Vulnerability

    Citrix ICA Basic Encryption

Affected

    Citrix

Description

    Dug  Song  found  following.    The  ICA  (Independent   Computing
    Architecture) protocol used in various Citrix products  (Winframe,
    Metaframe)  relies  on  a  trivially  cracked encryption scheme to
    protect user authentication.   The ICA basic encryption  algorithm
    is a  variant of  the simple  XOR scheme  used for  saved Winframe
    passwords:

        void decrypt(u_char key, u_char *p, int len)
        {
          int i;

          for (i = len; i > 0; i--)
            p[i] = p[i-1] ^ p[i] ^ key;

          p[0] ^= (key | 'C');
        }

    Demonstration code to decrypt Winframe passwords stored in
    appsrv.ini:

        http://www.monkey.org/~dugsong/icadecrypt.c.txt

    Demonstration   code   to   sniff   (and   decrypt)   ICA  network
    authentication:

        http://www.monkey.org/~dugsong/dsniff/

Solution

    Citrix offers  a secure  alternative called  SecureICA, which uses
    Diffie-Hellman for key exchange and RC5 to encrypt the  underlying
    transport  (now  at  128-bit  strength  worldwide).  While this is
    certainly better  than the  simple XOR  scheme outlined  above, it
    may still  be vulnerable  to an  active man-in-the-middle  attack.
    Caveat user.

    SecureICA is only  available for Windows  and DOS clients.   Unix,
    Macintosh, and Java  clients must use  the insecure protocol.  Due
    to the nature of the protocol it cannot be tunnelled through  ssh.
    A VPN is probably the  only solution for Unix, Macintosh  and Java
    clients.  However, the ICA  session can be tunnelled through  ssh.
    You need to forward port 1494 to the ICA server. However, the  ICA
    browser service  uses UDP  port 1604.  You can  get around this by
    using NAT techniques and  the ALTADDR command provided  by Citrix.
    From  a  security  point-of-view,  you  probably shouldn't NAT the
    browser service -  simply use the  ssh port forwarding  to connect
    to a known  server and known  application. The downside  is you're
    not able to use  seamless windows, as you  cannot get the list  of
    published applications from the ICA browser service.