Explore Universal Plug and Play

UPnP is a network technology that lets devices on your network set up rules on your router or modem automatically to allow the connections they need. UPnP is a really simple way to make sure you can connect to all kinds of services and is often recommended.

To me Universal Plug and Play (UPnP) alway was a mysterious protocol. When some cool program required an open port, UPnP made it happen. When some fancy program did not work, someone would ask me: “Did you enable UPnP”? Right, I forgot… I had no clue what magic UPnP did for me.

The problem UPnP solves, in this case, is simple. Almost every home network uses Network Address Translation (NAT), so your internal devices cannot be reached unless you map a port on the external IP interface, to a port of a computer on your internal network.

Quote from wikipedia:

Universal Plug and Play (UPnP) is a set of networking protocols that permits networked devices […] to seamlessly discover each other’s presence on the network and establish functional network services for data sharing, communications, and entertainment.

Now that’s pretty awesome! (For the security minded people: “Uh oh, Houston we have a problem…”) Ofcourse it’s nice that you can find your printer automatically and your media player shows your media storage device to pick your movies from.

The experiment

UPnP had its share of flaws. Unintended errors in UPnP software that were exploitable, but for now let’s focus on the really awesome features of UPnP. Let’s pick our initial case and see how we can use UPnP to make one or more internal unreachable ports, reachable for the world.

The UPnP stack consists of 6 layers:

Layer Description
Discovery finding UPnP devices on the network
Description Find what services the devices is offering
Control Ask for an action of that device
Eventing Subscribe to state changes
Presentation The human controllable interface
Adressing Of no DHCP is available, the device uses it’s on IP-range.

In this post we will only cover the first three layers: Discovery, Description and Control, because we only need to open up a port.

Discovery

Let’s discover some devices! UPnP uses HTTP over UDP (HTTPU) and broadcasts UPD packets on port 1900. The discovery protocol is known as Simple Service Discovery Protocol (SSDP).

From the top, we’ll do a M-SEARCH request to the broadcast address 192.168.1.255 on port 1900. We are querying for devices with the InternetGatewayDevice profile:

import socket

# Construct the message
msg = \
    'M-SEARCH * HTTP/1.1\r\n' \
    'HOST:192.168.1.255:1900\r\n' \
    'ST:urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\n' \
    'MX:2\r\n' \
    'MAN:"ssdp:discover"\r\n'

# Set up UDP socket
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
s.settimeout(2)
s.sendto(msg, ('192.168.1.255', 1900) )

# Listen for data until timeout
try:
    while True:
        data, addr = s.recvfrom(8192)
        print addr, data
except socket.timeout:
    pass

Discovery Response

Running the script gives the response below. What we are interested in is the “LOCATION” value:

('192.168.1.1', 1900) HTTP/1.1 200 OK
CACHE-CONTROL: max-age=120
ST: urn:schemas-upnp-org:device:InternetGatewayDevice:1
USN: uuid:A37351C5-8521-4c24-A43E-AC22055E489C::urn:schemas-upnp-org:device:InternetGatewayDevice:1
EXT:
SERVER: Compal Broadband Networks, Inc/Linux/2.6.39.3 UPnP/1.1 MiniUPnPd/1.7
LOCATION: http://192.168.1.1:5000/rootDesc.xml
OPT: "http://schemas.upnp.org/upnp/1/0/"; ns=01
01-NLS: 1
BOOTID.UPNP.ORG: 1
CONFIGID.UPNP.ORG: 1337

Query description

The desciption xml is a bit long. We’ll show the most important part below. From the service information, we are looking for the controlUrl:

  ...
  <service>
    <serviceType>urn:schemas-upnp-org:service:WANIPConnection:1</serviceType>
    <serviceId>urn:upnp-org:serviceId:WANIPConn1</serviceId>
    <controlURL>/ctl/IPConn</controlURL>
    <eventSubURL>/evt/IPConn</eventSubURL>
    <SCPDURL>/WANIPCn.xml</SCPDURL>
  </service>
  ...

Control device

We have the address, the port and the path of the control URL. Now let’s craft a soap message to open external port 8888 and map it to port 8888 on our internal ip 192.269.1.165.

import urllib2
soap_body = """<?xml version="1.0"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"
            s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
  <s:Body>
    <u:AddPortMapping xmlns:u="urn:schemas-upnp-org:service:WANIPConnection:1">
      <NewExternalPort>8888</NewExternalPort>
      <NewProtocol>TCP</NewProtocol>
      <NewInternalPort>8888</NewInternalPort>
      <NewInternalClient>192.168.1.165</NewInternalClient>
      <NewEnabled>1</NewEnabled>
      <NewPortMappingDescription>Added port via upnp</NewPortMappingDescription>
      <NewLeaseDuration>0</NewLeaseDuration>
    </u:AddPortMapping>
  </s:Body>
</s:Envelope>"""

headers = {
    'SOAPAction': 'urn:schemas-upnp-org:service:WANIPConnection:1#AddPortMapping',
    'Host': '192.168.1.1:5000/',
    'Content-Type': 'text/xml',
    'Content-Length': len(soap_body),
}

ctrl_url = "http://192.168.1.1:5000/ctl/IPConn"

request = urllib2.Request(ctrl_url, soap_body, headers)
response = urllib2.urlopen(request)

print response.read()

Now, send this message and show the response:

<?xml version="1.0"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"
            s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
  <s:Body>
     <u:AddPortMappingResponse xmlns:u="urn:schemas-upnp-org:service:WANIPConnection:1"/>
  </s:Body>
</s:Envelope>

Router forwarding result Wheeee, success! As result we can see a new forwarding rule is added in my router. Port 8888 on my external IP address is forwareded to port 8888 on my internal address 192.168.1.165.

Wrap up

With UPnP an attacker has a golden opportunity to gain access to your network. Vulnerable services could expose them self to the world or attackers might be able to open a port on your router. You better disable UPnP on your router.

Of course there are easier ways to forward a port using UPnP. A nice tool is MiniUPnPc which allows you to execute UPnP commands from the command line.

Forwarding a port 8888 of the current machine can be done by executing the following command.

$ upnpc -e 'Added port via upnp' -r 8888 TCP

To list all port redirections.

$ upnpc -l
upnpc : miniupnpc library test client. (c) 2005-2014 Thomas Bernard
Go to http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
for more information.
List of UPNP devices found on the network :
 desc: http://192.168.1.1:5000/rootDesc.xml
 st: urn:schemas-upnp-org:device:InternetGatewayDevice:1

Found valid IGD : http://192.168.1.1:5000/ctl/IPConn
Local LAN ip address : 192.168.1.165
Connection Type : IP_Routed
Status : Connected, uptime=3231463s, LastConnectionError : ERROR_NONE
  Time started : Sat Aug 19 14:33:19 2017
MaxBitRateDown : 10000000 bps (10.0 Mbps)   MaxBitRateUp 1000000 bps (1.0 Mbps)
ExternalIPAddress = **.**.**.**
 i protocol exPort->inAddr:inPort description remoteHost leaseTime
 1 TCP  8888->192.168.1.165:8888  'Added port via upnp' '' 0
GetGenericPortMappingEntry() returned 713 (SpecifiedArrayIndexInvalid)
$ 

Much easier than scripting it all yourself, but now you have learned how UPnP work!

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