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[inet-admins] CERT Advisory: CISCO bug: greater than or equal to 1000, on the AGS/AGS+/CGS/MGS, and on the CS-500, but not on Catalyst

,   ...


CERT* Vendor-Initiated Bulletin VB-97.09
October 2, 1997

Topic:  Vulnerabilities in Cisco CHAP Authentication
Source: Cisco Systems

To aid in the wide distribution of essential security
information, the
CERT Coordination Center is forwarding the following
information from
Cisco Systems.  Cisco urges you to act on this information
as soon as
possible. Cisco contact information is included in the
forwarded text
below; please contact them if you have any questions or
need further

=======================FORWARDED TEXT STARTS

 Cisco Systems Field Notice:
 Vulnerabilities in Cisco CHAP Authentication

 October 1, 1997, 09:00 AM US/Pacific, Revision 4

 A serious security vulnerability (bug ID CSCdi91594)
exists in PPP CHAP
 authentication in all "classic" Cisco IOS software
versions (the software
 used on Cisco non-switch products with product numbers
greater than or equal
 to 1000, on the AGS/AGS+/CGS/MGS, and on the CS-500, but
not on Catalyst
 switches or on 7xx or 9xx routers) starting with the
introduction of CHAP
 support in release 9.1(1). The vulnerability permits
attackers with
 appropriate skills and knowledge to completely circumvent
 authentication. Other PPP authentication methods are not

 A related vulnerability exists in Cisco IOS/700 software
(the software used
 on 7xx routers). A configuration workaround exists for
IOS/700, and a
 complete fix for 76x and 77x routers will be included in
software version
 4.1(2), due to be released by December, 1997. A fix for
75x routers is
 scheduled for the first half of 1998.

 This problem has been corrected in the following classic
Cisco IOS software

     Major         First Repaired             Recommended
Maintenance Releases
     Release       Maintenance Release        For
     -------       -------------------       
     Cisco IOS
     10.3          10.3(19a)                  10.3(19a)

     Cisco IOS
     11.0          11.0(17), 11.0(17)BT       11.0(17),

     Cisco IOS     11.1(13), 11.1(13)AA,      11.1(14),
11.1(14)AA, 11.1(14)CA,
     11.1          11.1(13)CA, 11.1(13)IA     11.1(14)IA

     Cisco IOS     11.2(8), 11.2(8)P,         11.2(8),
11.2(8)P, 11.2(4)F1.
     11.2          11.2(4)F1 (replaces        11.2(9) not
recommended for CHAP
                   11.2(4)F)                  users.

 Cisco Systems strongly recommends that all customers using
classic IOS PPP
 with CHAP authentication upgrade to one of these or to a
newer release, and
 that all users of IOS/700 PPP with CHAP authentication
install the
 configuration workarounds described in this document.

 The 11.2(4)F1 release will be available by Monday, October
6, 1997. Users of
 11.2F releases are encouraged to move to 11.2 or 11.2P
releases if at all
 possible. All the other releases mentioned above are
available immediately
 as of the release of this notice.

 The recommended release numbers listed above are expected
to be the best
 choices for most common situations, but it's very
important that customers
 evaluate their network configurations and other needs
before choosing which
 releases to use.

 Cisco is offering free software upgrades to all classic
IOS PPP users in
 order to address this vulnerability. Upgrade details are
at the end of this
 notice. Free upgrades will be offered to IOS/700 users
upon release of
 IOS/700 version 4.1(2).

 A moderately sophisticated programmer with appropriate
knowledge can set up
 an unauthorized PPP connection to any system that is
running vulnerable
 software, and that depends on CHAP for authentication. To
gain this
 unauthorized access, an attacker must have the following:

    * Knowledge of the details of this vulnerability

    * Access to modifiable code (generally meaning source
code) for a
      PPP/CHAP implementation, and sufficient programming
skill to make
      simple changes to that code. Note that such source
code is widely
      available on the Internet.

    * A modest amount of information about the
configuration of the network
      to be attacked, including such things as usernames
and IP addresses.

 This vulnerability cannot be exploited by an attacker who
is using an
 unmodified, properly functioning PPP/CHAP implementation;
the attacker must
 make modifications to his or her software to exploit this

 Who is Vulnerable
 All systems running "classic" Cisco IOS Software releases
older than those
 listed above, and which rely on CHAP for PPP
authentication, are vulnerable.
 Cisco believes that the greatest practical risk is to
dialin services using,
 for example, ISDN or POTS modems.

 Systems running IOS/700 software are vulnerable to a
related attack if they
 are using CHAP bidirectionally to authenticate both
calling and called

 Systems using PAP for PPP authentication are not
vulnerable. Systems not
 configured for PPP are not vulnerable. If the keywords
"ppp" and "chap" do
 not both appear in your system configuration file, you are
not vulnerable.

