RE: VNC Security

["Steve Bostedor" <Steveb () tshore com>]

Joshua, Please see my reply to Alexander.  It addresses some of what you said here.  I disagree that VNC should be 
avoided completely, though.  It's not THAT insecure!  I will go out on a limb and say that about 90% of the pop3 users 
in the world use plain text passwords.  Encrypted passwords aren't really that common and most ISP's don't require that 
home users encrypt their passwords.  

Do you use FTP?  Maybe you tripple encrypt your FTP data or just avoid FTP completely just like VNC, but I'll go out on 
a limb again and guess that at least 95% of FTP users in the world send the username and password in plain text and 
unencrypted.  I'll also guess that at least 30% of them use the same username and password for their FTP account as 
they do for numerous other functions.  Maybe even their encrypted Pop3 account. ;)

The reply to Alexander explains my question further.  


-----Original Message-----
From: Joshua Berry [mailto:jberry () PENSON COM]
Sent: Tuesday, April 19, 2005 6:43 PM
To: Andy Bruce - softwareAB; Steve Bostedor
Cc: security-basics () securityfocus com; vnc-list () realvnc com
Subject: RE: VNC Security


To the original poster:

It is my *opinion* that using VNC should be avoided completely.  The
last time that I used VNC it only support a password, and no user name.
This leaves only the password to brute-force, considerably lessening the
time needed to break in.  Also, you are making the assumption that
everyone uses plain text POP, I only use POP over SSL, IMAP over SSL or
HTTPS to access my email.  Also, this is not a good example because POP
user accounts/passwords only give you someone's email, a VNC password
will give you full access to the server/desktop it is running on.

The passwords can be sniffed on your local network or they can be
sniffed on the network that the server/desktop you are connecting to
resides on.  If this is a critical box, then now anyone that can sniff
the network can also gain a login to this box to do whatever they want.

I believe that VNC includes SSL or some other decent means of encryption
now.

To the first follow up poster:
a. Somebody just needs to get the password in that 20 minute
interchange, which is not too hard if they are only sniffing for X
sessions.  They can just dump that to a file and leave it running until
it picks something up.  Also, you can setup something to probe the box
on that port, so the next time VNC is enabled they can login.  I am
curious how you would notice someone sniffing the network?  I only see
this as being possible if the host was running linux/unix and forwarding
their syslogs to you, so that you could see when a NIC entered
promiscuous mode.

Lastly:
I have seen several VNC exploits available over the years, so this is
just a whole new service that you are exposing to risk that you often
don't need to (because if it is Linux you have SSH, and if it is a
windows box you have Terminal Services)


-----Original Message-----
From: Andy Bruce - softwareAB [mailto:andy () softwareab net] 
Sent: Tuesday, April 19, 2005 7:55 AM
To: Steve Bostedor
Cc: security-basics () securityfocus com; vnc-list () realvnc com
Subject: Re: VNC Security

This is a very interesting question to me. In my own case, I do have SSH

setup thru Cygwin (http://www.cygwin.com/) for my local network and I 
use VNC thru that connection when I need to manage my own stuff 
remotely. However, I have to admit that when I use VNC to aid remote 
clients (which happens quite frequently) I don't worry about encryption 
whatsoever.

FWIW, here's my approach:

1. I don't even try to explain setting up an SSH daemon to them. I 
simply have them install the VNC server in user-mode and start it.

2. If I can't explain to them in 5 min or less how to do port 
forwarding, I just have them connect directly to their cable/dsl modem.

3. Get the debugging and/or support done.

4. Have them stop the VNC server. Since it isn't running as a service, 
it won't start up next time and so won't be a security risk.

5. Tell them to turn off port forwarding from the router (if they could 
grok it), or just have them connect their PC back to the router and 
their router back to the cable/dsl modem. In either case, 5900 isn't 
available to the outside world so there's no risk even if they were 
running VNC in service-mode.

I have to agree with Steve that this is, for all practical purposes, a 
non-existent security risk. The only things that could go wrong:

a. "Somebody" is sniffing the packet stream while the VNC passwords are 
being exchanged, and, during that 20 minute interchange, cracks the 
password and logs onto the VNC server. Of course, we would notice this 
problem on both ends!

b. I have never captured the data shared between client and server 
(screen/UI deltas) and so have no idea if these pose a security risk or
not.

c. While the VNC server is running and they are connected to the 
internet (port forwarding has the same problem as direct connect) a port

sniffer detects that 5900 is available and immediately zooms in thru 
some VNC security hole. Wez would know a lot more about this possibility

than me, though!

Am I missing something here?

Steve Bostedor wrote:

> I'd like to know if anyone has any working examples of why an
> unencrypted VNC session over the Internet is seen as such a horrible
> security risk.  I understand that unencrypted ANYTHING over the
Internet
> lends the chance for someone to decode the packets (assuming that they
> capture every one of them) but in reality, what are the real risks here
> and has anyone successfully captured a VNC session from more than 2
> router hops away and actually gotten any meaningful information from
it?
> I've captured a big chunk of a LOCAL session using Ethereal and the
only
> thing that I can see that is usable is the password exchange.  Agreed
> that this could be a problem if someone just happened to be sniffing
> your local LAN segment at that exact moment and happened to capture
your
> encrypted VNC password, he could crack the password and log in himself.
> But how paranoid is it to go through all of the trouble of setting up
> SSH to avoid that when you could just change your VNC password often
and
> make sure that your local LAN is reasonably secure from prying eyes?
>
> How about once it gets out on the Internet?  Packets bounce all over
the
> place on the Internet.  What are the odds that someone out there will
> pick your VNC packets out of all of the millions of packets running
> through the back bone routers without being noticed, capture enough of
> them to possibly replay a session, and actually have the patience or
the
> tools to do so.  I've scoured the web out of this curiosity, looking
for
> a tool to put VNC packets together into something useful for a hacker.
> There's nothing.  Nada.  
>
> So, I guess that what I'm asking is; what all of the fuss is about?
> Your POP3 password likely gets passed unencrypted but we're being asked
> to be paranoid about an encrypted VNC password?  This is all coming
from
> a discussion that I had with someone over the merits of using SSH with
> VNC over the internet for a 10 minute VNC session.
>
> Does anyone have anything that's not hypothetical?  Is there a tool
that
> I'm missing out there that does more than just crack a VNC password?
> Does anyone know of any reported security breaches where VNC was a
> weakness?  
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