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TUCoPS :: Cisco :: m-050.txt

Data Leak with Cisco Express Forwarding (CIAC M-050)




             __________________________________________________________
                        U.S. Department of Energy
                     Computer Incident Advisory Center
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

                    Data Leak with Cisco Express Forwarding
                       [Cisco Security Advisory Rev 1.1]

February 28, 2002 18:00 GMT                                       Number M-050
______________________________________________________________________________
PROBLEM:       During the expansion to fit IP length, an information leak may 
               occur. 
PLATFORM:      All Cisco IOS releases that are supporting CEF are vulnerable. 
               In order to trigger this vulnerability CEF or dCEF must be 
               enabled on the device. The vulnerable Cisco IOS releases are 
               (this is not an exhaustive list): 
               11.1CC 
               12.0, 12.0S, 12.0T, 12.0ST 
               12.1, 12.1E, 12.1T 
               12.2, 12.2T 
DAMAGE:        By sending malformed packets, and capturing them after they 
               have been processed by CEF, an attacker may find a remnants of 
               a previous packets in them. The remnant data may contain 
               whatever the previous packet has carried. That may be parts of 
               a document, mail or any other content. 
SOLUTION:      Upgrade or use the workaround suggested in Cisco advisory. 
______________________________________________________________________________
VULNERABILITY  The risk is LOW. The Cisco PSIRT is not aware of any public 
ASSESSMENT:    announcements or malicious use of the vulnerabilities described 
               in this advisory. 
______________________________________________________________________________
LINKS: 
 CIAC BULLETIN:      http://www.ciac.org/ciac/bulletins/m-050.shtml 
 ORIGINAL BULLETIN:  http://www.cisco.com/warp/public/707/IOS-CEF-pub.shtml 
______________________________________________________________________________

[***** Start Cisco Security Advisory Rev 1.1 *****]

Cisco Security Advisory: Data Leak with Cisco Express Forwarding Enabled
Revision 1.1

For Public Release 2002 February 27 08:00 (UTC -0800)
Last Updated 2002 February 28 10:00 AM US/Eastern (UTC +0700)

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Please provide your feedback on this document.

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Summary

All Cisco devices running Cisco IOSŪ and having Cisco Express Forwarding 
(CEF) enabled can leak information from previous packets that have been 
handled by the device. This can happen if the packet length described 
in the IP header is bigger than the physical packet size. Packets like 
these will be expanded to fit the IP length and, during that expansion, 
an information leak may occur. Please note that an attacker can only 
collect parts of some packets but not the whole session. 

No other Cisco product is vulnerable. Devices that have fast switching 
enabled are not affected by this vulnerability. Cisco 12000 Series 
Internet Routers are not affected by this vulnerability. 

The workaround for this vulnerability is to disable CEF. 

This advisory is available at the 
http://www.cisco.com/warp/public/707/IOS-CEF-pub.shtml.

Affected Products
All Cisco IOS releases that are supporting CEF are vulnerable. In order 
to trigger this vulnerability CEF or dCEF must be enabled on the device. 
The vulnerable Cisco IOS releases are (this is not an exhaustive list): 

11.1CC

12.0, 12.0S, 12.0T, 12.0ST

12.1, 12.1E, 12.1T

12.2, 12.2T
 
No other Cisco products are affected. Specifically, Cisco 12000 Series 
Internet Routers are not affected by this vulnerability.

Details

When a router receives a packet where MAC level packet length is shorter 
than is indicated by the IP level, the router will "extend" the packet 
to the size indicated by the IP level. This extension will be done by 
padding the packet with an arbitrary data. The issue here is that padding 
may contain data from a previous packets that has not been erased. 

Although it is possible to trigger this vulnerability on command, it is not 
possible to predict what information would be collected this way. It is not 
possible for an attacker to selectively capture desired packets (for example, 
packets with username and password combination). 

This vulnerability is specific to CEF. Fast switching is not affected by it.

This vulnerability is documented as Cisco Bug ID CSCdu20643. For the 
Cisco IOS 11.1CC image, this vulnerability is described as Cisco Bug 
ID CSCdp58360. 

Impact

By sending malformed packets, and capturing them after they have been 
processed by CEF, an attacker may find a remnants of a previous packets 
in them. The remnant data may contain whatever the previous packet has 
carried. That may be parts of a document, mail or any other content. 

Note that in an interactive session such as typing a password, characters 
are sent one by one in separate packets. That drastically lowers the 
probability that all packets will be captured. In addition, it is almost 
certain that typed characters will be overwritten by the contents of the 
attacking packets.

Software Versions and Fixes

Each row of the table describes a release train and the platforms or products 
for which it is intended. If a given release train is vulnerable, then the 
earliest possible releases that contain the fix and the anticipated date of 
availability for each are listed in the "Rebuild", "Interim", and "Maintenance" 
columns. A device running any release in the given train that is earlier than 
the release in a specific column (less than the earliest fixed release) is known 
to be vulnerable, and it should be upgraded at least to the indicated release 
or a later version (greater than the earliest fixed release label). 

When selecting a release, keep in mind the following definitions: 

Maintenance 

Most heavily tested and highly recommended release of any label in a given 
row of the table.

Rebuild 

Constructed from the previous maintenance or major release in the same 
train, it contains the fix for a specific defect. Although it receives 
less testing, it contains only the minimal changes necessary to effect 
the repair.

Interim 

Built at regular intervals between maintenance releases and receives less 
testing. Interims should be selected only if there is no other suitable 
release that addresses the vulnerability, and interim images should be 
upgraded to the next available maintenance release as soon as possible. 
Interim releases are not available via manufacturing, and usually they are 
not available for customer download from CCO without prior arrangement with 
the Cisco Technical Assistance Center (TAC).