 Workarounds - Classic IOS
 Cisco knows of no generally usable workarounds for the
classic IOS
 vulnerability. Affected users who wish to protect
themselves must upgrade
 their software or stop using CHAP authentication.
Alternatives to CHAP
 authentication include PAP authentication and reliance on
"Caller ID"
 information. The security differences between these
methods are complex and
 situation dependent, and are beyond the scope of this

 Workarounds - IOS/700
 The IOS/700 vulnerability may be avoided by making any of
the following
 configuration changes:

    * Prevent the routers in question from receiving any
incoming calls,
      perhaps by changing the ISDN switch configuration, or
by relying on
      caller ID and using the "set callerid" and "set
callidreceive" commands

    * Prevent routers that receive calls from
authenticating themselves to
      the calling systems using CHAP. You can effectively
do this by using
      the "set ppp secret client" command to set the CHAP
secret that would
      be used for such authentication to some randomly
chosen "garbage"

    * Configure the routers such that different CHAP
secrets are used in each
      direction on each link. You can do this using the
"set ppp secret
      client" and "set ppp secret host" commands. Note that
this method
      cannot be used on 7xx routers that need to
communicate with classic IOS
      routers, since classic IOS does not support
asymmetric CHAP secrets.

 Any one of these changes should be sufficient. The changes
may be removed
 after the release and installation of IOS/700 software
version 4.1(2).

 Classic IOS Software Upgrade Notes
 You should upgrade your classic Cisco IOS software to one
of the releases
 mentioned in the first section of this notice, or to a
later release.
 Instructions for obtaining the new software are at the end
of this notice.
 Instructions for installing upgraded software are in the
standard system

 Before installing any Cisco IOS software upgrade, you
should always verify
 that the new software is compatible with your hardware.
It's especially
 important to make sure that you have enough memory to do
the upgrade.
 General assistance and full system documentation are
available via the
 Internet's Worldwide Web at http://www.cisco.com.

 Before installing any upgrade of any description, it's
always wise to make
 sure that the version you're installing has no bugs that
will negatively
 impact your configuration. Please check Cisco's Web site
for more
 information and advice on software upgrades in general.

 The new software has been changed in a number of ways in
order to make it
 more resistant to CHAP-related attacks. Some of those
changes may cause CHAP
 authentication to fail in certain customer networks. Cisco
believes the
 affected configurations to be rare. If you install
upgraded software, and
 legitimate CHAP connections stop working, please see the
 immediately following this one, which we believe describe
the failures that
 are likely to be be seen in real networks. If you still
can't get CHAP
 working after reading the paragraphs below, please call
the Cisco TAC for
 assistance in reconfiguring your software.

 The fix for this vulnerability was released in Cisco IOS
software version
 10.3(19), but an error in the implementation of the fix
caused almost all
 CHAP authentication between 10.3(19) systems to fail. This
error is
 corrected in 10.3(19a). 10.3(19) may be safely used if the
command "no ppp
 chap wait" is configured for each interface on which CHAP
is used. Because
 multiple fixes have been introduced for the potential
attack against which
 the modified behavior guards, using "no ppp chap wait"
will not appreciably
 increase your system's vulnerability.

 If an intermediate device, such as an ISDN switch,
establishes incoming
 calls to two separate systems running the modified IOS
software, and then
 places those two systems in contact with one another, CHAP
 between the two systems may fail. This is because each
system "thinks" that
 it's receiving a call, and neither system "thinks" that it
originated the
 call. If this is a problem in your configuration, use the
command "ppp
 direction dedicated" on the affected interfaces of both

 Cisco is not aware of these vulnerabilities having been
exploited by "system
 crackers", nor of any publicly available exploitation
code. Cisco does not
 believe that the details of the vulnerabilities are widely
understood in the
 cracker community. The theoretical possibility of these
vulnerabilities has,
 however, been discussed fairly openly among PPP security

 Even though Cisco does not know of active exploitation of
 vulnerabilities, Cisco expects that the cracker community
will eventually
 "discover" them, and that the issuance of this notice will
tend to
 accelerate that process. Vulnerable customers should
upgrade or install
 workarounds with all possible speed.

 Details of the Vulnerabilities
 Cisco will not release any further details of these
vulnerabilities at this
 time. Further details will be available to interested
parties after March
 31, 1998.

 The Cisco bug tracking number for the Cisco IOS software
vulnerability is
 CSCdi91594. The bug tracking number for the error in the
10.3(19) fix is

 Cisco customers and service partners can obtain the latest
releases of Cisco
 IOS software from the Software Center within Cisco
Connection Online (CCO),
 Cisco's Internet customer support service. CCO is located

   1. Existing Registered Users for CCO

      Customers and Partners with Cisco IOS software
service agreements who
      are already registered for CCO may proceed directly
to the Cisco IOS
      Software Center to obtain a new software release to
solve this issue.
      The Software Center includes Software Upgrade
Planners to inform you
      about new features, additional caveats, release
notes, and
      compatibility requirements to ensure a successful
upgrade. The Cisco
      IOS Software Center is located at

   2. New Registered Users for CCO

      Customers and Partners who have a software service
contracts directly
      with Cisco or a Cisco Partner, but have not yet
registered for Cisco
      Connection Online, and who know their contract
number, can proceed
      directly to register online at
Online registration
      takes effect immediately, after which customers may
proceed directly to
      the Software Center to obtain an upgrade.

   3. Assistance in Registering on CCO

      Customers and Partners who have a software service
contract, but need
      to confirm their contract number to register for CCO,
or require any
      other assistance registering for CCO access should
contact Cisco's
      Global Technical Assistance Center (TAC) at
      1-408-526-7209, or email "tac@cisco.com". Additional
worldwide contacts
      for Cisco support can be found at
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