|Archive of CERT general posting, CERT Vendor-Initiated Bulletin VB-97.09 - Cisco CHAP Vuls|
03/10/97, CERT Vendor-Initiated Bulletin VB-97.09 - Cisco CHAP Vuls|
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Subject: CERT Vendor-Initiated Bulletin VB-97.09 - Cisco CHAP Vuls
From: CERT Bulletin <email@example.com>
Date: Thu, 2 Oct 1997 11:43:39 -0400
Organization: CERT(sm) Coordination Center - +1 412-268-7090
-----BEGIN PGP SIGNED MESSAGE----- ============================================================================= 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 information. =======================FORWARDED TEXT STARTS HERE============================ Cisco Systems Field Notice: Vulnerabilities in Cisco CHAP Authentication October 1, 1997, 09:00 AM US/Pacific, Revision 4 ------------------------------------------------------------------------ Summary ------- 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 releases: 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). Impact ------ 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. 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" value. * 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 documentation. 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. Exploitation ------------ 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 CSCdj37314. Upgrades -------- 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 "http://www.cisco.com". 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 "http://www.cisco.com/kobayashi/sw-center/sw-ios.shtml". 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.shtml". 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 "firstname.lastname@example.org". Additional worldwide contacts for Cisco support can be found at "http://www.cisco.com/warp/public/687/Directory/DirTAC.html". 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 "http://www.cisco.com/public/sw-center/spc_req.shtml". 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. ========================FORWARDED TEXT ENDS HERE============================= If you believe that your system has been compromised, contact the CERT Coordination Center or your representative in the Forum of Incident Response and Security Teams (FIRST). See http://www.first.org/team-info/. We strongly urge you to encrypt any sensitive information you send by email. The CERT Coordination Center can support a shared DES key and PGP. Contact the CERT staff for more information. 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The CERT Coordination Center is part of the Software Engineering Institute (SEI). The SEI is sponsored by the U. S. Department of Defense. This file: ftp://ftp.cert.org/pub/cert_bulletins/VB-97.09.cisco -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBNDOmrXVP+x0t4w7BAQG+KwP/S7TH2tyEqNoextsShcxG+1a3A63naBU6 MUjLrFgJjBt8UWbPc/vXSyAf9xrskzR7XCwoDmzmOEXlnmZQMxOh9JKwYx8tqFbi jWPsgAu7NMgBZ5Oqaig4reOCYwmAJrDYB69YAvTZE+l9QlIYyOh9cILJu8FQpzq5 wTRPULwpDPI= =4Xxc -----END PGP SIGNATURE-----
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