Cisco Security Advisory-
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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19950601-key-packet-bypass.
-
This section provides details on affected products.
Vulnerable 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:
show version
Products Confirmed Not Vulnerable
No other Cisco products are currently known to be affected by these vulnerabilities.
-
This section provides detailed information about these vulnerabilities.
Description
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 "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 0.0.0.0 255.255.255.255 established
If you do not meet this test, then you are not vulnerable. You do not need to do anything.
Workaround
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.
or
-
Disable the interfaces to which the access list is applied using the
'shutdown' interface subcommand.
Example:
router(config)#interface ethernet 0 router(config-if)#shutdown
Solution
Obtain and install the appropriate release of IOS software as described above. For assistance, contact Cisco's TAC.
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 implementations.
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.
-
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".)
-
When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
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 assistance.
Software upgrades may be obtained via any of the following mechanisms:
World Wide Web (WWW)
For registered CCO users please open a URL to:
http://www.cisco.com/tacpage/sw-center/
and select the the version of software to download.
For non-registered users open a URL to:
https://www.cisco.com/cgi-bin/Software/SFA/sfa.cgi
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
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.
Character-based "CCO Classic"
For access, the following connection options are offered:
telnet cco.cisco.com
Dial-up modem
- 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 topic:
Software Updates Special Files
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 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 .
-
To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
-
Show LessRevision 1.0
1995-June-01
Initial public release.
-
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 is an uncontrolled copy, and may lack important information or contain factual errors. The information in this document is intended for end-users of Cisco products.