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TUCoPS :: Cisco :: ciaci002.txt

Cisco Chap Authentication Vulnerability


                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                    Cisco CHAP Authentication Vulnerability

October 1, 1997 17:00 GMT                                        Number I-002a
PROBLEM:       A serious security vulnerability exists Cisco's in PPP CHAP
               Authentication software.
PLATFORM 1:    Systems running all "classic" Cisco IOS software versions
               starting with release 9.1(1), including most Cisco router
PLATFORM 2:    Cisco 7xx routers running Cisco IOS/700 software.
DAMAGE:        The vulnerability permits attackers with appropriate skills and
               knowledge to gain unauthorized access.
SOLUTION:      Apply updates or workarounds listed below.
VULNERABILITY  Even though Cisco does not know of active exploitation of these
ASSESSMENT:    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 as soon as

[ Start Cisco Advisory ]

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 CHAP
 authentication. Other PPP authentication methods are not affected.

 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 Installation
     -------       -------------------        ----------------
     Cisco IOS
     10.3          10.3(19a)                  10.3(19a)

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

     Cisco IOS     11.1(13), 11.1(13)AA,      11.1(14), 11.1(14)AA,
     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 vulnerability.

 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 document.

 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

 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 paragraphs
 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 authentication
 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 systems.

 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 professionals.

 Even though Cisco does not know of active exploitation of these
 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 at

   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 1-800-553-2447,
      1-408-526-7209, or email "". Additional worldwide contacts
      for Cisco support can be found at

   4. Non Registered Users & CCO Special Access

      Customers who do not have Cisco IOS software service agreements with
      either Cisco or a Cisco partner can obtain software on an as-needed
      basis by contacting their Cisco Partner or Cisco TAC. Please see the
      above information for contacting Cisco's Global TAC. Files posted on
      CCO under special access code by a Cisco TAC representative may be
      obtained by customers at

   5. Physical Delivery

      If you would prefer to obtain software deliveries in hard media
      (Floppies, ROMs, etc.), please contact your Cisco Partner or Cisco's
      Global TAC. Alternately, registered CCO users may use the online
      Upgrade Agent to submit orders directly to Cisco:
      "". Physical media deliveries may take
      7-10 business days to ship. Depending on your contract status, there
      may be a charge for physical deliveries.

[ End Cisco Advisory ]


CIAC wishes to acknowledge the contributions of John Bashinski (Cisco) for the
information contained in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
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