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Why would home router have port 1900/tcp (upnp) open? What kind of purpose would that serve for the operator (service provider)?

I searched info about it and found this an article saying:

Security vulnerabilities in UPnP continue to crop up and continue to put millions of home networking devices at risk for compromise.

  • It serves the ISP by reducing the chance that they have to take support calls from their customers trying to get all their devices to communicate with the Internet. – schroeder Sep 18 '16 at 14:29
  • @schroeder which is sad as the proper solution to this is to deploy IPv6. – André Borie Sep 19 '16 at 1:20
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It serves the ISP no purpose at all. It is a convenience feature for home users.

UPnP allows services to automatically open ports on routers to allow traffic. A common example is for a media server.

However, UPnP, as others have said, has no authorisation on it, if you allow UPnP, anything with access to the UPnP port can open other ports at will.

For this reason, routers are meant to ensure that it is only available from the internal network and not from outside. Unfortunately, not all routers have been successful in ensuring this which is where the main vulnerabilities come from. The remainder of the vulnerabilities are if you get a rogue service running on your local network, it has more opportunity to open some ports.

If you don't need it, turn it off.

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UPnP (Universal Plug and Play) is basically a protocol designed to automate network settings manipulation or to discover other devices on the network (if they also use UPnP)

For example, if you had an Xbox360, and you would like to play online, the Xbox would automatically use UPnP to talk with your router, and tell him something like, 'open port X' or 'redirect traffic from external IP microsoft.com to my local IP in the NAT'. This is extremely useful for this kind of things, like video games, but unfortunately it's an insecure protocol.

UPnP has no authentication. Yes, this means that anybody in your network can force your router to perform those tasks described above (with a bad intention like opening your gateway(router) to the external network).

What kind of purpose would that serve for the operator (service provider)?

In theory, UPnP is useless for your ISP, since it is mostly used in the internal network and has no use from the outside.

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