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[Cisco] Schwachstelle in Catalyst 5000 Switches
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Liebe Kolleginnen und Kollegen,
soeben erreichte uns nachfolgende Warnung von Ciscos Product Security
Incident Response Team (PSIRT) ueber Sicherheitsprobleme in Cisco
Catalyst 5000 Switches. Wir geben diese Informationen unveraendert an
Das Spanning Tree Protokoll (STP) verhindert Zyklen im Routing von
Frames im Falle einer redundanten Netztopologie. In dem Cisco Advisory
wird ein Fehler in Catalyst 5000 Switches beschrieben, der das
fehlerhaften Weiterleiten eines 802.1x Frames durch einen vom STP
blockierten Port betrifft. Ein Angreifer kann diesen Fehler fuer
einen Denial of Service (DoS) Angriff nutzen (packet storm).
Es existieren mehrere Workarounds:
1.) Ausschalten des Switches mit den durch das STP blockierten Ports.
2.) Unterbrechen der STP Schleife durch Deaktivieren der blockierten
3.) Durch Konfigurierung der permanenten MAC Adresseintraege fuer die
STP Ports, so dass diese auf nicht verwendete Ports zeigen (Befehle im
Welche Cisco Catalyst 5000 Switches von der Schwachstelle betroffen
sind, entnehmen Sie bitte dem angefuegten Advisory.
Cisco stellt ueberarbeitete Software zur Verfuegung.
Mit freundlichen Gruessen,
Jan Kohlrausch, DFN-CERT
Jan Kohlrausch | mailto:firstname.lastname@example.org
DFN-CERT GmbH | http://www.cert.dfn.de/team/kohlrausch/
Oberstr. 14b. | Phone: +49(40) 808077 555
D-20144 Hamburg | FAX: +49(40) 808077 556
Germany | PGP-Key: finger email@example.com
Cisco Security Advisory: Catalyst 5000 Series 802.1x Vulnerability
For Public Release 2001 April 16 at 1500 UTC
When an 802.1x frame is received by an affected Catalyst 5000 series switch
on a STP blocked port it is forwarded in that VLAN instead of being
dropped. This causes a performance impacting 802.1x frames network storm in
that part of the network, which is made up of the affected Catalyst 5000
series switches. This network storm only subsides when the source of the
802.1x frames is removed or one of the workarounds in the workaround
section is applied. This vulnerability can be exploited to produce a denial
of service (DoS) attack.
This vulnerability is described in Cisco bug id CSCdt62732.
This notice will be posted at
Cisco Catalyst 5000 series switches based on any of the following EARL
(Encoded Address Recognition Logic) hardware revisions:
* EARL 1
* EARL 1+
* EARL 1++
and running any of the following switch software revisions:
* 4.5 (11) or earlier
* 5.5 (6) or earlier
* 6.1 (2) or earlier
are affected by this vulnerability.
This series includes the Catalyst models 5000, 5002, 5500, 5505, 5509,
2901, 2902 and 2926 switches.
To determine your hardware and software revision type sh mod on the console
prompt of the switch.
Products Not Affected
Catalyst 5000 series switches based on EARL 2 or later hardware revisions
are not affected by this vulnerability.
Catalyst 5000 series switches regardless of the EARL hardware revision,
running the following switch software revisions
* 4.5 (12) or later - expected general availability before 2001, May 1
* 5.5 (7) or later
* 6.1 (3) or later
are not affected by this vulnerability.
No other Cisco product is currently known to be affected by this
vulnerability. This includes the Catalyst 6000, 4000, 3500XL, 2900XL and
When an 802.1x (IEEE standard for port based network access control) frame
is received by an affected Catalyst 5000 series switch on a STP (Spanning
Tree Protocol) blocked port it is forwarded in that VLAN (Virtual Local
Area Network) instead of being dropped. This causes a performance impacting
802.1x frames network storm in that part of the network, which is made up
of the affected Catalyst 5000 series switches. This network storm only
subsides when the source of the 802.1x frames is removed or one of the
workarounds in the workaround section is applied.
