VNC challenge response password crack
#1
Hello everyone,

We wanted to crack a VNC challenge response using hashcat but could not find a complete guide.

So we started looking into how the VNC challenge response authentication works and here is what we understood:
- The client initiates a connection with the server.
- The server sends a unique/random 16-bytes challenge to the client.
- The client uses DES to encrypt (one round) the challenge with the input password and sends the response.
- The server receives the response and does the same encryption scheme to compare the results.
- The connection is established if it matches.

For info:
It is also known that DES encryption algorithm can only accept keys of 56 bits, since ASCII uses 7 bits long characters the key can be up to 8 characters long maximum. If it is shorter, it will be padded with zeros. This is making the assumption that the traditional VNC protocol is used with DES (some new VNC client may have modified this..).


The issue is that VNC doesn't use the password given by the user as is but performs a transformation first:
- the bits of each byte of the corresponding ascii value are reversed

Code:
Password : 12345678
Ascii values (HEX) : 31 32 33 34 35 36 37 38
Binary values:      00110001 00110010 00110011 ....
Binary reversed:  10001100 01001100 11001100 ....
Reversed (HEX): 8c 4c cc 2c ac 6c ec 1c

So the actual VNC user password used for encryption is : 8c4ccc2cac6cec1c (12345678 in ASCII)


John The Ripper has implemented this in the version 1.9.0 Jumbo-1

In order to crack VNC passwords with hashcat we implemented this transformation with a small bash script to create a modified charset of the ascii characters.

Code:
toHexVNC(){
  for ((i=0;i<${#1};i++));
  do
    ascii2binrev=`echo "${1:$i:1}" | perl -lpe '$_=unpack"B*",$_' | rev`
    binrev2hex+=`printf "%02x\n" "$((2#$ascii2binrev))"`
  done
  echo $binrev2hex
}

We are aware that this code could be optimized by using other conversion method (c.f. C++, comparison table,..).


We can now crack it with hashcat using:

- attack 3 (mask attack)

- type 14000 (DES)

- hash format : <cipher>:<plaintext> (in VNC: <response>:<challenge> and NOT <challenge>:<response>)
  --> the response and challenge needs to be truncated to 8bytes length (no need to waste resources on the whole 16 bytes and in any case hashcat accepts only 8 bytes of cipher/plaintext).

- reversed charset and option --hex-charset


note : it might be more interesting to generate a custom reversed charset using the function above as the full DES charset of hashcat uses more than the 95 main ascii characters. See "VNC_allascii.charset" below.


Code:
$ ettercap -Tq -r VNC.cap

ettercap 0.7.5.4 copyright 2001-2013 Ettercap Development Team
...
192.168.11.110-5901:$vnc$*a5d62a6cd58f41abe8785a4485811aac*248d3290ce533f028613f092f25834cf
...

$ cat toCrack.txt

248d3290ce533f02:a5d62a6cd58f41ab


$ cat VNC_allascii.charset (all 95 ascii characters transfomed for VNC)
8646c626a666e6169656d636b676f60e8e4ece2eae6eee1e9e5e8242c222a262e2129252d232b272f20a8a4aca2aaa6aea1a9a5a0c8c4ccc2cac6cec1c9c840224a4547ab4d4fabc7edabadebe5cdc3c7c3474fcf43a449414e46406043e



$ hashcat -a 3 -m 14000 toCrack.txt -1 VNC_allascii.charset ?1?1?1?1?1?1?1?1 --hex-charset



#Returns: 8c4ccc2cac6cec1c



The cracked password will be an HEX value and will need to be reversed again to find the password (in ASCII) with the following function:

Code:
toAscii(){
  for ((i=0;i<${#1};i+=2));
  do
    hex2binary=`perl -e 'printf "%08b\n", 0x'"${1:$i:2}"'' | rev`
    ascii2binrev+=`echo $hex2binary | perl -lpe '$_=pack"B*",$_'`
  done
  echo $ascii2binrev
}

Which will give the reversed password: 12345678



Some benchmarks:

Using 2x NVIDIA Quadro P4000 8GB

8 characters long loweralphanumspace -> max. ~2min
8 characters long mixalphanum -> max. ~2.2hours
8 characters long mixalphanumspace -> max. ~2.5hours
8 characters long allascii -> max. ~3days



Hope you'll find this interesting and useful.
Please feel free to give us feedback, thank you.

A&J
Reply
#2
Wow very cool, according to https://en.wikipedia.org/wiki/Virtual_Network_Computing there are different protocols used by different VNC clients. Which protocol / clients use this particular method of authentication?
Reply
#3
(12-18-2019, 04:34 PM)DanielG Wrote: Wow very cool, according to https://en.wikipedia.org/wiki/Virtual_Network_Computing there are different protocols used by different VNC clients. Which protocol/clients use this particular method of authentication?

The protocol it uses is RFB ("Remote FrameBuffer"). Even though it is not the most secure protocol, it is important to note that often VNC is used with another security layer. For example, RealVNC uses TLS which makes it harder for someone who is sniffing the traffic to retrieve the challenge-response and so, to crack the password. Also, as it is written on Wikipedia, VPN or SSH connection could be used to add security.
Reply