OSEC

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From: Richard van den Berg (richardtrust-factory.com)
Date: Wed Jul 03 2002 - 10:38:42 CDT

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    Trust Factory Security Advisory TF20020601

    Discovery Date: June 2, 2002
    Release Date: July 3, 2002
    ID: TF20020601
    Title: SunPCi II VNC weak authentication scheme vulnerability
    Impact: Remote attackers can gain access to the system
    Affected Technology: Solaris 2.6, 7, 8 Sparc PCI Platfroms using SunPCi 2.3
    Sun package: SUNWspvnc version 1.0
    Vendor Status: Vendor notified on June 2, 2002
                          Assigned Sunsolve Bug ID: 4698566
    Discovered By: Richard van den Berg <richardtrust-factory.com>
    Advisory URL: http://www.trust-factory.com/TF20020601.html

    Description:
    SunPCi II is a PCI daughterboard for Sun Sparc systems capable of running
    Microsoft Windows OS and applications using an Intel Celeron processor.
    Starting with version 2.3 of the SunPCi II drivers, Sun ships a modified
    copy of AT&T's Virtual Network Computing (VNC) client and server. One of
    the modifications is the authorization process between VNC client and VNC
    server. The new authentication scheme enables an attacker to discover the
    VNC password (which is a valid Solaris password) just by sniffing the
    network between VNC client and VNC server. Once the password is discovered,
    the attacker can gain access to the system using VNC or other protocols.
    By default the VNC server is running an X desktop as root.

    Technical details:
    The readme of the supplied source code of the altered VNC software
    mentions:

    - --------------------------Start Quote--------------------------------
    The original authorization code worked as follows:
        Server-> password was read/decrypted from file
        Server-> sent random bytes to client
        Client-> get password from user
        Client-> reads random bytes from server
        Client-> encrypt random bytes with password
        Client-> write encrypted random bytes to server
        Server-> reads encrypted random bytes
        Server-> encrypts original random bytes using password from file
        Server-> compares encrypted random bytes

    The new authorization code works as follows:
        Server-> sent random bytes to client
        Client-> get password from user
        Client-> reads random bytes from server
        Client-> encrypt password with random bytes as key
        Client-> write encrypted password to server
        Server-> reads encrypted password
        Server-> decrypts encrypted password using random bytes as key
        Server-> gets password of current user from system
        Server-> encrypts password using user password as salt
        Server-> compares encrypted passwords
    - ---------------------------End Quote---------------------------------

    Since the encryption used by VNC is the well known DES, it is easy to see
    how this change of code weakens the security significantly. In the original
    scheme it is difficult to reverse the encyption since the key is an unknown
    password. (An attacker would need to break into the system first and read it
    from the file mentioned in the first step.) In the new code, the key used
    for encryption is the readily available challange ("random bytes") sent by
    the server.

    Conclusion:
    Although encryption is being used, the way it is applied does not add any
    security to sending the password over the wire in plain text. The original
    VNC method is much more secure.

    Proof of concept:
    This requires merely an implementation of the DES algorithm. See attachment.

    Work arounds (pick at least one):
    a) Do not use the VNC software supplied by the SUNWspvnc package.
    b) Replace the modified VNC software with the original VNC package
    c) Only use the modified VNC software over a secure channel (i.e. ssh)

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    -- 
    Richard van den Berg, CISSP
    

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