The vulnerability is documented as Cisco bug id CSCdt62732.
When an affected Catalyst 5000 series switch network receives an 802.1x
frame it causes an 802.1x frames network storm. This network storm degrades
the performance of the network. Slower ports on the affected Catalyst 5000
series switches may stop passing user data. The affected Catalyst 5000
series switches may not respond to any management inquiries via SNMP,
Telnet or HTTP. However, management via the console port on the switches is
still possible and can be used to apply the workarounds.
Software Versions and Fixes
This vulnerability has been fixed in the following switch software
* 4.5 (12) or later - expected availability before 2001, May 1
* 5.5 (7) or later
* 6.1 (3) or later
and the fix will be carried forward in all future releases.
Software upgrade can be performed via the console interface.
Obtaining Fixed Software
Cisco is offering free software upgrades to remedy this vulnerability for
all affected customers. Customers with service contracts may upgrade to any
software release. Customers may install only the feature sets they have
Fixed software is currently available except where noted.
Customers with contracts should obtain upgraded software through their
regular update channels. For most customers, this means that upgrades
should be obtained via Cisco's Software Center at http://www.cisco.com/.
Customers without contracts or warranty should get their upgrades by
contacting the Cisco Technical Assistance Center (TAC) as shown below:
* (800) 553-2447 (toll-free in North America)
* +1 408 526 7209 (toll call from anywhere in the world)
* e-mail: firstname.lastname@example.org
See http://www.cisco.com/warp/public/687/Directory.shtml for additional TAC
contact information, including instructions and e-mail addresses for use in
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 "email@example.com" or
"firstname.lastname@example.org" for software upgrades; faster results will be
obtained by contacting the TAC directly.
The following workarounds will prevent the 802.1x frames from causing an
802.1x frames network storm in an affected Catalyst 5000 series switch
These workarounds can also be applied to a network experiencing an 802.1x
frames network storm.
1. Configure permanent MAC address entries for the entire reserved STP
range 01-80-c2-00-00-02 to 01-80-c2-00-00-0f to be directed out an unused
port for each VLAN on each affected switch in the network.
The commands to configure are given below.
set cam permanent 01-80-c2-00-00-02 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-03 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-04 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-05 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-06 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-07 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-08 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-09 <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0a <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0b <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0c <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0d <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0e <mod#>/<port#> <VLAN>
set cam permanent 01-80-c2-00-00-0f <mod#>/<port#> <VLAN>
2. Break the STP loop by either
a) Disabling the redundant (STP blocked ports) or
b) Disconnecting the cable from these ports
Remove all the sources of 802.1x frames before re-enabling the ports or
reconnecting the cables.
3. Power down the Catalyst 5000 switch(es) that create the spanning-tree
loop (any switch with STP blocked ports).
Remove all the sources of 802.1x frames before powering up the switches.
Exploitation and Public Announcements
The Cisco PSIRT is not aware of any malicious use of the vulnerability
described in this advisory.
A customer who discovered this vulnerability while using Microsoft Windows
XP BETA software reported this vulnerability to Cisco. Microsoft Windows XP
attempts 802.1x authentication during its boot-up phase. Following these
configuration steps for Microsoft Windows XP can disable this:
1. Click on the associated Local Area Connection under Network Connections
2. Click on the Authentication Tab at the top right.
3. Uncheck "Network Access Control using IEEE 802.1x"
This issue has been discussed in news articles regarding issues with
Microsoft Windows XP BETA program and the Cisco Catalyst 5000 series
Status of This Notice: FINAL
This is a FINAL notice. 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 does not anticipate issuing updated versions of this
notice unless there is some material change in the facts. Should there be a
significant change in the facts, Cisco may update this notice.
This notice will be posted on Cisco's Worldwide Web site at
In addition to Worldwide Web posting, a text version of this notice will be
clear-signed with the Cisco PSIRT PGP key and will be posted to the
following e-mail and Usenet news recipients:
* email@example.com (including CERT/CC)
* 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 Number 1.0 Initial Public Release
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.
This notice is Copyright 2001 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.
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