elem>3des-cbc</elem>
--   <elem>blowfish-cbc</elem>
--   <elem>cast128-cbc</elem>
--   <elem>aes192-cbc</elem>
--   <elem>aes256-cbc</elem>
-- </table>
-- <table key="mac_algorithms">
--   <elem>hmac-sha1</elem>
--   <elem>umac-64@openssh.com</elem>
--   <elem>hmac-ripemd160</elem>
--   <elem>hmac-sha2-256</elem>
--   <elem>hmac-sha2-512</elem>
-- </table>
-- <table key="compression_algorithms">
--   <elem>none</elem>
--   <elem>zlib@openssh.com</elem>
-- </table>

author = "Kris Katterjohn"

license = "Same as Nmap--See https://nmap.org/book/man-legal.html"

categories = {"safe", "discovery"}


portrule = shortport.ssh

-- Build onto lists{} and possibly modify parsed{} based on whether the
-- algorithm name-lists are identical between the server-to-client and
-- client-to-server types.  Note that this simply modifies the passed tables.
local combine_types = function(parsed, lists)
  local doubles = {
    "encryption_algorithms",
    "mac_algorithms",
    "compression_algorithms"
  }

  for _, i in ipairs(doubles) do
    local c2s = i .. "_client_to_server"
    local s2c = i .. "_server_to_client"

    if parsed[c2s] == parsed[s2c] then
      parsed[i] = parsed[c2s]
      parsed[c2s] = nil
      parsed[s2c] = nil
      table.insert(lists, i)
    else
      table.insert(lists, c2s)
      table.insert(lists, s2c)
    end
  end
end

-- Build and return the output table
local output = function(parsed, lists)
  local out = stdnse.output_table()

  for _, l in ipairs(lists) do
    local v = parsed[l]
    local a = v:len() > 0 and stringaux.strsplit(",", v) or {}
    if nmap.verbosity() > 0 then
      setmetatable(a, {
        __tostring = function(t)
          return string.format("(%d)\n      %s", #t, table.concat(t, "\n      "))
        end
      })
    else
      setmetatable(a, {
        __tostring = function(t)
          return string.format("(%d)", #t)
        end
      })
    end
    out[l] = a
  end

  return out
end

action = function(host, port)
  local sock = nmap.new_socket()
  local status = sock:connect(host, port)
  if not status then
    return
  end

  -- send the client banner
  -- NB: The protocol does not prescribe which side sends the banner first
  status = sock:send("SSH-2.0-Nmap_SSH2_Enum_Algos\r\n")
  if not status then
    sock:close()
    return
  end

  -- slurp the server banner
  status = sock:receive_buf("\r?\n", false)
  if not status then
    sock:close()
    return
  end

  local ssh = ssh2.transport

  -- send the client key exchange
  -- NB: The protocol does not prescribe which side sends the kex init first
  status = sock:send(ssh.build(ssh.kex_init()))
  if not status then
    sock:close()
    return
  end

  local response
  status, response = ssh.receive_packet(sock)
  sock:close()
  if not status then
    return
  end

  local parsed = ssh.parse_kex_init(ssh.payload(response))

  local lists = {
    "kex_algorithms",
    "server_host_key_algorithms"
    -- Other types will be added below in combine_types()
  }

  -- Modifies tables
  combine_types(parsed, lists)

  return output(parsed, lists)
end

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

106a0b78-e1fc-4d81-8573-7b12a78c5a92

Propose edits, view related rules, and engage with the community through comments.

Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

Community Discussion