o [NSE] ssh-hostkey now additionally has a postrule that prints hosts

that have the same hostkey. [Toni Ruottu]

CHANGELOG

@@ -1,5 +1,8 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [NSE] ssh-hostkey now additionally has a postrule that prints hosts
+  that have the same hostkey. [Toni Ruottu]
+
 o [NSE] Added dns-nsec-enum.nse, which quickly enumerates the domains
   of a DNSSEC server that uses NSEC records for nonexistent domains.
   [John Bond, David]

scripts/ssh-hostkey.nse

@@ -2,6 +2,8 @@ description = [[
 Shows SSH hostkeys.
 
 Shows the target SSH server's key fingerprint and (with high enough verbosity level) the public key itself.  It records the discovered host keys in <code>nmap.registry</code> for use by other scripts.  Output can be controlled with the <code>ssh_hostkey</code> script argument.
+
+The script also includes a postrule that check for duplicate hosts using the gathered keys.
 ]]
 
 ---
@@ -36,6 +38,18 @@ Shows the target SSH server's key fingerprint and (with high enough verbosity le
 -- 22/tcp open  ssh
 -- |  ssh-hostkey: 2048 xuvah-degyp-nabus-zegah-hebur-nopig-bubig-difeg-hisym-rumef-cuxex (RSA)
 -- |_ ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAwVuv2gcr0maaKQ69VVIEv2ob4OxnuI64fkeOnCXD1lUx5tTA+vefXUWEMxgMuA7iX4irJHy2zer0NQ3Z3yJvr5scPgTYIaEOp5Uo/eGFG9Agpk5wE8CoF0e47iCAPHqzlmP2V7aNURLMODb3jVZuI07A2ZRrMGrD8d888E2ORVORv1rYeTYCqcMMoVFmX9l3gWEdk4yx3w5sD8v501Iuyd1v19mPfyhrI5E1E1nl/Xjp5N0/xP2GUBrdkDMxKaxqTPMie/f0dXBUPQQN697a5q+5lBRPhKYOtn6yQKCd9s1Q22nxn72Jmi1RzbMyYJ52FosDT755Qmb46GLrDMaZMQ==
+--
+--@output
+-- Post-scan script results:
+-- | ssh-hostkey: Possible duplicate hosts
+-- | Key 1024 60:ac:4d:51:b1:cd:85:09:12:16:92:76:1d:5d:27:6e (DSA) used by:
+-- |   192.168.1.1
+-- |   192.168.1.2
+-- | Key 2048 2c:22:75:60:4b:c3:3b:18:a2:97:2c:96:7e:28:dc:dd (RSA) used by:
+-- |   192.168.1.1
+-- |_  192.168.1.2
+--
+
 author = "Sven Klemm"
 license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
 categories = {"safe","default","discovery"}
@@ -57,6 +71,7 @@ end
 
 portrule = shortport.port_or_service(22, "ssh")
 
+postrule = function() return (nmap.registry.sshhostkey ~= nil) end
 
 --- put hostkey in the nmap registry for usage by other scripts
 --@param host nmap host table
@@ -67,7 +82,10 @@ local add_key_to_registry = function( host, key )
   table.insert( nmap.registry.sshhostkey[host.ip], key )
 end
 
-action = function(host, port)
+--- gather host keys
+--@param host nmap host table
+--@param port nmap port table of the currently probed port
+local function portaction(host, port)
   local output = {}
   local keys = {}
   local _,key
@@ -107,3 +125,61 @@ action = function(host, port)
   end
 end
 
+--- check for the presence of a value in a table
+--@param tab the table to search into
+--@param item the searched value
+--@return a boolean indicating whether the value has been found or not
+local function contains(tab, item)
+  for _, val in pairs(tab) do
+    if val == item then
+      return true
+    end
+  end
+  return false
+end
+
+--- iterate over the list of gathered keys and look for duplicate hosts (sharing the same hostkeys)
+local function postaction()
+  local hostkeys = {}
+  local output = {}
+
+  -- create a reverse mapping key_fingerprint -> host(s)
+  for ip, keys in pairs(nmap.registry.sshhostkey) do
+    for _, key in ipairs(keys) do
+      local fp = ssh1.fingerprint_hex(key.fingerprint, key.algorithm, key.bits)
+      if not hostkeys[fp] then
+        hostkeys[fp] = {}
+      end
+      -- discard duplicate IPs
+      if not contains(hostkeys[fp], ip) then
+        table.insert(hostkeys[fp], ip)
+      end
+    end
+  end
+
+  -- look for hosts using the same hostkey
+  for key, hosts in pairs(hostkeys) do
+    if #hostkeys[key] > 1 then
+      local str = 'Key ' .. key .. ' used by:'
+      for _, host in ipairs(hostkeys[key]) do
+        str = str .. '\n  ' .. host
+      end
+      table.insert(output, str)
+    end
+  end
+
+  if #output > 0 then
+    return 'Possible duplicate hosts\n' .. table.concat(output, '\n')
+  end
+end
+
+local ActionsTable = {
+  -- portrule: retrieve ssh hostkey
+  portrule = portaction,
+  -- postrule: look for duplicate hosts (same hostkey)
+  postrule = postaction
+}
+
+-- execute the action function corresponding to the current rule
+action = function(...) return ActionsTable[SCRIPT_TYPE](...) end
+