Summary

• An error in Cisco IOS software makes it possible for untrusted, unauthenticated users who can gain access to the login prompt of a router or other Cisco IOS device, via any means, to cause that device to crash and reload.

  This applies only to devices running classic Cisco IOS software, including most, but not all, Cisco router products. The easiest way to determine whether your device is running classic Cisco IOS software is to use the show version command as detailed under Who Is Affected below.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19980810-ios-login.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  All users of classic Cisco IOS software versions 9.1 and later, but earlier than the repaired versions listed in the "Details" section of this notice, whose devices can be connected to interactively by untrusted users, are affected by this vulnerability. It is not necessary to be able to actually log in to exploit this vulnerability; simply establishing a terminal connection is sufficient. The vulnerability can be exploited from any interactive prompt issued by the router, including but not limited to, the login prompt.

  Note that some of the repaired software has been in the field for some time; you may already have installed it. Please check your software version number before assuming that you are affected.

  The vulnerability can be exploited using direct console or asynchronous serial connections (including dialup connections), TELNET connections, UNIX "r" command connections, LAT connections, MOP connections, X.29 connections, V.120 connections, and possibly others. Except in extraordinary security environments, administrators are strongly encouraged to assume that hostile users can find ways to make interactive connections to their Cisco IOS devices.

  It is impossible to list all Cisco products in this notice. If you are unsure whether your device is running classic Cisco IOS software, log into the device and issue the command show version. Classic Cisco IOS software will identify itself simply as "IOS" or "Internetwork Operating System Software," and affected software will have a version number greater than or equal to 9.1. Other Cisco devices either will not have the show version command, or will give different output.

  Products Confirmed Not Vulnerable

  If you are not running classic Cisco IOS software, then you are not affected by this vulnerability. Cisco devices which do not run classic Cisco IOS software include the following:

  • 7xx dialup routers (750, 760, and 770 series) are not affected.
    
  • Catalyst LAN switches (except for the Catalyst 2900XL) are not affected.
    
  • WAN switching products in the IGX or BPX lines are not affected.
    
  • The AXIS shelf is not affected.
    
  • The LS1010 or LS2020 ATM switches are not affected. Earlier versions of this notice said that some LS1010 switches were affected. This was an error; the 11.2WAx and 11.3WAx versions of Cisco IOS software used in LS1010 switches are based on repaired variants.
    
  • Any host-based software is not vulnerable.
    
  • The Cisco PIX Firewall is not vulnerable.
    
  • The Cisco LocalDirector is not vulnerable.
    
  • The Cisco Cache Engine is not vulnerable.
    

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• The Cisco IOS software error has been assigned Cisco bug ID CSCdj43337.

  Note: If you are a registered CCO user and you have logged in, you can view bug details.

Workarounds

• It is possible to work around this problem by preventing interactive access to the Cisco IOS device. If only IP-based interactive access is of concern, this can be done by using the ip access-class line configuration to apply an access list to all virtual terminals in the system. However, it is important to remember that non-IP-based means of making interactive connections to Cisco IOS devices do exist, and to eliminate those means as possible routes of attack. Interactive access can be prevented completely by applying the configuration command no exec to any asynchronous line, or the command transport input none to any virtual terminal line, that may be accessible to untrusted users.

Fixed Software

• This vulnerability affects all releases of Classic Cisco IOS software from 9.1 up to, but not including, the following corrected releases (including interim and beta software):

  • 11.3(1), 11.3(1)ED, 11.3(1)T
    
  • 11.2(10), 11.2(9)P, 11.2(9)XA, 11.2(10)BC, 11.2(8)SA3
    
  • 11.1(15)CA, 11.1(16), 11.1(16)IA, 11.1(16)AA, 11.1(17)CC, 11.1(17)CT
    
  • 11.0(20.3)
    

  It is not necessary to run the specific releases listed above; the fix is present in all subsequent versions of the same releases as well. For example, 11.2(9)P is fixed, so 11.2(10)P is also fixed.

  Releases of Cisco IOS software up to and including 10.3 have reached end of support, and no fixes are currently or planned to be available for those releases. All releases after 9.1 do, however, contain the problem.

  All planned fixes to Cisco IOS software have been completed and tested. Integration into regular released software is complete for all versions except 11.0. If you are running a version of software earlier than the ones listed above, please contact the Cisco TAC for assistance.

  As of the date of this notice, the fix for this problem is available for the 11.0 release only in the 11.0(20.3) version. This is an interim release, and has not been subjected to the same degree of testing as a regular Cisco IOS release. The first regular 11.0 release containing the fix will be 11.0(21). Release of 11.0(21) is tentatively scheduled for mid-September, 1998; this schedule is subject to change. Because of the relative maturity of the 11.0 Cisco IOS software, Cisco believes that installation of 11.0(20.3) carries less risk than would installation of an interim release for a newer Cisco IOS version, but customers are advised to use caution in installing 11.0(20.3), or any other interim release, in any critical device.

Exploitation and Public Announcements

• Cisco has had no actual reports of malicious exploitation of this vulnerability. However, there have been sporadic reports of unexplained crashes that have been consistent with the crashes caused by this vulnerability; the vulnerability was initially identified because of such a report. It is possible that the reported crashes could have been caused by random events, but it is also possible that they could have been deliberate. Cisco has essentially no information that would be useful in determining which is the case. None of the customers reporting the crashes indicated any suspicion of a deliberate attack.

  Cisco knows of no public announcements of this vulnerability before the date of this notice.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19980810-ios-login

Revision History

• Revision 1.4	1998-August-20	Error with respect to impact on LS1010 switches corrected. 11.2(8)SA3 added to repaired version list.
  Revision 1.3	1998-August-19	Various common misunderstandings corrected. More information about which Cisco IOS software versions are vulnerable and about which products run classic Cisco IOS software.
  Revision 1.2	1998-August-10	Initial public release.

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