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 obtain fragments of text entered by prior interactive users of the device. This text may contain sensitive information, possibly including passwords. This vulnerability exposes only text entered at prompts issued by the IOS device itself; the contents of data packets forwarded by IOS devices are not exposed, nor are data entered as part of outgoing interactive connections, such as TELNET connections, from the IOS device to other network nodes.

  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.

  Although the conditions under which it can be exploited are similar, this vulnerability is not related to the remote crash vulnerability announced in August, 1998.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19981014-ios-hist.

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. Note that all of the repaired versions are quite recent as of the date of this notice, and that it is unlikely that most Cisco users have installed them. The vulnerability affects the vast majority of systems running Cisco IOS software as of this date.

  The vulnerability can be exploited using direct console or asynchronous serial connections (including dialup connections), TELNET connections, UNIX "r" command connections, local-area transport (LAT) connections, Maintenance Operation Protocol (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 not necessary to be able to actually log in to exploit this vulnerability; simply establishing a terminal connection is sufficient.

  It is impossible to list all Cisco products in this notice; the lists below included only the most commonly used or most asked-about products.

  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". Other Cisco devices either will not have the show version command, or will give different output.

  Cisco devices that run classic Cisco IOS software include:

  • Cisco routers in the AGS/MGS/CGS/AGS+, IGS, RSM, 8xx, 1xxx, 25xx, 26xx, 30xx, 36xx, 40xx, 45xx, 47xx, AS52xx, AS53xx, 70xx, 72xx (including the ubr72xx), 75xx, and 12xxx series
    
  • Most recent versions of the LS1010 ATM switch
    
  • Some versions of the Catalyst 2900XL LAN switch
    
  • The Cisco DistributedDirector
    

  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, and are not affected by this vulnerability, include the following:

  • 7xx dialup routers (750, 760, and 770 series) are not affected.
    
  • Catalyst 19xx, 28xx, 29xx, 3xxx, and 5xxx LAN switches are not affected, except for some versions of the Catalyst 2900XL. However, optional router modules running Cisco IOS software in switch backplanes, such as the RSM module for the Catalyst 5000 and 5500, are affected.
    
  • WAN switching products in the IGX and BPX lines are not affected.
    
  • The MGX (formerly known as the AXIS shelf) is not affected.
    
  • No host-based software is affected.
    
  • The Cisco PIX Firewall is not affected.
    
  • The Cisco LocalDirector is not affected.
    
  • The Cisco Cache Engine is not affected.
    

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

Details

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

  • View CSCdk43920
    

Workarounds

• There are two major workarounds for this vulnerability:

  1. Prevent untrusted users from having interactive access to the Cisco IOS device. If only IP-based interactive access is of concern, access can be restricted by using the ip access-class line configuration command 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. The transport input command is particularly useful in controlling the protocols that can be used to get interactive access. 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.
     
  2. Overwrite any potentially sensitive information before logging out of any interactive session on an IOS device. This can be done by entering repeated spaces at an IOS command prompt until the command interpreter will accept no more input on the line, then pressing the "return" key. Follow this by entering a printing character, such as "q", repeatedly until no more input is accepted, then pressing control-A, followed by control-K, then "return" again. This procedure vastly reduces the probability of information leakage, but has not been verified to completely eliminate the possibility in all affected versions of Cisco IOS software.
     

  Cisco recommends installing upgraded software in preference to using either of these workarounds. The first workaround should be part of normal security configuration in any Cisco IOS device, but cannot usually be used to eliminate all possible risk, since some interactive access must be available for system management. The second workaround is prone to human error, and, although it greatly reduces the probability of an attacker's finding anything sensitive, it does not completely eliminate that possibility.

Fixed Software

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

  Earliest Regular Releases	Earliest Interim Releases
  11.0(22)	11.0(21.2)
  11.1(22), 11.1(22)CA, 11.1(21)CC1, 11.1(22)CE	11.1(22), 11.1(21.2)CA, 11.1(21)CC1, 11.1(21.1)CE
  11.2(16), 11.2(16)P, 11.2(16)BC,11.2(8)SA4	11.2(15.4), 11.2(15.4)P, 11.2(15.4)BC, 11.2(8)SA4
  11.3(6), 11.3(6)T, 11.3(6)AA, 11.3(1)MA6, 11.3(6)NA, 11.3(9)WA4	11.3(5.6), 11.3(5.6)T, 11.3(5.6)AA, 11.3(1)MA54, 11.3(5.6)NA
  12.0(1), 12.0(1)T, 12.0(1)S, other 12.0	Will be integrated in initial 12.0(1)x releases

  It is not necessary to run the specific versions listed above; the fix is present in all subsequent versions of the same releases as well. For example, 11.2(16)P is fixed, so 11.2(17)P will also be fixed.

  The fix is available in all regular releases as of the date of this notice. However, the fix has not yet been released for all "two-letter" early deployment software. Integration is under way for the unreleased "two-letter" versions.

  Some releases of Cisco IOS software have been obsoleted or have reached end of maintenance. The upgrade paths for the users of these releases are as follows:

  Obsolete Release	Upgrade To
  1.x - 8.x, 9.1, 9.14, 9.17, 9.21, 10.1, 10.2, 10.3 (all variants)	11.0 (be especially careful to check hardware compatibility)
  11.0BT	11.1
  11.1AA	11.2(16)P
  11.2(4)XA, 11.2(9)XA	11.2(16)P
  11.3(2)XA	11.3(3)
  11.2F	11.3(6)

Exploitation and Public Announcements

• Cisco knows of no public announcements or discussion of the details of this vulnerability prior to the date of this notice. An inadvertant preannouncement was made to certain Cisco customers during the week of October 5, but the only information given to those customers consisted of the bug ID and the bug headline, which was "Security Problem". In other words, they were told that a security problem existed in a version of Cisco IOS software, but were given absolutely no details. A later message to those same customers informed them that the vulnerability had been found by a trusted customer, that Cisco knew of no exploitation of the vulnerability, and that a formal notice would be forthcoming. Extreme care was taken to avoid giving information that could be used to localize the vulnerability to any particular part of the Cisco IOS software, or other information that might be useful in finding the details.

  Cisco knows of no malicious exploitation of this vulnerability. This vulnerability was found by a Cisco customer during laboratory testing.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19981014-ios-hist

Revision History

• Revision 1.0

  1998-October-14

  Initial public release

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