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RealVNC Authentication Bypass
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SUMMARY
"VNC (Virtual Network Computing) software makes it possible to view and
fully-interact with one computer from any other computer or mobile device
anywhere on the Internet."
Improper security measures allow attackers to bypass RealVNC
authentication.
DETAILS
Vulnerable Systems:
* RealVNC version 4.1.1
As documented in rfbproto.pdf by Tristan Richardson, the RFB (remote frame
buffer) protocol performs an initial handshake which allows clients and
servers to negotiate appropriate authentication measures. There are
several methods of authentication, including the standard DES
Challenge-Response, as well as an option to disable authentication
completely. Due to an incorrect implementation, clients are able to force
the server to disable authentication, and allow login without a password.
Proof of Concept:
1. Server sends its version, "RFB 003.008\n"
2. Client replies with its version, "RFB 003.008\n"
3. Server sends 1 byte which is equal to the number of security types
offered
3a. Server sends an array of bytes which indicate security types offered
4. Client replies with 1 byte, chosen from the array in 3a, to select the
security type
5. The handshake, if requested, is performed, followed by "0000" from the
server
In RealVNC 4.1.1 and possibly prior versions which implement RFB 003.008
(though not RealVNC 4.0), the server does NOT perform a check to determine
if the byte sent by the client in step 4 has actually been offered by the
server in step 3a. In effect, authentication is moved from the server side
to the client side. It is possible to force your client to simply request
"Type 1 - None" as the security type, and gain access to the server
without having to go through the time consuming and cumbersome password
entry field.
Here is a typical packet dump:
Server -> Client: 52 46 42 20 30 30 33 2e 30 30 38 0a <- Server version
Client -> Server: 52 46 42 20 30 30 33 2e 30 30 38 0a <- Client version
Server -> Client: 01 02 <- One field follows... and that field is 02 (DES
Challenge)
Client -> Server: 01 <- Ahh, the lovely 1 byte exploit! Beautiful, isn't
it?
Server -> Client: 00 00 00 00 <-- Authenticated!
Workaround:
Run VNC servers behind firewall, and use SSH tunnels for communication.
ADDITIONAL INFORMATION
The information has been provided by <mailto:iamjamesevans@gmail.com>
James Evans.
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