Re: VNC authentication weakness

["Ariel Waissbein" <Ariel_Waissbein () corest com>]

There is two things I want to point out. I address the confidentiality of
the packets
travelling over the net, and the design weaknesses of the protocol.

I do not know in deetail the VNC authentication algorithm, but if it uses
DES
as this thread implies, then it is suceptible to brute-force attacks. No
matter
how good the pseudo-random challenge value is.

> VNC uses a DES-encrypted challenge-response system to avoid passing
passwords
> over the wire in plaintext.

A secure channel is a pretty good solution to the replay attack. Or in any
event, to
prevent sniffers form reusing responses to the reused challenges, the
challenge packets
should have an always-changing padding value, e.g.

get            rnd
send         E_K ( timestamp | rnd)

This would keep the challenges repetitions unnoticed by sniffers. Notice
that
this doesn't prevent the brute-force attack (e.g., if an attacker decrypts
challenge
and response he gets the key and will be forever happy).

There are also some minor drawbacks of this authentication procedure, e.g.,
the
packets integrity is never checked, the server is not authenticated. This
problems
do not pose a direct threat on the use of VNC, and in turn can be solved
using
secure channels. Ssh is then a good option.

Ariel Waissbein


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Ariel Waissbein
Senior Researcher - Corelabs

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