__________________________________________________________
U.S. Department of Energy
Computer Incident Advisory Center
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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
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Government or the University of California, and shall not be used for
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