Brute force VNC crack

Ingecom - SERRE Jean-Christophe jcs "at"
Wed, 16 Aug 2000 20:33:42 +0000

"Kenneth Foster" <fosterk "at"> wrote:
> brute forcing of passwords will always work.  There are two ways to do stop
> this.
> 1:      Use passwords that don't show up in dictionaries.  This may be more
> difficult to remember, but it makes it less likely to be cracked.  The code,
> as written, uses a dictionary attack.  Not quite what it says in the title
> of the crack.

Yes and no: in "hacker toolz" you'll find a lot of little programs for
generating you a (huge) textfile with the complete list of possible
passwords based on your provided character set and mini/maxi lengths --
then, using that textfile as the "dictionnary" will actually perform the
brute-force attack.

This kind of attack tool often just provide support for an external
textfile, because it allows to reuse the wealth of available
dictionnaries as well as a brute-force generated textfile, while being a
simple quick-n-dirty program...

> 2:      The other way to stop this is to change the encryption key used by your
> company and recompile your server and client tools.  By changing the key no
> password, even the correct one, from a non-company VNCviewer will work. At
> least from my testing.

As a note, since most people don't have VC++6 or time for recompiling
VNC, this can also be easily done, as always, by a mere hex-patch.

>From the WinVNC source file "vncauth.c" we see that VNC's DES key is:
  unsigned char fixedkey[8] = {23,82,107,6,35,78,88,7};
which is, in hex:
  17 52 6B 06 23 4E 58 07

Using any hex-editor, one will find a unique occurence of that hex
pattern in the WINVNC.EXE and VNCVIEWER.EXE binaries, and will be able
to hex-change it for a custom key w/o any recompilation.

JCS - Jean-Christophe SERRE - INGECOM France - +33 (0)
Microsoft: the 51st State of America -- 52nd coming soon!
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