Summary

• Trivial File Transfer Protocol (TFTP) is a protocol which allows for easy transfer of files between network connected devices. A vulnerability has been discovered in the processing of filenames within a TFTP read request on IOS devices and PXM-1 based MGX switches.

  The following products are identified as affected by this vulnerability:

  • MGX 8230, 8250 and PXM-1 based MGX 8850 switches running versions 1.2.10 or below
    
  • Cisco IOS devices running versions 11.1, 11.2, 11.3
    

  Unless explicitly stated otherwise, all other Cisco products are not affected.

  There is no workaround on MGX switches.

  On IOS devices, a simple workaround exists,which is detailed in the Workarounds section below.

  This advisory is available at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020730-ioc-tftp-lfn.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  The following products are affected:

  • MGX 8230, 8250 and PXM-1 based MGX 8850 switches running versions 1.2.10 or below
    
  • Cisco IOS devices running versions 11.1, 11.2, 11.3
    

  Products Confirmed Not Vulnerable

  The following products are not affected:

  • MGX 8830 switches
    
  • MGX 8850 switches that are not PXM-1 based
    
  • Cisco IOS software versions 11.1, 11.2, 11.3 when running on a 68040 based architecture such as a Route Processor.
    Only this specific architecture is not vulnerable to a reload with the above generally affected versions. Other devices such as Route Switch Processors are affected. To verify which type of route processor you have, issue the command show version at the prompt on the router and look for a string similar to:
    

    cisco RP1 (68040) processor (revision A0) with 16384K bytes of memory.

  • IOS devices running IOS software versions 12.0 and above
    

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

Details

• The original report is located at http://online.securityfocus.com/archive/1/284634 . Cisco responded with the following, which is also archived at http://online.securityfocus.com/archive/1/284688 . A researcher report can also be found at http://www.phenoelit.de/stuff/Cisco_tftp.txt.

  By sending a crafted TFTP read request it is possible to trigger a buffer overflow in the TFTP server. On IOS devices this can only happen if no alias for all files being served has been defined.

  This vulnerability can be exploited remotely.

  On MGX switches, a successful exploitation will suspend the tftp service, but the switch will continue working.

  On IOS devices, a successful exploitation may cause a software reset of device.

  This vulnerability has been documented as CSCdy22809 for MGX switches and CSCdy03429 for IOS.

Workarounds

• There is no workaround on MGX switches.

  There are two workarounds on IOS devices to address this issue.

  Disable the TFTP server entirely

  Cisco IOS provides TFTP server functionality to facilitate the transfer of Cisco IOS images when another TFTP server may not be available. If the TFTP server functionality is not currently needed, the following steps may be taken to disable the TFTP server.

  1. While in enable mode on the router, issue the command show running-config and look for lines starting with tftp-server.
     
  2. For each line in the config starting with tftp-server, prepend the word no followed by a space followed by the full text of the matching line in config mode to remove that entry. This step must be repeated for each matching line of the config.
     
  3. Once this task has been completed, verify that there are no lines starting with tftp-server by issuing the command show running-config from the enable prompt.
     
  4. Once verified, save the new configuration so that the server will be disabled upon the next reset of the device.
     

  Provide aliases for TFTP server filenames

  Cisco IOS provides the ability to alias a long filename to a shorter filename. If the tftp-server entries in the configuration have the keyword "alias" in them, the router will not be vulnerable to exploitation of this vulnerability. To implement this workaround, follow the directions above for disabling the TFTP server, and then add any configuration lines back to the config by appending the keyword "alias" followed by a short filename such that the command resembles:

  tftp-server flash rsp-jv-mz.111-24a alias CiscoIOS

  Note that this must be done for every line starting with "tftp-server" in the configuration. The existence of a single line in the configuration beginning with "tftp-server" without an alias defined while running affected versions of software is all that is needed to become subject to this vulnerability.

Fixed Software

• This problem is fixed in the 1.2.11 version of WAN switching software for MGX switches.

  The affected IOS releases, 11.1, 11.2, and 11.3, are all at End of Life, which means they do not have a maintenance version scheduled, and will not be fixed. It is recommended to use the documented workarounds if these versions must be used.

Exploitation and Public Announcements

• This vulnerability was initially reported to the Cisco PSIRT by Phenoelit and was later announced on the BUGTRAQ mailing list on 2002-07-27 (http://online.securityfocus.com/archive/1/284634 ). Cisco responded with the message at http://www.securityfocus.com/archive/1/284688 and this notice.

  The Cisco PSIRT is not aware of any malicious use of the vulnerability described in this advisory.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20020730-ioc-tftp-lfn

Revision History

• Revision 1.3	2003-August-20	Corrected ID number for MGX switches
  Revision 1.2	2003-August-14	Added the information about MGX switches
  Revision 1.1	2002-August-20	Credited vulnerability reporter in Public Announcements
  Revision 1.0	2002-July-30	Initial Public Release

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