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Home : Advisories : Vulnerabilities in Cisco CHAP Authentication

Title: Vulnerabilities in Cisco CHAP Authentication
Released by: CISCO
Date: 1st October 1997
Printable version: Click here
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(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 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 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 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

      "http://www.cisco.com/public/registration.html". 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 "tac@cisco.com". 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:

      "http://www.cisco.com/upgrade/". Physical media deliveries may take

      7-10 business days to ship. Depending on your contract status, there

      may be a charge for physical deliveries.

(C) 1999-2000 All rights reserved.