At 12:38 PM 7/1/96 -0400, Russell L. Jones wrote:
>What are the known bugs which leave Cisco routers running the 10.X version
of the management software vulnerable to IP based attacks?
The only one that I'm aware of is the fragmentation problem when
the ACK bit is set with 'established' parameter [below].
Cisco Security Advisory
Thu Jun 1 16:27:08 PDT 1995
The following describes a vulnerability in Cisco's IOS software
when the 'established' keyword is used in extended IP access control lists.
This bug can, under very specific circumstances and only with certain IP host
implementations, allow unauthorized packets to circumvent a filtering router.
This vulnerability is present in the following IOS software versions:
10.3(1) through 10.3(2)
10.2(1) through 10.2(5)
10.0(1) through 10.0(9)
and all previous versions of Cisco software.
If you are running any of these IOS versions on a product that uses IP
extended access lists, and you are using the 'established' keyword in these
lists, then Cisco strongly recommends that you take immediate action to
remove the vulnerability. You can determine what version of IOS you
are running by issuing the following command:
The recommended action is to upgrade to a more recent version of IOS,
or take one of the immediate workaround actions described below. The
vulnerability is fixed by in the following official software releases:
10.0(10) or later
10.2(6) or later
10.3(3) or later
(For reference, the Cisco update identifier for this fix is "CSCdi34061".)
Customers may obtain software upgrades without going through the Cisco's
Technical Assistance Center via Cisco's Customer Information On-Line
service, instructions for downloading are available at the end
of this message.
You may also contact your Cisco distributor or contact Cisco's
Technical Assistance Center (TAC) for more information. TAC can be reached
by phone at 800-553-2447, by E-Mail to tac @
com or via the
World-Wide-Web at http://www.cisco.com. In Europe you can contact TAC by
phone at 32-2-778-42-42 or via E-Mail to euro-tac @
A bug in Cisco's extended IP access list implementation can, under
very specific circumstances, allow a user to bypass IP packet filtering.
This may permit unintended IP traffic to pass through your firewall
To determine if you are vulnerable, look through your configuration.
The configuration can be displayed by enabling and then entering the
command "write term".
If you see an access list line using a list number in the range of 100
through 199 that permits or denies TCP traffic and contains the word
'established' near the end of the line, you may be vulnerable.
An example line might look like:
In IOS 10.3:
access-list 100 permit tcp any any established
In IOS 10.2 or earlier:
access-list 100 permit tcp 0.0.0.0 255.255.255.255 0.0.0.0
If you do not meet this test, then you are not vulnerable. You
do not need to do anything.
The following actions will remove the vulnerability:
- Rewrite the access list parameters so the 'established' keyword is
not necessary. This does not simply mean that you may remove the
'established' keyword, but rather that you will need to re-design
your access lists to provide similar functionality without using
the established mechanism.
- Disable the interfaces to which the access list is applied
using the 'shutdown' interface subcommand:
router(config)#interface ethernet 0
Obtain and install the appropriate release of IOS software as
described above. For assistance contact Cisco's TAC.
D) Technical Comments
This problem is caused by an obscure but common design flaw, that
we believe, exists in many router/firewall vendor's packet filtering
Owners of non-Cisco hardware who use IP packet filtering features similar
to Cisco's "extended access lists" as part of a firewall system may wish
to contact their vendor to confirm that this vulnerability does not exist
in their system. (Technical discussions about the problem have already
occured in the appropriate forum.)
This vulnerability can only be exploited with certain IP host
implementations (we do not have information on which implementations
are susceptible). Cisco suggests that all routers configured to
filter IP packets based upon the 'established' mechanism be upgraded.
Paul Ferguson || ||
Consulting Engineering || ||
Reston, Virginia USA |||| ||||
tel: +1.703.716.9538 ..:||||||:..:||||||:..
e-mail: pferguso @
com c i s c o S y s t e m s