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 TAC for assistance 
as shown in the following section. 

More information on Cisco IOS software release names and abbreviations is 
available at http://www.cisco.com/warp/public/620/1.html. 

Train Description of Image or Platform Availability of Fixed Releases* 
11.1-based Releases Rebuild Interim** Maintenance 
11.1CC ED release for 7000 series 11.1(36)CC3     
12.0-based Releases Rebuild Interim** Maintenance 
12.0 GD release for all platforms   12.0(20.4)   
12.0S ED release for all platforms 12.0(17)ST4 12.0(18.3)S 12.0(19)S 
12.0ST ED release for all platforms   12.0(18.3)ST 12.0(19)ST 
12.0T ED release for all platforms To be decided 
12.0W5 ED release for all platforms   12.0(20.4)W5(24.7)   
12.1-based Releases Rebuild Interim** Maintenance 
12.1 LD release for all platforms   12.1(9.2) 12.1(10) 
12.1E ED release for all platforms 12.1(8.5)E2 12.1(9.5)E 12.1(8a)E 
12.1EC ED release for all platforms 12.1(7.5)EC1 12.1(9.5)EC   
12.1T ED release for all platforms To be decided 
12.1XM ED release for all platforms 12.1(5)XM6     
12.2-based Releases Rebuild Interim** Maintenance 
12.2 LD release for all platforms   12.2(2.5) 12.2(3) 
12.2S LD release for all platforms   12.2(3.3)S   
12.2T ED release for all platforms   12.2(2.4)T 12.2(4)T 


Obtaining Fixed Software
 
Customers with contracts should obtain upgraded software through their 
regular update channels. For most customers, this means that upgrades 
should be obtained through the Software Center on Cisco's Worldwide 
Web site at http://www.cisco.com. 

Customers whose Cisco products are provided or maintained through prior 
or existing agreement with third-party support organizations such as Cisco 
Partners, authorized resellers, or service providers should contact that 
support organization for assistance with the upgrade, which should be 
free of charge. 

Customers who purchase direct from Cisco but who do not hold a Cisco 
service contract and customers who purchase through third party vendors 
but are unsuccessful at obtaining fixed software through their point of 
sale should get their upgrades by contacting the Cisco Technical Assistance 
Center (TAC). TAC contacts are as follows: +1 800 553 2447 (toll-free from 
within North America) +1 408 526 7209 (toll call from anywhere in the world) 
or by email: tac@cisco.com. 

Please have your product serial number available and give the URL of this 
notice as evidence of your entitlement to a free upgrade. Free upgrades for 
non-contract customers must be requested through the TAC. 

Please do not contact either "psirt@cisco.com" or "security-alert@cisco.com" 
for software upgrades. 

Workarounds

The workaround is to disable CEF on a router.

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious use 
of the vulnerabilities described in this advisory. 

Status of This Notice: INTERIM

This is an interim security advisory. Cisco anticipates issuing updated 
versions of this notice at irregular intervals as there are material changes 
in the facts, and will continue to update this notice as necessary. The 
reader is warned that this notice may contain inaccurate or incomplete 
information. Although Cisco cannot guarantee the accuracy of all statements 
in this notice, all of the facts have been checked to the best of our ability. 
Cisco anticipates issuing monthly updates of this notice until it reaches 
FINAL status. 

A standalone copy or paraphrase of the text of this security advisory that 
omits the distribution URL in the following section is an uncontrolled copy, 
and may lack important information or contain factual errors.

Distribution

This notice will be posted on Cisco's Worldwide Web site at 
http://www.cisco.com/warp/public/707/IOS-CEF-pub.shtml. In addition to 
Worldwide Web posting, a text version of this notice is clear-signed with 
the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet 
news recipients: 

cust-security-announce@cisco.com 
bugtraq@securityfocus.com 
first-teams@first.org (includes CERT/CC) 
cisco@spot.colorado.edu 
comp.dcom.sys.cisco 
firewalls@lists.gnac.com 
Various internal Cisco mailing lists 
Future updates of this notice, if any, will be placed on Cisco's Worldwide 
Web server, but may or may not be actively announced on mailing lists or 
newsgroups. Users concerned about this problem are encouraged to check the 
URL given above for any updates.

Revision History
Revision 1.0 2002-February-27 08:00 UTC-0800  Initial public release 
Revision 1.1 2002-February-28 10:00 UTC +0700  
     Added Cisco 12000 Series Internet Routers to products not affected 

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco 
products, obtaining assistance with security incidents, and registering 
to receive security information from Cisco, is available on Cisco's 
Worldwide Web site at 
http://www.cisco.com/warp/public/707/sec_incident_response.shtml. 
This includes instructions for press inquiries regarding Cisco security 
notices. All Cisco Security Advisories are available at 
http://www.cisco.com/go/psirt. 

--------------------------------------------------------------------------------

This notice is Copyright 2002 by Cisco Systems, Inc. This notice may be 
redistributed freely after the release date given at the top of the text, 
provided that redistributed copies are complete and unmodified, and 
include all date and version information.

[***** End Cisco Security Advisory Rev 1.1 *****]

_______________________________________________________________________________

CIAC wishes to acknowledge the contributions of Cisco Systems, Inc. for the 
information contained in this bulletin.
_______________________________________________________________________________


CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
    Voice:    +1 925-422-8193 (7x24)
    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604
    E-mail:   ciac@ciac.org

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:      http://www.ciac.org/
   Anonymous FTP:       ftp.ciac.org

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.org/.

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation or favoring by the United States Government or the
University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.

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