Advisory ID: cisco-sa-19950601-key-packet-bypass
For Public Release 1995 June 1 23:37 UTC (GMT)
This document 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 advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19950601-key-packet-bypass
This section provides details on affected products.
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:
No other Cisco products are currently known to be affected by these vulnerabilities.
This section provides detailed information about these
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 setup.
To determine if you are vulnerable, look through your configuration.
The configuration can be displayed by enabling and then entering the command
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.
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 occurred
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.
Cisco has not provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). At the time of original publication, CVSS scoring in Cisco security publications was not yet adopted.
Successful exploitation of the vulnerability may, under very specific
circumstances and only with certain IP host implementations, allow unauthorized
packets to circumvent a filtering router.
When considering software upgrades, also consult
and any subsequent advisories to determine exposure and a complete upgrade
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 Technical
Assistance Center ("TAC") or your contracted maintenance provider for
Software upgrades may be obtained via any of the following mechanisms:
For registered CCO users please open a URL to:
and select the the version of software to download.
For non-registered users open a URL to:
When prompted for a code, please enter the special access code you are
given by your Cisco
Technical Assistance Center support representative for a list of
available files to download.
ftp cco.cisco.com and at the initial (username) prompt, enter the
special access code you are given by your Cisco support representative as a
userid. At the password prompt, enter your e-mail address.
For access, the following connection options are offered:
In Europe +33 1 64 46 40 82
In the US (408) 526 8070
vt100, N81, up to 14.4Kbps
Enter either as a guest or registered user and navigate to the
At the prompt for a code, please enter the special access code you are
given by your Cisco support representative for a list of available files to
download. A list of files will be displayed for you to select and download.
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
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set compatibility
and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
Customers with contracts should obtain software through their regular update channels. For most customers, software patches and bug fixes should be obtained through the Software Center on Cisco's worldwide website at http://www.cisco.com
Customers whose Cisco products are provided or maintained through prior or existing agreements with third-party support organizations, such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific customer situations, such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.
Customers who purchase direct from Cisco but do not hold a Cisco
service contract, and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes 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)
Customers should have the product serial number available and be prepared to provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.
for additional TAC contact information, including localized telephone numbers,
and instructions and e-mail addresses for use in various languages.
The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.
This vulnerability was reported to Cisco by .
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This advisory is posted on Cisco's worldwide website at:
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.
Future updates of this advisory, if any, will be placed on Cisco's
worldwide website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.
Initial public release.