Cisco Security Advisory
Cisco CHAP Authentication Vulnerabilities
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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 advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19971001-chap.
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Vulnerable Products
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 dial-in services that use, 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.
Products Confirmed 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.
No other Cisco products are currently known to be affected by these vulnerabilities.
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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.
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This section describes workarounds for these vulnerabilities.
Classic Cisco IOS
Cisco knows of no generally usable workarounds for the classic Cisco 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.
IOS/700
The IOS/700 vulnerability may be avoided by making any of the following configuration changes:
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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.
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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.
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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 Cisco IOS routers, because classic Cisco 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).
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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.
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When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco Technical Assistance Center ("TAC") or your contracted maintenance provider for assistance.
This problem has been corrected in the following classic Cisco IOS software releases:
Major Release
First Repaired Maintenance Release
Recommended Maintenance Releases 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
11.1(13), 11.1(13)AA, 11.1(13)CA, 11.1(13)IA
11.1(14), 11.1(14)AA, 11.1(14)CA, 11.1(14)IA
Cisco IOS 11.2
11.2(8), 11.2(8)P, 11.2(4)F1 (replaces 11.2(4)F)
11.2(8), 11.2(8)P, 11.2(4)F1. 11.2(9) not recommended for CHAP users.
Cisco Systems strongly recommends that all customers using classic Cisco 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 Cisco 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).
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 Cisco 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.
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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.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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