Bugtraq mailing list archives
Re: VNC authentication weakness
From: Jack Lloyd <lloyd () acm jhu edu>
Date: Wed, 24 Jul 2002 16:43:41 -0400 (EDT)
On Wed, 24 Jul 2002 jepler () unpythonic net wrote:
If your server will give the same challenge repeatedly, and you can sniff somebody else's challenge and response, it appears that you could authenticate without knowing the password simply by connecting within the 1-second window to get the same challenge, and then send the same response as the legitimate client. Another weakness in the challenge is that it uses 'random()%256'. Many implementations of random() have highly predictable low bits. It's not clear that this leads to as easy a compromise as the repeated challenge problem, but it's something that warrants consideration..
While looking at this, I noticed (in 3.3.3r2) that VNC seems to use the password directly as a key to DES (truncating if the size is > 8 and padding with NULL if it's < 8). Since DES ignores the low bit of each byte of the key, this seems to mean that there are many different passwords which will be accepted in place of the "real" password. (Can someone confirm this is actually the case?) Between that and the fact that challenges are based only on time(0) [ie you can easily precompute every possible challenge the VNC server would send out over the next week], it _may be a bad idea to rely on VNC's built in authentication. Regards, Jack
Current thread:
- VNC authentication weakness jepler (Jul 24)
- Re: VNC authentication weakness David Frascone (Jul 24)
- Re: VNC authentication weakness Iván Arce (Jul 24)
- Re: VNC authentication weakness Jack Lloyd (Jul 25)
- Re: VNC authentication weakness Constantin Kaplinsky (Jul 26)
- Re: VNC authentication weakness Andreas Beck (Jul 25)
- Re: VNC authentication weakness David Wagner (Jul 25)
- Re: VNC authentication weakness Mitch Adair (Jul 26)
- Re: VNC authentication weakness Jose Nazario (Jul 26)
- Re: VNC authentication weakness Ariel Waissbein (Jul 27)
- Re: VNC authentication weakness David Wagner (Jul 25)
- Re: VNC authentication weakness David Frascone (Jul 24)
- <Possible follow-ups>
- RE: VNC authentication weakness Andrew van der Stock (Jul 26)
- Re: VNC authentication weakness Kragen Sitaker (Jul 28)
- Re: VNC authentication weakness Theo de Raadt (Jul 29)
- Re: VNC authentication weakness Nate Lawson (Jul 29)
- Re: VNC authentication weakness Theo de Raadt (Jul 29)