Cisco Security Advisory-
The following describes an error in Cisco's IOS software 10.3 release when the 'tacacs-ds' or 'tacacs' keyword is used in extended IP access control lists.
The solution is to obtain and install the appropriate release of IOS software as described above. For assistance contact Cisco's TAC.
This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19950731-acl-packet-bypass.
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This section provides details on affected products.
Vulnerable Products
This vulnerability is present in the following IOS software versions:
10.3(3.4) through 10.3(4.2)
If you are running any of these IOS versions on a product that uses IP extended access lists, and you are using the 'tacacs-ds' or 'tacacs' keyword in these lists, then Cisco strongly recommends that you review your access lists to insure that they have been parsed correctly. You can determine what version of IOS you are running by issuing the following command:
show version
If your access list has been parsed incorrectly, the recommended action is to upgrade to a more recent version of IOS or perform the workaround described below. The bug is fixed by in the following official software releases:
10.3(4.3) or later
(For reference, the Cisco update identifier for this fix is "CSCdi36962".)
Products Confirmed Not Vulnerable
No other Cisco products are currently known to be affected by these vulnerabilities.
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A bug in certain versions of IOS can cause extended IP access lists to be parsed incorrectly. Under some circumstances, this may allow packets to bypass IP packet filtering. This may permit unintended IP traffic to pass through a filtering router.
IP extended access lists between versions 10.3(1) through 10.3(3.3) used the keyword 'tacacs-ds'. This keyword could be saved as part of the router configuration either in non-volatile memory on the router or on an external TFTP server.
Configuration files written by these versions which are read by versions 10.3(3.4) through 10.3(4.2) will not have the 'tacacs-ds' keyword parsed correctly. The result will be that the entire line in the access list will be ignored. An error message will be generated when this occurs. Loss of such a line from the access list may create a vulnerability if the access list is used as part of a packet filter.
To determine if you are vulnerable, examine your current configuration and compare it to your intended configuration.
If the access lists in your current configuration and your intended configuration do not use the keyword 'tacacs-ds', you are not vulnerable. You do not need to do anything.
If your current configuration contains the keyword 'tacacs-ds', you should NOT upgrade that router to any version of IOS between 10.3(3.4) and 10.3(4.2). You are not currently vulnerable.
If your intended configuration contains the keywords 'tacacs-ds', 'tacacs', or filters on TCP or UDP port 49, and your current configuration does NOT contain this line of the access list, you are currently vulnerable. You should perform the workaround described below.
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The following actions will remove the vulnerability:
Delete the access list and re-enter it based upon your intended configuration. Do not enter the 'tacacs-ds' keyword. Use the keyword 'tacacs' instead.
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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:
certjuly31
for a list of available files to download.
FTP
ftp cco.cisco.com and at the initial (username) prompt, enter:
certjuly31
At the password prompt, enter your e-mail address. Then:
get README.certjuly31
This file contains a list of files available that close this vulnerability. Please examine this list to determine which files you need and then download them.
Character-based "CCO Classic"
For access, the following connection options are offered:
telnet cco.cisco.com
Dial-up modem
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In Europe +33 1 64 46 40 82
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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:
certjuly31
A list of files will be displayed for you to select and download.
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In Europe +33 1 64 46 40 82
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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 .
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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.
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Revision 1.0
1995-July-31
Initial public release.
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