[NEWS] Multiple Vulnerabilities in Access Control List Implementation for Cisco 12000 Series Internet Router

From: support@securiteam.com
Date: 11/21/01 

From: support@securiteam.com
To: list@securiteam.com
Subject: [NEWS] Multiple Vulnerabilities in Access Control List Implementation for Cisco 12000 Series Internet Router
Message-Id: <20011121142233.1C18C138BF@mail.der-keiler.de>
Date: Wed, 21 Nov 2001 15:22:33 +0100 (CET)

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  Multiple Vulnerabilities in Access Control List Implementation for Cisco
12000 Series Internet Router
------------------------------------------------------------------------

SUMMARY

Six vulnerabilities involving Access Control List (ACL) has been
discovered in multiple releases of Cisco IOS Software Release for Cisco
12000 Series Internet Routers. Not all vulnerabilities are present in all
IOS releases and only line cards based on the Engine 2 are affected by
them.
No other Cisco product is vulnerable.

The workarounds are described in the Workarounds section.

DETAILS

Affected products:
Only Cisco 12000 Series Internet Routers with line cards based on Engine 2
are affected with these vulnerabilities. Not all line cards of a Cisco
12000 Series are affected by all vulnerabilities. Vulnerabilities are
present in the underlying technology an individual line card is based
upon. That technology is called "Engine". Currently Cisco is shipping line
cards based on the following Engines: 0, 1, 2, 3 and 4.

To determine what Engine your card is based on, you need to log on the
Cisco 12000 router and issue "sh diag" command while in enable mode. The
engine type will be displayed as "L3 Engine: x" where x will be the
corresponding number.

The following example shows the output for an Engine 2 based line card.

c12000#sh diag
SLOT 1 (RP/LC 1 ): 1 Port Packet Over SONET OC-48c/STM-16 Single Mode/SR
SC-SC connector
MAIN: type 41, 800-5271-01 rev A0 dev 0
HW config: 0x04 SW key: 00-00-00
PCA: 73-3295-05 rev A0 ver 5
HW version 1.1 S/N SDK034004AY
MBUS: Embedded Agent
Test hist: 0x00 RMA#: 00-00-00 RMA hist: 0x00
DIAG: Test count: 0x00000000 Test results: 0x00000000 L3 Engine: 2 -
Backbone OC48 (2.5 Gbps)
^^^^^^^^^^^ <- Note the engine type

[further output truncated]

These vulnerabilities are affecting line cards based on Engine 2.

Details:
Six vulnerabilities were found in IOS releases that are supporting Cisco
12000 platforms. Only line cards based on Engine 2 are affected.

CSCdm44976
ACL will not block non-initial fragments of a packet. This Cisco bug ID is
adding a support for "fragment" keyword in the ACL. The White Paper Access
Control Lists and IP Fragments describe how keyword fragment modifies
behavior of ACL.

CSCdu57417
The keyword "fragment" in the compiled ACL (Turbo ACL) is ignored if a
packet is destined to the router itself.

CSCdu03323
The implicit "deny ip any any" rule at the end of an ACL is ignored if an
ACL of exactly 448 entries is applied on an interface as an outgoing ACL.
An ACL with any other number of rules, greater or less than 448, is
unaffected by this vulnerability.

CSCdu35175
A support for "fragment" keyword in an outgoing ACL is added. Previously,
only incoming ACL supported this keyword and outgoing ACL was ignoring it.

CSCdt96370
An outbound Access Control List (ACL) may not block all intended traffic
on a router when an input ACL is configured on some, but not all,
interfaces of a multi port Engine 2 line card. The prerequisite is that,
the traffic in question, was not filtered by an inbound ACL on the ingress
port. An ACL applied at the ingress point will work as expected and block
desired traffic.

CSCdt69741
Packet fragments are not filtered by the ACL despite using "fragment"
keyword. The White Paper Access Control Lists and IP Fragments describe
how keyword fragment modifies behavior of ACL.

Impact:

CSCdm44976
The router will not block all traffic. By sending an offending traffic in
packet fragments, it is possible to circumvent the protection offered by
ACL and cause Denial-of-Service for the protected IP address.

CSCdu57417
It is possible to cause the Denial-of-Service on the router itself if
sufficient amount of traffic is sent to the router. This offending traffic
should be send as packet fragments.

CSCdu03323
If an outgoing ACL contains exactly 448 entries and if explicit rule "deny
ip any any" is not present as the last statement, the ACL will fail to
drop packets. Our tests show that only 50% of packets are dropped. This
may allow some undesired traffic to pass into the protected network thus
violating security policy.

CSCdu35175
Fragmented packets may be allowed into the protected network if the
keyword "fragment" was applied to an outgoing ACL.

CSCdt96370
This vulnerability can cause unwanted traffic to be allowed in and out of
the protected network. The security based on an ACL will be breached
completely.

CSCdt69741
This vulnerability can be exploited to attack systems that are supposed to
be protected by the ACL on the router.

Software versions and fixes:
For a complete listing of all software fixes, please refer to:
 <http://www.cisco.com/warp/public/707/GSR-ACL-pub.shtml#software>
http://www.cisco.com/warp/public/707/GSR-ACL-pub.shtml#software

Obtaining fixed software:
Cisco is offering free software upgrades to eliminate this vulnerability
for all affected customers.
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)
 * e-mail: 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:

CSCddm44976
There is no direct workaround for this vulnerability. If feasible, packet
fragments may be filtered before they reach the GSR.

CSCdu57417
There is no direct workaround for this vulnerability. If feasible, packet
fragments may be filtered before they reach the GSR.

CSCdu03323
The workaround is to either shorten the ACL to be less than 448 lines in
length or to explicitly add rule "deny ip any any" as the last statement.

CSCdu35175
The workaround is to transform an ACL to an incoming ACL instead of the
outgoing one.

CSCdt96370
Apply an ACL on all ports on the ingress line card. If a particular port
is supposed to not block any traffic, then apply the ACL of the form,
access-list xy permit ip any any.

CSCdt69741
There is no direct workaround for this vulnerability. It is possible to
block the fragments on an intermediate router, if such exists, that should
be placed between the affected Cisco 12000 and the final target. The
intermediate router must not be another Cisco 12000 affected by the same
vulnerability.

ADDITIONAL INFORMATION

The information has been provided by <mailto:psirt@cisco.com> Cisco
Systems Product Security Incident Response Team.

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